In a longitudinal wave, the particles in the medium move parallel to the direction of propagation. Compression and rarefaction (expansion) are formed in the medium. An example of longitudinal wave is sound waves. For a longitudinal wave, the distance between two consecutive compressions or two consecutive rarefactions measured in metres (m) is known as the wavelength, λ. The measurement of how fast the wave propagates through a medium is known as the speed of wave. The speed of wave, v is the distance a wave travels per second. The relationship between speed, v, wavelength, λ, period, T and frequency, f is given by the following equation: The medium in which the waves travel through affects the speed of the waves. For example, sound waves take a shorter time to travel in water than in air. The stages in the formation of longitudinal waves with the compressions and rarefactions seen moving towards the wall Longitudinal wave Direction of vibrations Wave direction Compression Compressions Rarefaction Compression Rarefaction Direction of propagation of wave Wavelength v = λ T = fλ Chapter 4 Vibrations, Waves and Light 95 ©Praxis Publishing_Focus On Science
Based on the ability to travel through a vacuum, waves can be categorised into electromagnetic waves and mechanical waves. Sound Sounds of birds chirping, traffic and the wind howling are produced in the same way although they are from different sources. Sound is produced by vibrations and detected by our ears. A laser (Light Amplification by Stimulated Emission of Radiation) is a device that gives out a highly concentrated narrow beam of electromagnetic waves. Laser beams can be made up of infrared rays, visible light and ultraviolet rays. There are many uses for laser beams such as: Science Facts Musical instruments produce sound through vibrations. When you strike the prongs of a tuning fork and bring it near your ear, you can hear a sound caused by the vibrations of the prongs, but you cannot observe the vibrations easily like in some other objects. If you touch your throat while you speak, you can feel the vibrations. The sound we make comes from the vibrations of the vocal cords in the voice box. • to create special effects in the entertainment industry • to calculate distances • to read CDs • to cut, drill and engrave metals • to read barcodes • to carry information in fibre optic cables • to be used in laser printing Electromagnetic waves • Electromagnetic waves do not require a medium to travel through, they can travel through a vacuum. • The waves contain an electric field and a magnetic field, and travels at the speed of light which is 3 × 108 m s–1. • Radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and gamma rays are examples of electromagnetic waves. Mechanical waves • Mechanical waves require a medium to travel through from one point to another as they cannot travel through a vacuum. • Mechanical waves can be longitudinal or transverse. • Sound waves (longitudinal waves), ocean waves (transverse waves) and seismic waves are examples of mechanical waves. Prongs 96 ©Praxis Publishing_Focus On Science
Sound is propagated through a medium from one point to another point. Sound can only propagate through a liquid, solid and gas. It cannot propagate through a vacuum. Activity 2 Aim: To investigate whether sound waves require a medium to propagate Materials and apparatus: Bell jar, electric bell and vacuum pump Procedure: 1 Set up the apparatus as shown in the diagram. 2 Switch on the bell and record your observation. 3 Then, switch on the vacuum pump. Record your observation. Observation: Write down what you observe. Discussion: 1 What is the purpose of using the vacuum pump? 2 Do sound waves require a medium to propagate? If yes, what is the medium in this activity? Conclusion: Write down your conclusion. Sound waves require a medium to propagate To vacuum pump Electric bell Bell jar To electric connection Sound travels at different speeds in different mediums. Sound waves pass through solids, liquids and gases in the same way. The vibrations are passed from one particle to another. In a liquid, the particles are closer to one another compared to that in the air. Thus, vibrations will be passed on much faster compared to air. In a solid, the particles are much closer to one another compared to that in the liquid. Thus, vibrations will be passed on much faster in a solid compared to in a liquid. Therefore, sound travels the fastest in solids, fast in liquids and the slowest in gases. Science Facts The speed of sound in is approximately 340 m/s at 20ºC. is approximately 1482 m/s. is approximately 5640 m/s. Air Water Glass Wood is approximately 3960 m/s. Chapter 4 Chapter 4 Vibrations, Waves and Light Vibrations, Waves and Light 97 ©Praxis Publishing_Focus On Science
We can measure the speed of sound in air with the following method. Microphone 2 Microphone 1 1 m Metal Hammer 0.003 Start Stop Digital timer Two microphones are placed 1 m apart and connected to a digital timer as shown in the diagram above. A loud sound is created by hitting a hammer on a metal block. Microphone 1 starts the timer when it detects the sound and microphone 2 stops the timer when the sound is detected. The sound travelled 1 m in 0.003 s. The speed of sound can then be calculated using the speed equation: Speed of sound = Distance travelled Time taken = 1 0.003 ≈ 333 m/s How Does Sound Travel? Do you know how sound travels from its source to your ears? The diagram below shows how the sound from a tuning fork travels when its prongs vibrate. Compression Rarefaction Compression Rarefaction Tuning fork Close-up to show air particles Direction of sound wave Ear 98 ©Praxis Publishing_Focus On Science
When the prongs of the tuning fork move outwards, the air molecules in the surroundings next to the prongs are pushed and compressed forming a region of high pressure called compression. When the prongs of the tuning fork move inwards, the air molecules in the surroundings next to the prongs are spread out forming a region of low pressure called rarefaction. The air molecules around the prongs compress and spread out repeatedly as the prongs vibrate. The vibrating air molecules will vibrate against any nearby molecules causing the vibrations to move outwards. The sound travels through the air as sound waves, which are a series of rarefactions and compressions of air molecules. When the sound waves are detected by our ear, the eardrum vibrates and the brain interprets this vibration as sound. The strings of a violin vibrate when plucked, and sound waves are produced. When a saxophone is blown, the air column inside vibrates, and sound waves are produced. A drum skin vibrates when it is hit, and sound waves are produced. Chapter 4 Chapter 4 Vibrations, Waves and Light Vibrations, Waves and Light 99 ©Praxis Publishing_Focus On Science
How Do We Hear Sound? Do you know how we hear sounds? The human ear can be divided into the outer ear, middle ear and inner ear. The ears will convert sound into electrical signals which are then sent to the brain to be interpreted. The chart below shows how our ears detect sounds. Semicircular Ossicles canals Ear canal Eardrum Auditory nerve Cochlea Oval window Eustachian tube Pinna Outer ear Middle ear Inner ear Subcutaneous layer Sound waves from the air enter through the ear pinna of the outer ear and travel through a narrow passageway called the ear canal, which leads to the eardrum. The eardrum which is a thin sheet of membrane vibrates from the incoming sound waves. The vibrations of the ear drum are then transferred to the ossicles, the three tiny ear bones in the middle ear. The ossicles amplify the sound vibrations and send them to the cochlea, a snail-shaped structure filled with fluid and tiny hairs, in the inner ear. The tiny hairs respond to the vibrating fluid by producing tiny electrical signals. These signals are sent along the auditory nerve to the brain, which turns it into a sound that we recognise and understand. 100 ©Praxis Publishing_Focus On Science
Pitch and Loudness of Sound Women usually have voices of a higher pitch than men do. Do you know what is meant by pitch? The pitch of a sound refers to how high or low a sound seems to a listener. The pitch depends on the frequency of sound waves. Frequency is the number of vibrations per second. It is measured in hertz (Hz). An object that vibrates fast produces a sound with high frequency, therefore the sound is of high pitch. An object that vibrates slowly produces a sound with low frequency, therefore the sound is of low pitch. Some sounds are loud and some are soft. The loudness of a sound depends on the amplitude of the sound waves. The amplitude of a sound wave is a measure of how much energy it carries. Sounds of higher amplitude are louder compared to sounds of lower amplitude. Sounds of higher amplitude are produced by larger vibrations and carry more energy. The harder a person plucks a guitar string, the louder the sound it produces. This is due to more energy being transferred by that person to the string, thus larger sound waves (sound waves of higher amplitude) are produced. When the person plucks the guitar string very lightly, a softer sound is produced. This is due to less energy being transferred by that person to the string. As a result, much smaller sound waves (sound waves of lower amplitude) are produced. The rate at which sound energy passes through a unit area is called the sound intensity. Sound waves with greater amplitude carry more energy, have greater intensity and is louder. As the sound waves travel farther from the source, the same amount of energy is spread over a greater area, causing the intensity and loudness of the sound to reduce. This is the reason sound fades away over distance. Amplitude Sounds of higher amplitude are louder Amplitude Sounds of lower amplitude are softer A child usually has a voice of a higher pitch than an adult does. Chapter 4 Vibrations, Waves and Light 101 ©Praxis Publishing_Focus On Science
