3.3 Sources and Forms of Energy Energy is defined as the ability to do work. Living things need energy to grow and perform life processes and activities. Which activity do you think requires more energy, walking or swimming? On the other hand, non-living things such as electrical appliances need energy to work. As science and technology advance, more energy is required to power equipment, automobiles and buildings. We get this energy from a number of sources which we can classify into non-renewable energy sources and renewable energy sources. In 2020, renewable energy in Indonesia made up 11.2% of the country’s energy mix, with hydro and geothermal power plants making up the majority of this contribution. By the year 2025, and 2050, respectively, Indonesia wants 23% and 31% of its energy to come from renewable sources. Let’s take a look at the pie chart below which shows the world’s total consumption by source in 2018. According to the pie chart, oil, coal and natural gas make up the bulk of usage. They are examples of fossil fuels. They are formed from the remains of extinct species from millions of years ago. Considering how long it takes for fossil fuels to form and how much of them are consumed globally, these resources cannot be quickly restored once used up. They are therefore referred to as non-renewable sources. Source: IEA (2020), Global share of total final consumption by source, 2018. https://www.iea.org/data-and-statistics/charts/global-share-of-total-final-consumption-by-source-2018. All rights reserved; as modified by Praxis Publishing Singapore Pte. Ltd. World’s total consumption by source, 2018 Electricity 19.3% Others 3.5% Coal 10% Oil Biofuels 40.8% and waste 10.2% Natural gas 16.2% Chapter 3 Work, Energy and Simple Machines 59
Coal comes in different forms, from hard black rocks to soft brown dirt. Some forms burn hotter and cleaner than others. Coal is used to produce electricity. Many coal beds are near the ground’s surface. We get coal by mining. Most coal is used by power plants where it is burned to make steam. The steam turns turbines to produce electricity. Coal is also used in steel manufacturing. It is refined to produce a substance called coke which burns at very high temperatures to smelt iron into steel. Coke is better as a fuel than coal because coke burns with no smoke whereas coal burns with smoke. Non-renewable Energy Sources Coal, petroleum and natural gas are the types of fossil fuels currently in use. They are made into fuels for different kinds of equipment. The majority of non-renewable energy sources are fossil fuels. Nuclear energy, is also considered non-renewable because it uses uranium extracted from the Earth, but the amount of uranium reserve is limited. The coal we use today was formed millions of years ago in swampy areas where plentiful huge plants grew. When the plants fell and decayed in the swamps, they were covered by mud, soil and other plants. Layers upon layers of sediments piled up. Over millions of years, intense heat and pressure of the Earth converted the plant matter into a mixture of carbon and hydrocarbon compounds, known as coal. Petroleum is formed over millions of years from the decay of algae and tiny ocean animals known as plankton. The tiny animals and plants were buried in the sediments on the ocean floor. These sediments and organic matter are buried deep in the Earth, under pressure and heat, eventually becoming oil-bearing shale and finally, crude oil. Gasoline and diesel are used as fuels in cars. Petroleum, also known as crude oil, is a yellowish-black liquid mixture of mostly hydrocarbons that occur naturally. Gasoline, diesel and kerosene, are the refined products of crude oil. Gasoline and diesel are major sources of energy for transportation. Mining activities to obtain coal Coal Petroleum 60
Similar to coal and petroleum, natural gas is a fossil fuel that was formed when dead plants and bacteria settled at the bottom of marshes and oceans and began to degrade over time. These organisms were turned into gas after millions of years of exposure to heat and pressure. A rock traps natural gas underground, where it remains until it is extracted. Natural gas, like the previously mentioned fossil fuels, coal and petroleum, is a source of energy, but it has an advantage over them in terms of the amount of usable energy it can generate. However, it is a non-renewable fossil fuel, just like the others. Natural gas is delivered to our homes to use for heating. It is used in stoves to boil water. Nuclear energy is produced from uranium, a non-renewable energy source. Uranium is extracted and mined to produce nuclear energy. Just like coal and other fossil fuels, viable uranium supplies are limited. Nuclear energy is not burned, as compared to fossil fuels. As a result, carbon dioxide and other harmful gases are not produced when using nuclear power. Compared to energy from fossil sources, nuclear energy is significantly cleaner. About 10% of the world’s electricity is generated from uranium inside the reactor of a nuclear power plant. Natural Gas Nuclear Energy Chapter 3 Work, Energy and Simple Machines 61