Activity 3 Aim: To study the effect of amplitude on the loudness of sound, and the effect of frequency on the pitch of sound A Effect of amplitude on the loudness of sound Materials and apparatus: Tuning fork, cathode ray oscilloscope (C.R.O.), microphone and connecting wires Procedure: Oscilloscope Rubber hammer Tuning fork Microphone 1 Connect a microphone to an oscilloscope and switch it on. 2 Hold the microphone in front of the tuning fork and hit the tuning fork gently with a rubber hammer. 3 Listen to the loudness of the sound produced and observe the trace produced on the screen of the oscilloscope. Draw a diagram of the trace. 4 Repeat steps 2 and 3 using the same tuning fork but this time hit the tuning fork harder than previously. Observation: Action Loudness of the sound produced Trace on screen Hit gently Hit hard Discussion: 1 Which action produces a loud sound? 2 Which action produces sound waves of a higher amplitude? Conclusion: Write down your conclusion. Loudness and pitch of sound 102 ©Praxis Publishing_Focus On Science
B Effect of frequency on the pitch of sound Materials and apparatus: Tuning fork with short prongs, tuning fork with long prongs, cathode ray oscilloscope (C.R.O.), microphone and connecting wires Procedure: 1 Connect a microphone to an oscilloscope and switch on. 2 Hold the microphone in front of the tuning fork with the short prongs and hit the tuning fork. 3 Observe the trace produced on the screen of the oscilloscope. Draw the diagram of the trace. 4 Repeat steps 2 and 3 using the tuning fork with the long prongs. Observation: Tuning fork Tuning fork with short prongs Tuning fork with long prongs Trace on screen Discussion: Which tuning fork produces a sound of higher pitch? Give the reason. Conclusion: Write down your conclusion. Doppler Effect At a racing track, do you know why the race car’s engine sounds different when the car approaches you versus when it is moving away from you? This is due to the Doppler effect. The Doppler effect is the apparent change in the frequency of the sound waves as the sound source and observer move towards or away from each other. One common example of Doppler effect is shown in the diagram below where the ambulance is the sound source while P and Q are two stationary observers. Sound waves from the ambulance siren travel outward in all directions. Low frequency High frequency P Q As the ambulance moves forward and approaches observer Q, sound waves get closer together in front of the ambulance and its frequency increases. Hence, the pitch of the sound heard by observer Q is higher. As the ambulance drives away from observer P, the sound waves spread out behind the ambulance and its frequency decreases. Hence, the pitch of the sound heard by observer P is lower. Chapter 4 Vibrations, Waves and Light 103 ©Praxis Publishing_Focus On Science
Bat Emitted waves Reflected sound waves Dolphin Emitted waves Reflected sound waves Reflection of Sound When sound travels through a medium and hits the surface of an object, it is reflected. Echoes are produced by the reflection of sound from a surface. The reflection of sound waves is affected by the types of surfaces. Hard, flat and smooth surfaces like marble tiles and wooden panel floors are good sound reflectors. That is the reason you hear echoes in empty rooms. Marble tiles Plank Good sound reflectors Echolocation is a technique used by some animals to determine the location of distant or invisible objects using the reflection of sound. Why are the floor of cinemas or auditoriums covered with carpets and the walls are covered by curtains of soft materials? Think About It Bats use echolocation to navigate and find prey in the dark. A bat sends out sound waves of very high frequency. When the sound waves hit an object such as an insect, an echo is produced. The echo bounces off the insect and returns to the bat’s ear. The bat listens to the echo to figure out the location of the insect so that it can catch the insect even in total darkness. Dolphins use echolocation to find fish and avoid obstacles underwater where there is very little light. As sound travels faster in water than in air, dolphins can send sound waves over long distances. 104 ©Praxis Publishing_Focus On Science
Types of Sounds Do you think human beings can hear all the sounds in their surroundings? Human beings can only hear sounds with a frequency between about 20 Hz (20 vibrations per second) and 20 000 Hz (20 000 vibrations per second), which we call audible sound. Some examples are the sounds of people talking, school bell ringing and cars honking. As we grow older, our hearing deteriorates, and the frequency range becomes smaller. Sounds with frequencies below 20 hertz are called infrasound. Infrasound is a low-pitched sound. Our ears cannot detect this sound because it is below the normal human hearing range. Infrasound only can be heard by certain animals such as crickets and dogs. Some animals such as elephants and giraffes use infrasound to communicate over long distances. Volcanoes and earthquakes produce sounds we can hear as well as sounds below the range of human hearing known as infrasound. Bats and dolphins use ultrasound for communication and for echolocation. The approximate hearing ranges of some animals Sound Navigation and Ranging (SONAR) is a technique that uses sound waves to detect underwater objects or the seabed. In this technique, a device sends out ultrasound waves from a ship. The waves are reflected by objects. The reflected waves are commonly known as echoes. The time interval between the emission of the ultrasound waves and when the echo is received depends on the depth of the object below the ship. The distance of the object from the ship can be determined using the speed of sound in water and the time difference. This is how submarines can travel underwater safely. Science Facts Sounds with frequencies above 20 000 hertz are called ultrasound. Ultrasound is a high-pitched sound. Our ears cannot detect this sound because it is not within the normal human hearing range. Ocean Seabed Transmitter Receiver Animal Hearing range (Hz) Elephant 16 – 12 000 Sea lion 450 – 50 000 Killer whale 800 – 13 500 Cat 45 – 65 000 Rat 200 – 76 000 Animal Hearing range (Hz) Chicken 125 – 2000 Horse 55 – 33 500 Rabbit 360 – 42 000 Cow 23 – 35 000 Goat 78 – 34 000 Chapter 4 Vibrations, Waves and Light 105 ©Praxis Publishing_Focus On Science
Dangers of Loudness of Sound A louder sound carries more energy compared to a soft sound. Loudness of sound is measured with a sound meter in decibel (dB). Our ears will be painful if we hear sounds above 110 dB. This can cause permanent damage to the ears, such as rupture of the eardrum if we are exposed to the sound for more than an hour. Here are how loud some sounds are in decibels. If you are unable to reduce or avoid the noise in your surroundings, it is advisable to wear ear protectors to insulate your ears from loud sounds. Whisper 20 dB Light rainfall 50 dB Fighter jet flying 140 dB Space shuttle takes off 180 dB Electric drill 95 dB Lawn mover 90 dB Chainsaw 100 dB Subway train 100 dB 106 ©Praxis Publishing_Focus On Science
4.3 Light and Optical Instruments Light is an electromagnetic wave. Light can travel through a vacuum and through different mediums such as air and water. We can see the moon, distant stars and galaxies because light from them travels through outer space (mostly vacuum) and the Earth’s atmosphere to reach our eyes. Some of the sources of light are the Sun, a fire and a lamp. The path along which light travels is known as a ray. When we draw a ray, we use a straight line with an arrow. The arrows show the direction of the light ray. Our shadow becomes shorter at noon compared to in the morning or the evening. Why? Think About It Luminous objects are objects that are light sources giving out their own light. For example: the Sun, stars, a lamp and fire. Non-luminous objects are objects that do not give out their own light. They only reflect the light given out by light sources. For example, the Moon, planets, trees, cats and tables. Science Facts The beam of lights coming from spotlights at a podium show that light travels in a straight line. Light travels at 3.0 × 108 m s–1, which is faster than sound, therefore we see the lightning before we hear the sound of thunder during a thunderstorm. The Sun is a natural source of light energy. The shadow of the bicycle is formed when sunlight is blocked by the bicycle. Some facts about light The speed of light is much higher than the speed of sound When light is blocked by an opaque object, a shadow is formed behind the object Light travels in a straight line Light is a form of energy Chapter 4 Chapter 4 Vibrations, Waves and Light Vibrations, Waves and Light 107 ©Praxis Publishing_Focus On Science