Renewable Energy Sources Renewable energy sources are energy sources that can be replaced after being used. Examples of renewable energy sources are wind, the Sun, moving water, biomass, waves and heat from inside the Earth. Wind Wind is moving air. Wind is used to spin the blades of wind turbines. A wind turbine converts the kinetic energy of wind into electrical energy. It is a clean source of energy, free from pollution. The Sun gives out heat and light energy. We use it to keep warm and to dry ourfood.Now, we have devices to collect solar energy for water heating. Some devices such as solar panels convert it directly into electricity. This energy can be used to heat water and power homes, buildings and even cars. The advantages of using solar energy are that it does not cause pollution and it incurs low maintenance costs. However, it is weather dependent and expensive in energy storage. In ancient times, wind was used to turn the blades of windmills. A windmill converts wind energy directly into mechanical energy to grind grains or pump water. Science Facts Wind turbines are constructed in areas with consistent and strong wind. Solar panels are used to convert solar energy into heat energy and electrical energy to power home appliances. Sun 62
Moving water in streams and waterfalls has energy. The energy from water can be used as a source of energy in different ways. Hydroelectric energy is the energy released when water rushes from a dam into a turbine. The fast-moving water turns the turbine to generate electricity. This energy source is reliable and efficient. It is also flexible as we can control the flow of water. However, there are limited places to build the hydroelectric power stations. They can only work in hilly areas. Biomass is the organic materials that come from plants and animals. Wood, unwanted agricultural waste such as dried plants and husks, animal manure, vegetable oils and food waste are examples of biomass. When biomass is burned directly, or converted to liquid biofuels or biogas that can be burned as fuels, biomass energy is released. Biomass is cheaper than fossil fuels and reduces landfills. However, it is not entirely clean and there is a risk of deforestation. Biofuels obtained from plants such as corn and beans are used as alternative sources of energy. Alcohol of the type known as ethanol is produced by fermenting corn, sugarcane or other plants. Ethanol can be used to power vehicles. It is a renewable energy source because we can always grow more plants. Moving Water Biomass Chapter 3 Work, Energy and Simple Machines 63
Steam Turbine Cooling tower Generator Injection well Hot water Geothermal energy is the heat energy beneath the surface of the Earth. This energy can be drawn from the hot water below the Earth’s surface or by pumping cold water onto the hot rocks and returning the heated water to the surface. This can drive steam turbines to produce electricity. It is free of pollution and the supply is constant. However, this type of energy can be obtained only in specific locations and these sites are usually prone to earthquakes. Geothermal energy is released to the surface of the Earth by geyser. The steam rises from the geothermal power station above a rocky lake coastline Cold water is pumped onto the hot rocks and the heated water is returned to the surface. Wind blowing over the ocean produces the vertical movement of the surface water known as waves. We can capture the wave energy by using special floating devices called Salter’s ducks which are connected in a chain on the water. By bobbing up and down on the sea, the generators inside the Salter’s ducks convert wave energy into electricity. Waves Heat from Inside the Earth Indonesia is one of the countries in the world that has the highest geothermal energy resources. Give a reason. Think About It 64
Different Forms of Energy There are different forms of energy: chemical energy, light energy, thermal energy, sound energy, electrical energy, potential energy, kinetic energy and mechanical energy. Chemical energy is the energy stored in food and chemical compounds such as batteries, natural gas and coal. It is released in other forms of energy during chemical reactions. Wood contains chemical energy because when we burn it, it gives out heat and light energy. Science Facts The food we eat is an example of stored chemical energy, the energy is released during digestion where the molecules in the food are broken down into small pieces. Fuel stores chemical energy. When it is burned, the chemical energy is released and converted into heat and light energy. The chemical energy stored in a car battery is released in the form of electrical energy when we turn on the ignition switch. Light energy is produced by hot objects such as bulbs and the Sun, and can be seen by human eyes. Without light, we cannot see in the dark. The sun gives out light, it travels at a speed of 300 000 km/s to reach us. We can see illuminating objects as these objects give out light, for example fireflies. Non-illuminating objects such as trees do not give out light. We can see them because they reflect the light that falls on them into our eyes. A firefly has an abdomen that illuminates when its body undergoes a chemical reaction that makes it glow. This type of light production is called bioluminescence. Chemical Energy Light Energy Chapter 3 Work, Energy and Simple Machines 65
Thermal energy is the total energy in an object due to the movement of particles within the object. The faster the particles in an object move, the greater the thermal energy and the higher the temperature of the object. The movement of water particles becomes faster when the water is heated. Have you ever experienced an electrical outage? The flow of electric charges is what generates electrical energy. Electrical energy is produced when electric charges flow through a conductor such as an electric wire. The faster the electric charges move, the more electrical energy they carry. Electrical energy is widely used in our home appliances. For example, batteries are used to power the remote control that we use to turn on/off the television. Batteries convert chemical energy into electrical energy. Electrical energy is used to power up the lights in our homes. Lightning is a natural phenomenon and it causes thunder. Would lightning produce electrical energy? What energy does a television produce when it is turned on? Sound is a form of energy produced by vibrations. Sound cannot travel through vacuum. However, for sound to travel from one place to another, a medium like a solid, liquid or gas is required. People can hear what we talk because sound energy move from our vocal chords to them in the form of vibrations. Thermal Energy Sound Energy Electrical Energy When a drum is struck, the skin vibrates and produces sound. Some musical instruments produce sound when we tap them, pluck their strings or blow the columns and so on. The girl enjoys listening to music with her headphones. Headphones are basically speakers that use an electromagnet to vibrate air to create sound. Are electricity and electrical energy the same? Electricity is the moving of electrical charges, either mechanically in the case of static electricity, or by flowing in a closed loop in the case of current electricity. Electrical energy is a form of energy resulting from the moving of the electric charges. It has the ability to do work, such as lighting up a bulb. Science Facts 66
Potential energy is the energy stored in an object due to its position or condition. Examples of potential energy are gravitational potential energy and elastic potential energy. Gravitational potential energy is the energy stored in the object due to its vertical position or height from the ground. For example, the fruit on a tree positioned well above the ground stores gravitational potential energy due to its position and the Earth’s gravitational force acting on the fruit. The higher the fruit is above the ground, the more gravitational potential energy it possesses. Gravitational potential energy can be calculated using the formula: Gravitational potential energy = m × g × h where, m = mass of the object in kg g = gravitational field strength (9.8 N/kg on Earth) h = the height of the object in m and the SI unit of energy is Joule (J). Fruit A possesses greater gravitational potential energy than fruit B because its position is higher above the ground. What are other examples or situations that show gravitational potential energy? Think About It A B Potential Energy Activity 1 1 Carry out the following activities with additional activities of your own in groups of four: (a) Push a hockey ball on the table to the floor. (b) Walk up the steps to the stage in the school hall. (c) Lift a basketball from the floor and hold it over your head. 2 Calculate the gravitational potential energy involved in each activity using the formula ‘Gravitational potential energy = mgh’. Calculating the gravitational potential energy The bird on an electric pole possesses gravitational potential energy. Chapter 3 Work, Energy and Simple Machines 67
Example 7 An object that has a mass of 40 kg is lifted to the height of 5.2 m using a pulley. What is the gravitational potential energy gained by the object? Solution: Gravitational potential energy = mgh = 40 × 9.8 × 5.2 = 2038.4 J The amount of gravitational potential energy increases when the: • mass of the object increases. • height of the object from the surface of the Earth increases. • gravitional strength increases. The gravitational potential energy for an object on the surface of the Earth is zero. Elastic potential energy is the energy stored in an object due to it being stretched or compressed. An elastic band needs a force to be stretched. The elastic band gains energy when a stretching force is applied. When the elastic band is stretched by a force to a particular length, work is said to be done, affected by the force applied and the length stretched. The work that is done is transferred as energy and stored in the elastic band as elastic potential energy. Elastic potential energy is calculated with this formula: Elastic potential energy = 1 2 Fx where, F = force applied in N x = displacement in m where displacement is the length stretched in the direction of the force applied. A force is used to stretch the elastic band to a particular length. The work done is transferred as elastic potential energy stored in the elastic band. The pole vaulter is doing work by applying his body weight to bend the pole. The elastic potential energy is stored in the pole. 68