Shadow of the apartment Apartment Activity 4 Aim: To study how light travels Materials and apparatus: Three cardboards, A, B and C, a pin and a candle Procedure: A B C I B A C II 1 Make a small hole at the same height using a pin on each of the three cardboards, A, B and C. 2 Fix the three cardboards vertically in such a way that their holes are aligned in a straight line as shown in diagram I. 3 Place a lit candle near cardboard C. 4 Look through the hole in cardboard A. 5 Record your observation. 6 Now, push cardboard B slightly out of its position so that the three holes do not remain in a straight line as shown in diagram II. 7 Look through the hole in cardboard A. 8 Record your observation. Observation: Write down your observation. Discussion: 1 When the holes on the cardboards are in a straight line, can you see the candle flame? Give a reason. 2 When the holes on the cardboards are not in a straight line, can you see the candle flame? Give a reason. Conclusion: Write down your conclusion. Light travels in a straight line Shadows The shadow of an object is formed when the path of light is blocked by the object. This is because light travels in a straight line. Since light cannot bend to go around the apartment, a shadow forms in front of the apartment. The apartment is opaque and does not allow light to pass through. 108 ©Praxis Publishing_Focus On Science
Reflection of Light When light reaches the surface of an opaque object, it may be absorbed, transmitted or reflected. Reflection of light occurs when light bounces off the surface of an object it strikes. What is the angle of incidence and angle of reflection when a light ray strikes the surface at a right angle? Think About It If a surface is smooth like in a plane mirror, then the image formed by reflection is sharp and clear. This is because all the reflected rays travel in the same direction. This type of reflection is called regular reflection. However, if the surface is uneven or rough, an image may not form, or if there is an image, it will be blurry. This is because light is scattered from the uneven and rough surface. This type of reflection is called diffused reflection. The reflection of light follows the law of reflection that states • the incident ray, the reflected ray and the normal to the surface lie on the same plane • the angle of incidence is equal to the angle of reflection. Reflection from a mirror Reflection from a smooth surface Reflection from a rough surface The angle of incidence is the angle between the incident ray and the normal. The incident ray is the light ray that strikes the surface. Mirror The reflected ray is the light ray that bounces off the surface. The angle of reflection is the angle between the reflected ray and the normal. Reflected rays are parallel Incident rays Reflected rays are not parallel Incident rays The normal is a line drawn perpendicularly to the surface of the mirror. Chapter 4 Vibrations, Waves and Light 109 ©Praxis Publishing_Focus On Science
Activity 5 Aim: To study the law of reflection in light Materials and apparatus: A ray box, power supply, a plane mirror, white paper and protractor Procedure: Angle of reflection Reflected ray First line Incident ray Ray box Angle of incidence Second line Paper Mirror Normal 1 Draw a straight line on a piece of paper and draw a second line perpendicular to the first line. The second line is the normal. 2 Place the edge of a plane mirror on the paper along the first line. 3 Shine a light ray on the mirror at an angle. 4 Use a pencil to draw the path of the both the incident and reflected rays. 5 By using a protractor, measure both the angles of incidence and reflection from the normal. 6 Repeat the activity with different angles of incidence. 7 Record the angles in the table provided. Observation: Angle of incidence Angle of reflection Discussion: What is the relationship between the angle of incidence and the angle of reflection? Conclusion: Write down your conclusion. Law of reflection Caution Do not touch the lamp in the ray box because it may be very hot. Example 2 The diagram on the left shows an incident ray on a plane mirror. Find the angle of reflection. Solution: Angle of incidence = 140° – 90° = 50° Angle of reflection = Angle of incidence = 50° 140° 50° 50° 110 ©Praxis Publishing_Focus On Science
Activity 6 Aim: To study characteristics of the image formed by a plane mirror Materials and apparatus: Plane mirror, graph paper and mirror holder Procedure: 1 Write the word ‘SWEET’ on the graph paper. 2 Place a plane mirror vertically on the line PQ. 3 Record your observation about the image formed. 4 Measure the distance between the word on the graph and the mirror and the distance between the image formed and the mirror by counting the squares on the graph paper. Observation: Write down your observations. Discussion: 1 How is the image formed in the plane mirror? 2 The image formed in a plane mirror is virtual. What does this mean? Conclusion: Write down your conclusion. SWEET SWEET Image formed by a plane mirror Reflection from a plane mirror Image is upright Image is virtual (cannot be reproduced on a screen) Size of image = Size of the object Image is laterally inverted (left to right inversion) Distance of image from mirror = Distance of object from mirror Characteristics of image formed by a plane mirror P Q Chapter 4 Vibrations, Waves and Light 111 ©Praxis Publishing_Focus On Science
A ray diagram enables a person to view a point on the image of an object. The chart below shows the steps to draw the ray diagram of a dot reflected by a plane mirror. Why is the word ‘AMBULANCE’ printed laterally inverted on the front of an ambulance? Think About It A kaleidoscope is a tube with two or more plane mirrors placed at an angle to each other, usually forming a V-shape or a triangle. Small pieces of coloured objects are placed in between the mirrors. A kaleidoscope works on the principle of multiple reflection. What we see through the kaleidoscope is the light from the objects that is reflected from one mirror to another, forming beautiful images. Science Facts Some examples of beautiful images formed in a kaleidoscope Kaleidoscope A dotted line is drawn from object, O perpendicular to mirror M and the line is extended into the mirror. The distance between object, O and the mirror is the same as the distance between image, I and the mirror. Therefore, we can obtain the location of image, I. M I O Image, I is joined to the eye by drawing dotted lines within the mirror and then with solid lines from the surface of the mirror onwards . These solid lines represent the reflected rays. Object, O is joined to the positions of the reflected rays on the mirror by drawing solid lines. These solid lines represent the incident rays. M I O M I O Eyepiece Circular tube that contains plane mirrors Coloured objects inside 112 ©Praxis Publishing_Focus On Science
Refraction of Light Light can pass through transparent mediums such as water, air and glass. Refraction of light is a phenomenon in which the direction of the light changes as it travels from one transparent medium to different medium, due to the change in the speed of light. The refraction of light follows the law of refraction that states • The incident and refracted rays are on opposite sides of the normal at the point of incidence and all of them lie on the same plane. • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always a constant. Light travels at different speeds in mediums of different densities. The diagrams below show the refraction of light when light rays pass through mediums of different densities. The light ray is not refracted when the incident ray is parallel to the normal whether it moves from a less dense medium to a denser medium or from a denser medium to a less dense medium. Light slows down when the incident ray moves from a less dense medium to a denser medium. Therefore, the light changes direction and is refracted towards the normal. The angle of refraction is less than the angle of incidence. Light speeds up when the incident ray moves from a denser medium to a less dense medium. Therefore, light changes direction and is refracted away from the normal. The angle of refraction is greater than the angle of incidence. Normal Incident ray Angle of incidence Angle of refraction Refracted ray Air (less dense) Water (more dense) Normal Incident ray Angle of incidence Angle of refraction Air (less dense) Water (more dense) Refracted ray Normal Incident ray Refracted ray Air (less dense) Water (more dense) Normal Incident ray Refracted ray Air (less dense) Water (more dense) The spoon appears bent in the water because of the refraction of light. Chapter 4 Vibrations, Waves and Light 113 ©Praxis Publishing_Focus On Science