Kinetic Energy 0.5 m 25 N Example 8 The archer pulls an arrow on an elastic bow string. The bow string is pulled back a distance of 0.5 m with a force that increases uniformly from zero to 25 N. Calculate the elastic potential energy that is stored in the bow string. Solution: Elastic potential energy = 1 2 Fx = 1 2 × 25 × 0.5 = 6.25 J Kinetic energy is the energy of a moving object. It depends on its mass and velocity. If both the mass of the object and its velocity increase, the kinetic energy increases too. The kinetic energy of an object can be calculated using the formula: Two moving cars may collide due to the kinetic energy each of them possesses. Kinetic energy = 1 2 mv2 where, m = mass of the moving object in kg v = the velocity of object in metre per second (m/s) and the SI unit of kinetic energy is Joule (J). Example 9 Calculate the kinetic energy of a ball with a mass of 200 g that moves at 12.2 m s−1. Solution: Kinetic energy of the ball = 1 2 × 0.2 × 12.22 = 14.88 J Which has more kinetic energy - an airplane during takeoff or a rocket during launch? Give a reason. Think About It Chapter 3 Work, Energy and Simple Machines 69
Mechanical energy is the sum of the kinetic energy and the potential energy in an object used to do work. It is energy in an object due to its motion or position, or both. Mechanical energy = potential energy + kinetic energy When a person lifts his hand to push the door open, he possesses potential chemical energy (energy stored in him) and kinetic energy (energy in the motion of his hand). Both are transferred into mechanical energy which causes work to be done (door opens). Kinetic energy Mechanical energy Potential energy When a hammer is lifted, the potential energy increases because of its high position. Then, when the hammer moves at great speed towards the nail, it possesses kinetic energy. The mechanical energy which is the combination of kinetic energy and potential energy in the hammer, will drive the nail into the wall (work is done). A barbell lifted above this man’s head possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). The moving ball possesses mechanical energy due to both its speed (kinetic energy) and its position above the ground (gravitational potential energy). A compressed spring possesses mechanical energy due to its condition (elastic potential energy). Mechanical Energy 70
When an object falls, its height from the ground decreases and its velocity increases. Its potential energy changes into kinetic energy. Thus, the potential energy of a falling object decreases while its kinetic energy increases. However, the mechanical energy of the falling object is constant (provided there is no loss of energy due to friction) because it is the sum of the potential energy and the kinetic energy of the object. The decrease in the potential energy of the object during falling equals the increase in its kinetic energy. Although there is an increase in its kinetic energy as the object falls, the value of the kinetic energy does not exceed the value of mechanical energy because the mechanical energy of an object is the sum of the potential energy and the kinetic energy of the object. Activity 2 1 Work in groups. 2 Identify three examples of situations whereby mechanical energy is involved. State the sources of the mechanical energy in each situation. 3 Each group is to prepare a poster to present their findings. 4 Discuss all the sources with the members of other groups. Identifying the sources of mechanical energy KE = 0, PE = mgH So, ME = mgH C If velocity of object = v: PE = mgh, KE = – mv2 So, ME = mgh + – mv2 B A h H 1 2 1 2 If velocity of object = V: PE = 0 (because h = 0), KE = – mV2 So, ME = – mV2 1 2 1 2 The kinetic energy, KE at the maximum height before falling equals to zero (because velocity of object = 0), therefore the mechanical energy, ME at the maximum height equals to the potential energy, PE only. The potential energy, PE just before the object hits the ground equals to zero (because the height = 0), therefore the mechanical energy, ME equals to the kinetic energy, KE of the object only. Chapter 3 Work, Energy and Simple Machines 71
Energy Transformation Energy can be changed from one form into another in a system, but it cannot be destroyed or created. This is known as the law of conservation of energy. Energy changes take place all around us. When energy is used, it often converts from one form to another. Let us look at these situations. Do your palms get warm when rubbed against one another, and can you hear the rubbing sound? What are the energy changes involved? A little girl is on a swing in the playground. What is the energy change involved? Think About It A hair dryer produces sound and hot air when we blow our hair. We can say that electrical energy has changed into heat energy and sound energy. Electrical energy → heat energy + sound energy Resource Activity 3 1 Work in groups of five. 2 Set up an electric circuit as shown. 3 Observe what happens when the switch is turned on to complete the circuit. Touch the bulb. 4 Describe the energy transformation that occurs. Identifying the energy transformation 72