Activity 7 Aim: To study the refraction of light Materials and apparatus: Glass block and ray box with a single slit plate, plastic ruler, power supply, white paper and protractor Procedure: 1 Shine a ray of light at an angle into a glass block as shown in diagram I. 2 Observe the movement of the ray of light. 3 Next, shine the ray of light vertically into the glass block so that it travels at a right angle with the glass block as shown in diagram II. 4 Observe the movement of the ray of light. Observation: Record your observations. Discussion: 1 When light is directed at an angle to the glass block, was it refracted? 2 When light is directed vertically to the glass block, was it refracted? Conclusion: Write down your conclusion. Glass block Ray box I Glass block Ray box II Refraction of light Effects of Refraction Refraction of light gives us a false impression of the actual depth and position of an object in the water. Actual depth Observer Apparent depth Air Water Bottom of the swimming pool Actual depth Observer Apparent depth A swimming pool looks shallower than it really is. This is because the light from the bottom of the swimming pool is refracted away from the normal when it emerges from the water into the air. When the refracted light enters the eyes of an observer, he sees an image of the bottom of the swimming pool somewhere above the actual depth of the bottom of the pool. The depth viewed by the observer from above is called the apparent depth. A pencil in a glass of water looks bent when viewed from the air. This is because the light from the pencil is refracted away from the normal when it emerges from the water into the air. When the refracted light enters the eyes of an observer, he sees the image of the pencil somewhere above the bottom of the glass. Thus, the pencil looks bent. 114 ©Praxis Publishing_Focus On Science
On a sunny day, if you stare long enough, you can see a pool of water in the distance as you are travelling on the road. However, when you reach the spot, the pool of water is not there. This phenomenon is called a mirage. The mirage is caused by layers of air of different temperatures and thicknesses on a sunny day. The sun heats up the ground which in turn heats up the air just above it. When light travels through the layers of air, it refracts, fooling our eyes into seeing something which does not exist, which resembles water. The mirage of pools of water normally seen in deserts or roads is actually the image of the clouds in the sky. White light is not a single colour, it is actually a combination of several colours which can be separated by refraction. When light passes through a prism (triangular glass block) as shown in the diagram on the right, the light is refracted twice: when it enters, and leaves the prism. Both these surfaces are not parallel. Each colour is refracted by a different amount, thus the colours spread out slightly and this effect is known as dispersion. Red light which has the highest speed is refracted the least from the normal whereas violet light which has the slowest speed is refracted the most from the normal. The light is dispersed into a spectrum of seven colours which are red, orange, yellow, green, blue, indigo and violet. White light Normal Glass prism Rainbows are produced when light from the Sun is both reflected and refracted by the tiny water droplets in the air usually after a shower of rain. The water droplets act like prisms. We can only see a rainbow when the sun is low and behind us. The light that enters and leaves the water droplet is refracted twice. This causes the dispersion of the light into seven different colours to form the rainbow. Sunlight Violet Red Water droplet Very hot air Hot air Warm air Cool air Refracted light Pool of water (apparent image) Cloud Formation of a mirage Refraction of light through a glass prism Formation of rainbow Resource Chapter 4 Vibrations, Waves and Light 115 ©Praxis Publishing_Focus On Science
Activity 8 In this activity, use a ray box and a prism to separate out the colours of white light into a spectrum. The prism must be placed carefully if you want to see all the colours clearly. Observe which colour is refracted the most and the colour refracted the least. Can you identify all seven colours in the spectrum? Dispersion of light Why is light dispersed when it passes through a prism, but not a rectangular glass block? Think About It Optical Lenses Optical lenses are made of transparent materials such as glass and plastic with curved sides. As a ray of light enters a lens, it is refracted. It is refracted again when it leaves the lens. There are two main types of lenses: convex or converging lenses and concave or diverging lenses. Focal point, F Parallel rays passing through a convex lens Principal axis Optical centre, C f Parallel rays passing through a concave lens Focal point, F Principal axis Optical centre, C f • A convex lens is thicker in the middle. • Light rays that are parallel and close to the principal axis are refracted inwards and converge to a focal point, F after passing through the lens. • The thicker the lens is at the centre, the shorter is the focal length, f of the lens. • A concave lens is thicker around the edge. • Light rays that are parallel and close to the principal axis are refracted outwards and appear to diverge from the focal point, F after passing through the lens. • The thinner the lens is at the centre, the shorter is the focal length, f of the lens. Convex lens Concave lens Principal axis → a straight line passing through the centres of both the curvatures of the lens. Optical centre, C→ a point on the principal axis of a lens through which light passes without any deviation. Focal point, F → a point on the principal axis where all rays originally parallel and close to the principal axis converge to it after passing through a convex lens or appear to diverge from it after passing through a concave lens. Focal length, f → the distance between the focal point, F and the optical centre, C of the lens. 116 ©Praxis Publishing_Focus On Science
Steps to draw a ray diagram for a convex lens Draw a straight line which is the principal axis and curved lines which are the convex lens. 1 Mark the position of the optical centre as well as the positions of focal point, F and 2F. 2 Draw an upright arrow which is the object, on the principal axis at the given distance. 3 Draw a ray from the top of the object, parallel to the principal axis until it reaches the centre of the lens. From there, draw the refracted ray passing through the focal point, F on the opposite side. 4 Draw a ray from the top of the object that passes through the optical centre without any deviation. This is because the ray that passes through the optical centre will not change its course. 5 Draw a ray from the top of the object, passing through the focal point, F on the object’s side until it reaches the centre of the lens. From there, draw the refracted ray parallel to the principal axis on the opposite side until it meets the other refracted ray. 6 Draw the image using an arrow. The head of the arrow is the point where the refracted rays meet. The image is real, which means that the image can be projected onto a screen. 7 Principal axis Object distance, u 2F F F 2F Real image Image distance, v Object F F 2F 2F 1 2 3 4 5 6 7 Chapter 4 Vibrations, Waves and Light 117 ©Praxis Publishing_Focus On Science
The characteristics of the image formed by a convex lens depend on the object distance. The table below shows the characteristics and position of each image formed by a convex lens. Object distance, u Ray diagram Image distance, v Characteristics of image Uses At infinity: u = ∞ Parallel rays from distant object Image F F v = f At F and on the opposite side of the lens: v = f Real, inverted and diminished Objective lens of a telescope Beyond 2F: u > 2f Image F 2F 2F F u > 2f Object v < 2f Between F and 2F and on the opposite side of the lens: f < v < 2f Real, inverted and diminished Single-lens camera At 2F: u = 2f Object F 2F 2F u = 2f v = 2f F Image At 2F and on the opposite side of the lens: v = 2f Real, inverted and same size as object Photocopier to make copy of equal size Between F and 2F: f < u < 2f 2F 2F f < u < 2f v > 2f Image Object F F Beyond 2F and on the opposite side of the lens: v > 2f Real, inverted and magnified Projector and photograph enlarger At F: u = f 2F 2F u = f F F Image at infinity Parallel rays Object At infinity: v = ∞ Virtual, upright and magnified Spotlight Between F and C: u < f v > f F F Image Object u < f Behind the object and on the same side of the lens: v > f Virtual, upright and magnified Magnifying glass *Solid lines are used for real images and dotted lines are used for virtual images. *Real image can be projected onto a screen whereas virtual image cannot be projected onto a screen. 118 ©Praxis Publishing_Focus On Science
Vision Vision is one of our senses and we rely greatly on vision to understand the world around us. We use our eyes to see things in the surroundings and gather information about them. The chart below shows the mechanism of vision. Object Image F F As for a concave lens, there are three main rays in the construction of the ray diagram. The image is situated on the same side of the object, between the object and the lens. The image formed by a concave lens is always virtual, upright and diminished, regardless of the position of the object from the mirror. Light enters our eyes by passing through the cornea, a transparent surface in front of the eye. Light passes through the pupil, an aperture-like structure located at the centre of the iris (the coloured part of the eye). In bright light, the pupil becomes smaller and in the dark, the pupil becomes larger. Light passes through the lens behind the pupil and through refraction, the rays of light are focused onto the retina. (Lens thickness, governed by the ciliary muscles, is responsible for the focus of the eye. To focus nearby objects, ciliary muscles contract causing the lens to become thicker and to focus on distant objects, ciliary muscles relax causing the lens to become thinner.) When light is focused onto the retina, a real, inverted and diminished image is formed. Retina contains lightsensitive cells, rod and cone cells. The rod cells enable vision in very dim light but do not allow detection of colours, while the cone cells enable vision in bright light and allow detection of colours. When light falls onto these cells, the light is converted to electrical signals that are sent to the brain through the optic nerve. The brain collects all the signals and interprets the image, giving it its actual size and details of the object. Cornea Pupil Iris Lens Retina Ciliary body and muscles Optic nerve Resource Structure of human eye Chapter 4 Vibrations, Waves and Light 119 ©Praxis Publishing_Focus On Science
Insects can see and they have many eyes known as compound eyes. It is made up of ommatidia (singular: ommatidium) where each of it functions as a separate light receptor. Although the compound eyes are unable to focus, they can alert insects of the presence of their enemy because they can look in many different directions at the same time. The structure of a compound eye is different from the human eye, but both have lenses and light-sensitive cells. The insect eye has many tiny lenses with one lens per ommatidium whereas the human eye has a single large lens. For an insect, the lens of each ommatidium focuses light on a few light-sensitive cells without any adjustments. For human eye, there are many light-sensitive cells that work together to form an image. Science Facts Optical Instruments Optical instruments are devices with a single or a combination of lenses that process light waves to enhance an image for clearer viewing. Pinhole Camera The pinhole camera is an ancient form of camera. There is no lens used in a pinhole camera. There is only a small hole at the front for the rays of light to enter and a simple screen (photo-sensitive paper) at the back where an inverted image can be seen. Activity 9 In this activity, use a cuboid-shaped box and tracing paper. The box must not have any gaps that allow light in so that the box is completely dark inside. Procedure: 1 Cut out the back surface of the box and cover it with tracing paper to create a screen. 2 Use a sharp pin to make a small hole at the front of the box. 3 Point the hole towards a bright object like a candle in a darkened room and you will see an image of the object on the screen. 4 Move the camera closer to the object, what do you observe? 5 Move the camera further from the object, what do you observe? Discussion: 1 What do you notice about the image formed on the screen? 2 In what way does the image change as the camera is moved towards the objects? Pinhole Object Image Screen X X Y Y Making a pinhole camera A dragonfly has compound eyes containing up to 30 000 little lenses per eye. Secondary pigment cells Rhabdom Retinula cells Crystalline cones An ommatidium Primary pigment cells Lenses Axons 120 ©Praxis Publishing_Focus On Science
Camera In a camera, the image of an object is captured in a similar way to the human eye. The light passes through a lens and focuses to form an image. The characteristics of the image formed in a camera are similar to the one formed in our eyes. The converging lens in our eyes brings rays of light together towards a point known as the focal point to create an image. Similarly, a camera uses a converging lens to focus light. Since the lens in a camera is solid glass and its shape cannot be changed, the distance between the lens and the film needs to be changed to focus the light. The lens of the camera moves inwards or outwards unlike the eye which changes the shape of the lens, to focus the light. In a simple non-digital camera, the light from the lens falls onto a photographic film. Nowadays, electronic components that detect light levels directly, known as charged couple devices (CCDs) are used in digital cameras to record images. Magnifying Glass A magnifying glass uses a single lens to magnify objects. The object to be magnified is placed between the focal point, F and a convex lens. A magnified, upright and virtual image is produced. The image can only be seen by looking through the lens, and it appears on the same side of the lens as the object. Spectacles When the lenses in the eyes cannot focus light onto the retina, the lenses in spectacles can be used to rectify certain defects in vision. In people with long-sightedness (unable to see near objects clearly), the image would form behind the retina, thus a blur image forms on the retina. To rectify this condition, a convex lens is used to help focus the image on the retina. In people with short-sightedness (unable to see distant objects clearly), the image would form in front the retina, thus a blur image forms on the retina. To rectify this condition, a concave lens is used to help focus the image on the retina. Contact lenses work the same way as spectacles, but they are placed directly onto the cornea. Telescope A telescope is an instrument that uses a combination of lenses to increase the magnification of an image. It is used to magnify distant objects. A simple astronomical telescope used to view planets and stars contains two lenses, an eyepiece and an objective lens to receive light. In a modern telescope, images are captured on a CCD like the one in a digital camera. Principal axis Object Convex lens Focal point Film or CCD Real, inverted image Object Image Image of object is highly magnified and on the same side as the object fe I Parallel rays from distant object Objective lens Final image at infinity Eyepiece LO = f O + f e Chapter 4 Vibrations, Waves and Light 121 ©Praxis Publishing_Focus On Science
1 motion is a motion repeated at equal intervals of time. 2 is a periodic motion in which an object moves back and forth repeatedly about the equilibrium position. 3 is the maximum displacement of an object from its equilibrium position. 4 is the time taken for one complete cycle. 5 is the number of complete cycles in one second and its SI unit is . 6 A wave is a travelling disturbance from a source. 7 In a transverse wave, the particles in the medium move to the direction of propagation of the wave. 8 For a transverse wave, the horizontal distance between two consecutive crests or between two consecutive troughs is known as the . 9 In a longitudinal wave, the particles in the medium move to the direction of propagation of the wave. 10 Electromagnetic waves do not require a to travel through, they can travel through a vacuum. 11 Mechanical waves cannot travel through a . 12 Sound is produced by and detected by our ears. 13 An object that vibrates slowly produces a sound with low frequency, therefore the sound is of a pitch. 14 The loudness of a sound depends on the of the sound waves. 15 The effect is the apparent change in frequency of sound waves as the sound source and observer move towards (or away from) each other. 16 is the use of sound waves and echoes to determine distant objects. 17 Sounds with frequencies below 20 hertz are called . 18 Loudness of sound is measured with a sound meter in . 19 Light travels in a line. 20 of light occurs when light bounces off the surface of an object it strikes. 21 of light is the bending of a light ray due to the change in the speed of the light when it enters a medium. When light is focused onto the retina in our eye, the image formed is real, inverted and diminished. 22 There is no lens in a camera. 23 In people with long-sightedness, the image would form the retina. 24 A telescope is used to magnify objects like the stars. RECALL Fill in the missing words. 122 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 You cannot clearly hear people who are talking far away from you. Give the reason. 2 When two people who are speaking with their backs facing each other, they find it hard to hear each other’s voices. Give the reason. 3 Ani saw a large piece of cucumber in a jar of cucumber pickle but when she pulled it out, she felt disappointed because it was actually much smaller than what it appears to be. Give the reason. 4 We cannot see colours in the dark. Give the reason. Chapter 4 Chapter 4 Vibrations, Waves and Light Vibrations, Waves and Light 123 ©Praxis Publishing_Focus On Science