3.4 Simple Machines Do you know that the staircase that helps you move between floors in your house or a building, is a machine? How is a staircase a machine? A machine is a device that makes work easier to be carried out. When we use a machine, we apply a force over some distance. The force applied on the machine is called the input force. The work we do on the machine is called the input work. The machine also does work by applying a force to move an object over some distance. The force that the machine applies is called the output force. The work that the machine does is called the output work. In an ideal machine, the input work equals to the output work. However, in actual machines, the output work is lesser than the input work because some of the input work is used to overcome friction. The mechanical advantage of a machine is the measure of its performance. The actual mechanical advantage is the ratio of the output force to the input force of the machine. Mechanical advantage (MA) = Output force (N) Input force (N) Generally, it is difficult to measure the input and output forces because an unknown amount of input force is used to overcome friction. Therefore, the distance measurements are used to calculate the ideal mechanical advantage. Mechanical advantage (MA) = Input distance Output distance Let’s say the ideal mechanical advantage of a machine is 3 (in the absence of friction), this means that the machine multiplies the input force by a factor of 3. For example, if we apply 100 N as input force, the machine will multiply 100 N by 3 to generate an output force of 300 N. The trade-off is that the input force must be applied over a greater distance than the distance the object is moved. A simple machine is a device or tool that helps us to do work easier. There are six types of simple machines: levers, inclined planes, wedges, screws, wheels and axles and pulleys. Levers A lever is a bar that rests or rotates around a fixed point called the pivot or fulcrum. Effort is the force applied at a certain point on the bar. A lever is used to lift a weight, known as the load. Effort Bar Fulcrum Load When the fulcrum is placed closer to the load, the effort used is lesser, but the effort must move through a greater distance. The mechanical advantage of a lever is calculated by dividing the input arm (the distance from the fulcrum to the input force) by the output arm (the distance from the fulcrum to the output force). Output force Input force Input arm = 20 cm Output arm = 5 cm Mechanical advantage = 20 cm 5 cm = 4 The mechanical advantage is greater than one if the fulcrum is nearer to the output force compared to the input force. Chapter 3 Work, Energy and Simple Machines 73
Claw hammer The see-saw, a pair of scissors, and a claw hammer are examples of first-class levers. Can you identify the fulcrum, effort and load of each lever? Scissors • In a first-class lever, the fulcrum is positioned between the load and the effort. • Based on the position of the fulcrum, load or effort, a lever can magnify the force applied and make it easier to do work. • The input force is the force that a person applies to a lever to make it move while the output force is the force that the lever applies on the load. • The direction of the output force and that of the input force are always different. In other words, if the effort is “down”, the load moves “up”. • The mechanical advantage can be more or less than one and this depends on the location of the fulcrum relative to the load and effort. → When the fulcrum is near the load and further away from the effort, as in a claw hammer that is used to pull out nail, the lever provides a mechanical advantage that is more than 1. The lever with a mechanical advantage more than 1 usually makes tasks easier to be carried out with little effort. → When the fulcrum is near the effort and further away from the load, as in a pair of scissors with its blades longer than its handle, the mechanical advantage is less than 1. The lever provides no mechanical advantage but increases the speed with which the load can move. Fulcrum Load Effort Based on the positions of the effort, fulcrum and load which are interchangeable, levers are grouped into three categories: first-class lever, second-class lever and third-class lever. First-class Lever 74 See-saw
Fulcrum Load Effort Nutcracker • In a second-class lever, the load is positioned between the fulcrum and the effort. • The input force (effort) is always further away from the fulcrum compared to the distance of the output force (load) from the fulcrum. The output force will always be greater than the input force, thus the mechanical advantage is always greater than one. The wheelbarrow, nutcracker and bottle opener are examples of second-class levers. Can you identify the fulcrum, effort and load of each lever? Bottle opener Second-class Lever Wheelbarrow Chapter 3 Work, Energy and Simple Machines 75