Project Making a Periscope Activity objective: To design and create a periscope Problem statement: A periscope is an optical instrument people use to look at things over walls, corners, or other obstacles from a hidden position. Submarines have periscopes so that those who are inside the submarines can see objects above the surface while still underwater. We can demonstrate how a periscope works by making our own periscope. Concept applied: Reflection of light to enhance the ability to see objects over walls, corners or other obstacles Procedure: 1 Sketch a few designs of your periscope. 2 Prepare a variety of building materials (plane mirrors, shoe box/milk cartons). 3 Choose a design. Build the model that you have sketched. 4 Test your model by holding the periscope up to your eye and looking through it. 5 Think about improvements that you can make to your model such as adjusting the position of the plane mirrors to make the image clearer to the observer. Make the necessary changes to your model and test it again. You can choose another design, if required. Solution: Design of the model of periscope and its features Presentation: Compare your own model of periscope with your friends. Share the information with the class about what you learned about the application of the concept of reflection of light. 124 ©Praxis Publishing_Focus On Science
You have seen your mother cooking using a pot like this. What is the pot made of? Are all the parts of the pot made of the same material? How does the heat get to the food being cooked? What role does each part play to help your mother cook? Elements, Compounds and Mixtures CHAPTER 5 What will you learn? Differentiate between atoms and molecules Identify the chemical symbols of elements Grouping of elements in the Periodic Table of Elements according to their similarities Differentiate between metals, non-metals and metalloids Explain what a compound is, the chemical formulae of some common compounds and their properties Explain what a mixture is, and the different types of mixture Differentiate between solution, suspension and colloid Explain different ways to separate mixtures ©Praxis Publishing_Focus On Science
Atoms Matter is made up of tiny and discrete particles known as atoms. Atoms are the basic building blocks of all matter. Atoms are extremely small and we can only see them with an electron microscope by enlarging them to a million times. The diameter of an atom is about 10-10 m (0.1 nm). An atom is made up of even smaller particles, known as subatomic particles. There are three types of subatomic particles, namely protons, electrons and neutrons. Protons and neutrons make up the nucleus of the atom, while the electrons orbit the nucleus. Most of the volume in an atom is space. The neutrons are neutral or do not have any charges. The protons are positively charged whereas the electrons are negatively charged. The nucleus of an atom has an overall positive charge due to the presence of protons in it. Every atom has the same number of protons and electrons. Thus, an atom is neutral, in other words, without any charges. The masses of the subatomic particles are measured in atomic mass unit (amu). A proton has a mass about 1 amu. A neutron has about the same mass as a proton whereas an electron is 1 1840 amu. Therefore, an electron is very much lighter than a proton or a neutron. Atoms of different elements differ in mass and size and can be represented by circles or spheres. Different atoms are normally represented by different colours in drawings. An atom of hydrogen H An atom of carbon C An atom of oxygen O An atom of nitrogen N Electron Neutron Nucleus Proton Structure of an atom 5.1 Atoms and Molecules Matter is anything that has mass and takes up space. It is made up of atoms. When two or more atoms combine chemically, a molecule is formed. The iron tin is made up of iron atoms. Iron atom Argon atom The argon gas in the bulb is made up of argon atoms. 126 ©Praxis Publishing_Focus On Science
Molecules A molecule is a neutral particle made up of two or more atoms bonded together. When a molecule is made up of a fixed number of the same type of atoms, it is a molecule of an element, for example an oxygen molecule is made up of two oxygen atoms whereas a sulphur molecule is made up of eight sulphur atoms. Most non-metallic elements such as oxygen, sulphur, phosphorus, hydrogen, ozone and chlorine exist as molecules. Metals and noble gases such as helium, neon and argon, exist as individual atoms, not as molecules. Each molecule of oxygen gas consists of two oxygen atoms Oxygen cylinder filled with oxygen gas The number and types of atoms in a molecule is shown by the chemical formula of the molecule. For example, • O represents oxygen. • 2 shows that two oxygen atoms are chemically bonded together in one oxygen molecule. • O2 represents the chemical formula of an oxygen molecule. O2 • S represents sulphur. • 8 shows that eight sulphur atoms are chemically bonded together in one sulphur molecule. • S8 represents the chemical formula of a sulphur molecule. S8 Nanotechnology is the field of research and application of structures, device and systems by controlled manipulation of matter at a molecular or atomic level to produce materials and devices with extraordinary properties. The production of nanoparticles with specific properties is a branch of nanotechnology. Nanoparticles are tiny particles that ranges between 1 to 100 nanometres in size. Products of nanotechnology are smaller, cheaper, lighter with less requirement of energy and raw materials to manufacture them. It is used in various fields such as electronics, agriculture, energy and medicine. In the field of medicine, nanoparticles are used as drug delivery vehicles by helping the medication to travel exactly where it needs to go in the body. Science Facts Carbon nanotubes, a type of nanoparticle commonly used in the field of medicine as a drug delivery vehicle. Carbon nanotubes Chapter 5 Elements, Compounds and Mixtures 127 ©Praxis Publishing_Focus On Science
We can use model kits to build molecules of elements. The model comes with coloured plastic balls with holes in them and plastic sticks that fit into the holes to join the balls to each other. The plastic balls represent the atoms, and the sticks represent the bonds between the atoms. Bonds Nitrogen Sulphur Hydrogen Carbon Phosphorus Oxygen Chlorine Model kits showing how atoms are joined to one another to form molecules H H S S S S S S S S O O N N Cl Cl P P P P Phosphorus molecule Oxygen molecule Sulphur molecule Chlorine molecule Nitrogen molecule Hydrogen molecule Diamond molecule 128 ©Praxis Publishing_Focus On Science
When a molecule is made up of two or more types of atoms, it is a molecule of compounds, for example a water molecule is made up of two hydrogen atoms and one oxygen atom whereas a carbon dioxide molecule is made up of one carbon atom and two oxygen atoms. The chemical formula of water and carbon dioxide molecules are as follows. Model kits to show how atoms are joined to one another to form molecules of compounds. A sulphur trioxide molecule (SO3 ) A methane molecule (CH4 ) Matter can be divided into two main groups which are pure substances and mixtures. There are two kinds of pure substances which are elements and compounds. • O represents oxygen. • H represents hydrogen. • 2 shows that two hydrogen atoms are present in one water molecule. H2O Chemical formula of a water molecule • O represents oxygen. • C represents carbon. • 2 shows that two oxygen atoms are present in one carbon dioxide molecule. CO2 Chemical formula of a carbon dioxide molecule An ammonia molecule (NH3 ) A carbon dioxide molecule (CO2 ) H C H H H O S O O H H N H O C O H O H A water molecule (H2 O) Resource Chapter 5 Elements, Compounds and Mixtures 129 ©Praxis Publishing_Focus On Science
47% oxygen 28% silicon 8% aluminium 4.5% iron 3.5% calcium 9% others 5.2 Elements Elements are pure substances that cannot be broken down into other simpler substances by physical or chemical means. The most abundant element that makes up our Earth’s mass is oxygen (47%), followed by silicon (28%) and aluminium (8%). There are 118 elements that have been discovered as of 2016, with most of them found naturally and some made in the laboratory. Each element has a name and a symbol. The chemical symbol of each element consists of one or two letters. For elements with two letters in their symbol, the first letter is a capital letter while the second letter is a small letter. The table below shows some examples of elements with their chemical symbols. Element Symbol Hydrogen H Oxygen O Boron B Nitrogen N Helium He Calcium Ca Aluminium Al Copper Cu Sodium (Natrium) Na Lead (Plumbum) Pb Magnesium Mg Zinc Zn Elements such as hydrogen and oxygen are represented by the first letter of their English names. Hydrogen → H Oxygen → O Elements such as helium and calcium are represented by the first two letters of their English names. Helium → He Calcium → Ca Elements such as sodium and lead are represented by one or two letters in other language such as Latin. Sodium → Na (Natrium is in Latin) Lead → Pb (Plumbum is in Latin) Magnesium → Mg Zinc → Zn Elements such as magnesium and zinc are represented by the first letter and another letter in their names. Periodic Table of Elements All the elements are classified in a table called the Periodic Table of Elements to enable the study of elements systematically. Elements with similar chemical properties are grouped together. Some of the elements were named after the person who discovered them such as Rutherfordium (Rf) which was discovered in 1969 by Ernest Rutherford, a nuclear physicist and Bohrium (Bh) which was discovered in 1976 by Niels Bohr, a physicist. Abundance of elements in the Earth 130 ©Praxis Publishing_Focus On Science
The Periodic Table of Elements 1 1 2 3 4 5 6 7 8 9 10 Average Atomic Mass Atomic Number Name Symbol 11 12 13 14 15 16 17 18 2 3 4 5 6 7 Period Group Chapter 5 Elements, Compounds and Mixtures 131 ©Praxis Publishing_Focus On Science
Kawah Ijen, a volcano found in eastern Java, Indonesia is one of the places in the world where the mining of sulphur is carried out. Sulphur is a valuable element. In Java, it is used in the making of matches. Science Facts The elements in the Periodic Table are arranged in order of the increasing atomic (proton) number. The atomic number increases across the period and down the group. The elements in the Periodic Table of Elements are arranged into columns called groups and rows called periods. The elements in Group 1 alkali metals [lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr)] The elements in Group 2 alkaline earth metals [beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra)] The elements in Group 14 a non-metal [carbon(C)] • metalloids or semi-metals [silicon (Si) and germanium (Ge)] • metals [tin (Sn), lead (Pb) and flerovium (Fl)] The elements in Group 17 halogens [fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At) and tennessine (Ts)] The elements in Group 18 noble gases [helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and oganesson (Og)] The property of elements in the Periodic Table of Elements changes from being metallic to non-metallic from left to right. Elements to the left of the Periodic Table are metals (except hydrogen) and elements to the right are non-metals. Hydrogen, a non-metal, cannot be placed in any group in the Periodic Table because its properties does not match any of the groups. Hydrogen is placed in Group 1 only because it has 1 electron in the valence shell like the rest of the alkali metals. The blocks of elements known as metalloids or semi-metals form a staircase in the Periodic Table which divides the metals and non-metals. They are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po). They have properties of both metals and non-metals. 132 ©Praxis Publishing_Focus On Science
Metals, Non-Metals and Metalloids Iron is a metal, and it has similarities with other metals such as silver and gold. Sulphur is a non-metal and it has similarities with other non-metals such as chlorine and bromine. The physical states of metals and non-metals at room temperature are different. Metals and nonmetals also have different chemical properties as well as physical properties such as density, melting and boiling points, and thermal and electrical conductivities. The differences in the physical properties between metals and non-metals are as follows. Metals Shiny High density Ductile (can be drawn into wire) Malleable (can be hammered into thin sheets without cracking) Sonorous (rings when struck) High melting and boiling points Good conductors of heat Good conductors of electricity Non-metals Dull Low density Non-ductile Non-malleable Non-soronous Low melting and boiling points Poor conductors of heat or thermal insulators Poor conductors of electricity or electrical insulators Aluminium tins Gold bars Copper pipes Silver bracelet Sulphur Phosphorus Carbon There are many more metal elements compared to non-metal elements on Earth. Science Facts Chapter 5 Elements, Compounds and Mixtures 133 ©Praxis Publishing_Focus On Science
Metalloids or semi-metals are considered unique in the sense that they possess some properties of metals and some properties of non-metals. Silicon is a metalloid which has a physical appearance similar to metals but chemical properties more similar to non-metals. Silicon is a semiconductor; therefore it conducts electricity. It is widely used in electronic devices. The table below shows the properties of some metals, non-metals and metalloids. Element Appearance Melting point (°C) Boiling point (°C) Density (g/cm3) Electrical conductivity Heat conductivity Hydrogen Gas -259 -253 0.000082 No No Oxygen Gas -218 -183 0.0014 No No Nitrogen Gas -210 -195 0.0012 No No Carbon Solid, black 3550 3825 2.26 Yes Yes Calcium Solid, dull gray 842 1484 1.55 Yes Yes Sodium Solid, shiny 97.80 883 0.968 Yes Yes Magnesium Solid, silvery-white 650 1090 1.738 Yes Yes Aluminum Solid, silvery-white 660.3 2519 2.7 Yes Yes Silicon Solid 1414 3265 2.33 Slightly Yes Mercury Liquid, silvery-white -38.89 356.73 13.53 Yes Yes There are two forms of carbon: graphite and diamond. Unlike other non-metals, they have a very high melting point. Graphite which is a smooth and slippery substance, is a good conductor of both heat and electricity and commonly used as electrodes. On the other hand, a diamond which is the hardest naturally occurring substance, is a poor conductor of electricity but a good conductor of heat. Science Facts 134 ©Praxis Publishing_Focus On Science
The comparison of the general properties of metals, non-metals and metalloids are shown in the following table. Properties Metals Non-metals Metalloids State of element at room temperature Solid (except mercury) Solid, liquid or gas Solid Appearance Shiny Dull Some shiny and some dull Malleability Malleable Brittle/ non-malleable Brittle/ non-malleable Ductility Ductile Non-ductile Non-ductile Tensile strength Highly tensile Non-tensile Non-tensile Melting point High (except mercury) Low (except carbon) Varies Boiling point High (except mercury) Low (except carbon) Varies Heat conductivity Good heat conductor Poor heat conductor Some conduct heat Electrical conductivity Good electrical conductor Poor electrical conductor Some conduct electricity Activity 1 Aim: To differentiate the properties of metals and non-metals Materials and apparatus: Copper rod, lead powder, pencil lead, sulphur rod, iodine powder, wax, sandpaper, carbon rod, hammer, connecting wires, thermometer, batteries, crucible, crocodile clips, ammeter, switch, Bunsen burner, wood, thumbtacks and tripod stand Procedure: Carry out Activities A to F to study each property and record your observations. Investigating the properties of metals and non-metals Activity A Surface appearance 1 Rub a copper rod with a piece of sandpaper. 2 Record your observation. 3 Repeats steps 1 and 2 by using a carbon rod. Copper rod Sandpaper B Malleability 1 Hit a copper rod with a hammer. 2 Record your observation. 3 Repeat steps 1 and 2 by using a carbon rod. Hammer Copper rod Chapter 5 Elements, Compounds and Mixtures 135 ©Praxis Publishing_Focus On Science
C Ductility 1 Try to bend a copper rod. 2 Record your observation. 3 Repeat steps 1 and 2 by using a pencil lead. Copper rod D Electrical conductivity 1 Set up an electrical circuit as shown in the diagram. 2 Connect the ends of the copper rod, X and Y with crocodile clips. 3 Observe the deflection of the ammeter needle. 4 Repeat steps 1 to 3 using a sulphur rod. Ammeter Switch Crocodile clip Copper rod X Y A E Heat conductivity 1 Set up an electrical circuit as shown in the diagram. 2 Attach a thumbtack to one end of a copper rod and a carbon rod with wax. 3 Heat the other end of the copper rod and carbon rod with a Bunsen burner. 4 Record your observation. Thumbtacks Wood Bunsen burner Copper rod Carbon rod F Melting point 1 Set up the apparatus as shown in the diagram. 2 Heat the lead powder strongly until it melts. 3 Record the melting point of the lead powder. 4 Repeat steps 1 to 3 by using iodine powder. 70 60 50 20 10 90 100 Thermometer Lead powder Bunsen burner Crucible Tripod stand Observation: Activity Metal Non-metal A: Surface appearance B: Malleability C: Ductility D: Electrical conductivity E: Heat conductivity F: Melting point Conclusion: Write down your conclusion. 136 ©Praxis Publishing_Focus On Science
Iron is used in making vehicles and bridges because it is strong and can be easily pressed into the desired shapes. Copper is used to make wires because it can conduct electricity efficiently. Metals such as aluminium are used to make kettles, pots and pans because they are good heat conductors that enable food to be cooked fast. Tungsten is used in making the filaments in bulbs as it does not melt atvery high temperatures. Metals and non-metals are widely used in our daily life. Metalloids are used mainly in the chemical, electronics and alloy industries. Iodine is used as an antiseptic to prevent wounds from getting infected. As helium is lighter than air, it is used in balloons to gain lift. Nitrogen is used in the manufacture of fertilisers. Neon is used in colourful glowing signboards. Chlorine is used as disinfectant to purify drinking water. Uses of metals Uses of non-metals Chapter 5 Elements, Compounds and Mixtures 137 ©Praxis Publishing_Focus On Science
5.3 Compounds Compounds are pure substances that are formed when two or more elements combine chemically in a chemical reaction. The smallest particle in a compound is a molecule. For example, magnesium oxide is a compound, made up of the elements, magnesium and oxygen. When we burn magnesium in the air, magnesium will combine chemically with oxygen in the air to form magnesium oxide. Magnesium oxide is a completely different substance from the two elements it is made up of. Magnesium is a silvery metal and oxygen is a colourless gas whereas the compound they formed is a white powder. A compound is made up of different elements combined in a fixed ratio. For example, water is made up of the elements oxygen and hydrogen. In each water molecule, one oxygen atom is bonded with two hydrogen atoms. All water molecules are similar. We say that water has a fixed composition. O O H H H H Water (compound) H2 O Two atoms of hydrogen (element) One atom of oxygen (element) is made up of A word equation can be used to describe the chemical reaction of the formation of water. Metalloids such as silicon are used to make tiny electric circuits in computer chips. Metalloids can conduct electricity at certain conditions, allowing them to control the flow of electricity. Magnesium Oxygen Magnesium oxide Magnesium reacts with oxygen to form magnesium oxide Hydrogen + Oxygen chemically combine to form Water 138 ©Praxis Publishing_Focus On Science
Sugar (made up of carbon, hydrogen and oxygen) Marble and chalk (made up of calcium, carbon and oxygen) Table salt (made up of sodium and chlorine) Dry ice (made up of carbon and oxygen) Sand (made up of silicon and oxygen) Salt is the basic ingredient which can be used as seasoning in our food and is also considered as one of the most effective preservatives. Salt is a chemical compound made by one atom of sodium and one atom of chlorine. It can lower the freezing point of water and prevents it from freezing. It can increase the boiling point of the water as well. Science Facts Baking soda (made up of sodium, hydrogen, carbon and oxygen) Common compounds and the elements they are made up of Chapter 5 Elements, Compounds and Mixtures 139 ©Praxis Publishing_Focus On Science
The name of a compound that has two elements usually ends with ‘ide’. Do you know how compounds are named? They are named based on the elements that constitute the compounds. The following table shows the chemical names and formulae of some common compounds. Name the compounds with these chemical formulae: (a) CaO (b) KCl (c) MgCO3 (d) H2 S (e) NaNO3 Think About It Chemical name Chemical formula Calcium chloride CaCl2 Sodium oxide Na2 O Calcium hydroxide Ca(OH)2 Potassium hydroxide KOH Potassium nitrate KNO3 Aluminium nitrate Al(NO3 ) 3 Sodium sulphate Na2 SO4 Calcium sulphate CaSO4 Sodium carbonate Na2 CO3 Calcium carbonate CaCO3 Sodium hydrogen carbonate NaHCO3 The name of the metal always comes first when naming a compound that contains a metal. Hydroxides consist of the elements oxygen and hydrogen. Nitrates consist of the elements oxygen and nitrogen. Sulphates consist of the elements oxygen and sulphur. Carbonates consist of the elements oxygen and carbon. Hydrogen carbonates consist of the elements hydrogen,carbon and oxygen. These are the volcanic rocks formed from the lava erupted from Mount Merapi in Yogyakarta, Indonesia. There is more than one compound in these rocks such as silicon dioxide and aluminum oxide. Oxygen and silicon are the most abundant elements found in volcanic rocks. Science Facts 140 ©Praxis Publishing_Focus On Science
Example Example Example Example The elements in a compound combine in a fixed proportion by mass. Properties of a Compound All compounds have general properties as shown below. A compound is formed by chemical reactions. A new substance, iron sulphide (compound), is formed when a mixture of iron filings and sulphur powder is heated. A compound can only be broken down into its constituent elements by chemical reactions. Heat Silver metal formed A glowing splinter re-ignites due to the presence of oxygen. Silver oxide (compound) breaks down into silver and oxygen when heated. The properties of a compound and its constituent elements are different. + Hydrogen Oxygen Water + Sodium is a highly reactive metal with a shiny appearance and is a solid at room temperature. Chlorine is a greenish-yellow gas at room temperature and is poisonous. Sodium chloride, the compound formed from the chemical reaction between sodium and chlorine, is a white crystalline solid at room temperature. • 1 g of hydrogen combines with 8 g of oxygen to produce 9 g of water. • Proportionately, if there is 10 g of hydrogen, it reacts with 80 g of oxygen to produce 90 g of water. Sodium chloride Chapter 5 Elements, Compounds and Mixtures 141 ©Praxis Publishing_Focus On Science
Activity 2 Aim: To produce a compound by heating two elements Materials and apparatus: Sulphur powder, iron powder, Bunsen burner, mineral wool, glass rod, ignition tube and the holder Procedure: 1 Put one spatula of sulphur powder and one spatula of iron powder into an ignition tube. 2 Stir well with a glass rod. Record the colour of the mixture. 3 Insert a plug of mineral wool into the mouth of the ignition tube. 4 Heat the mixture at the base of the ignition tube using a blue flame until an orange glow is seen. Immediately stop heating. The glow continues and moves steadily through the mixture. 5 Let the product cool and record the colour of the mixture. Discussion: 1 What is the purpose of using the mineral wool? 2 What observation shows that the reaction has started? 3 Describe the product formed and name it. 4 Write a word equation of the chemical reaction occurred. 5 Is the substance formed in this activity a mixture or a compound? Explain why. Conclusion: Write down your conclusion. Producing a compound Caution Carry out this activity in a fume chamber and do not inhale any gas released during the heating. Mixture Heat Mineral wool Red glow as the iron and sulphur react together Ways to Form Compounds There are various ways to form compounds. 1 Elements are combined bychemical reactions to form a new compound. Example:The reaction between carbon and oxygen forms carbon dioxide. Carbon (element) + Oxygen (element) Carbon dioxide (compound) 2 Compounds are combined bychemical reactions to form a new compound. Example:The reaction between magnesium oxide and water forms magnesium hydroxide. Magnesium oxide (compound) + Water (compound) Magnesium hydroxide (compound) 3 Compounds and elements are combined bychemical reactions to form a new compound. Example:The reaction between carbon monoxide and oxygen forms carbon dioxide. Carbon monoxide (compound) + Oxygen (element) Carbon dioxide (compound) 142 ©Praxis Publishing_Focus On Science
5.4 Mixtures Have you seen your mother preparing sugar syrup? Sugar syrup is a concentrated mixture of sugar and water, used as a sweetener in beverages. Do you think that the proportion of sugar to water is always fixed in the syrup? Mixtures have two or more components that are physically combined. There are no chemical bonds between the components in the mixtures. The mixtures do not have any fixed composition too. Other examples of mixtures are air, blood, sea water and soil. As the components in a mixture are not bonded chemically, they can be separated by physical means. Air is a mixture of gases, dust particles and watervapour. There is no fixed composition among the components. There could be more watervapour when the air is damp. Mineralwateris amixture thatcontainsminerals. Besides water, a bottle of mineral watercontains other substances such as sodium, potassium and calcium. The blood is a mixture of both solid and liquid parts.The liquid part which is the plasma consists of salts, water and proteins. The solid part consists of red blood cells, white blood cells and platelets. Soil is a mixture of inorganic minerals, water, air, living organisms and decaying organic matter called humus. The pie chart shows the composition of air when it is dry. Does the composition remain the same always? Other gases 0.17% Nitrogen 78% Oxygen 20.9% Argon 0.90% Carbon dioxide 0.03% Examples of mixtures Seawateris amixture of 96.5%water,2.5% salts and smaller amounts of suspended impurities and atmospheric gases. Chapter 5 Elements, Compounds and Mixtures 143 ©Praxis Publishing_Focus On Science
There are two types of mixtures, homogeneous and heterogeneous mixtures. Homogeneous Mixture A homogeneous mixture is a mixture with uniform composition throughout the mixture. In a homogeneous mixture, only one phase of matter is visible. Metal gamelan instruments are made of either bronze, brass or iron. If it is made of bronze, it is a mixture of tin and copper. A jug made of pewter is a mixture of tin, copper and antimony. Coffee, rainwater, air and vinegar are some examples of homogeneous mixtures. Alloys such as steel, brass, bronze and pewter are also homogeneous mixtures. An alloy is formed by mixing a metal with small amounts of other metals or non-metals. Coffee Air Vinegar Examples of homogeneous mixtures Rainwater Examples of heterogeneous mixtures Heterogeneous Mixture A heterogeneous mixture is a non-uniform mixture with visible, individual components. It contains two or more components that mix but remain physically separated. The components’ chemical properties do not change and the individual component can be observed with the naked eye. Thus, a heterogeneous mixture consists of two or more phases. A glass of soft drink with ice cubes A bowl of salmon and avocado salad A pizza with mozzarella cheese 144 ©Praxis Publishing_Focus On Science