• In the third-class lever, the effort is positioned between the fulcrum and the load. • The input force is always closer to the fulcrum compared to the distance of the output force from the fulcrum, so the output arm always moves farther than the input arm. The output force will always be smaller than the input force, thus the mechanical advantage is always less than one. • Since third-class levers do not give a mechanical advantage, they are used mainly to increase the speed (or distance covered per unit time) of the load. Fulcrum Load Effort Activity 4 1 Work in groups. 2 Identify levers used in daily life and determine the classes of the levers. 3 Prepare a poster on the different classes of levers with the position of the load, effort and fulcrum labelled on every lever. Describe the benefits of using these levers. 4 Each group will present their findings. Identifying different classes of levers used in daily life The shovel, broom, tong and fishing rod are examples of third-class levers. Can you identify the fulcrum, effort and load of each lever? Tongs Fishing rod Shovel Third-class Lever Broom 76
Inclined Planes An inclined plane is a flat, sloping surface without any moving parts. It is used to move an object from a lower level to a higher level. The longer the inclined plane, the lesser the force needed to move the object. An example of an inclined plane is a ramp. The ramp reduces the force needed to move an object from a lower to a higher level. Comparing the force needed when lifting a box straight into the back of the lorry to the force needed when a ramp is used to move the box. Weight = 1200 N Force = 360 N Force = 1200 N h = 1.2 m 4 m P Q Without using a ramp A box weighing 1200 N is lifted to the back of the lorry that is 1.2 m off the ground. The force exerted which is 1200 N (the weight of the box), over a distance of 1.2 m, equals to 1440 J of work. Using a ramp with a length of 4 m The amount of work that needs to be done is the same, it does not change. However, the distance, over which force is exerted, becomes 4 m. Therefore, the force that needs to be exerted is 360 N only. Force = work distance = 1440 4 = 360 N Work = force × distance = 1200 × 1.2 = 1440 J When the load is being pushed up on an inclined plane, the mechanical advantage of the machine is the ratio of the length of the inclined plane (PQ) to its height, h. In this situation, the ramp has a mechanical advantage of about 3.3. Which ramp has a greater mechanical advantage if both have the same height? Think About It Mechanical advantage = length of inclined plane height of inclined plane = 4 1.2 = 3.3 Another way to calculate mechanical advantage of the ramp is as follows: Mechanical advantage = output force or load input force or effort = 1200 360 = 3.3 Chapter 3 Work, Energy and Simple Machines 77
Front teeth Wedges A wedge is an inclined plane that moves, and it has one or more sloping sides. A wedge can be used to split things apart. The mechanical advantage of a wedge increases as it becomes longer with a thinner tip. Input force Output force Output force Wedge The output force is in a different direction from the input force due to the shape of the wedge. Which wedge requires lesser input force to do the same amount of work? Think About It The teeth of carnivores (meat eaters) are more wedge shaped compared to herbivores (plant eaters). This is because carnivores use their teeth to cut and tear meat while herbivores use their teeth to grind plant tissues. Axe Knife P Q In humans, the front teeth are wedge shaped. When we bite a fruit, our teeth is pushed into the fruit. The teeth change the downward effort force into sideways force which pushes the skin of the fruit apart. The teeth of a herbivore The teeth of a carnivore Some of the examples of wedges 78
Screws A screw is an inclined plane wrapped around a cylinder. A screwdriver is used to drive the screw into an object or flat surface. When the screw is rotated many times, it moves downward into the object following the threads on the screw. The force applied (input force) is changed by the threads to a force that pulls the screw into the object (output force). Only a small effort is applied on the screw to produce a large force downwards. The mechanical advantage of a screw can be obtained by dividing the output force by the input force. Threads on the screw The force applied by the screw (output force) is always greater than the force applied to the screw (input force). Therefore, the mechanical advantage of a screw is always greater than one. The mechanical advantage of a screw can also be calculated by taking the ratio of the circumference of the screw head to the distance between the screw thread, that is the distance travelled by the thread after each revolution. Mechanical advantage (MA) = output force (N) input force (N) Why is it harder to turn a screw when the threads on the screw are more widely spaced? Think About It Bolt and nut Jar lid Drill bits Light bulb Guitar tuning keys Cork opener Chapter 3 Work, Energy and Simple Machines 79
Wheels and Axles A wheel and axle consists of a large circular object called the wheel which is connected to a smaller rod-shaped object called the axle. The axle allows the wheel to rotate around it and they rotate together. The wheel and axle rotate together because the force applied to the wheel is transferred to the axle, and vice versa. Wheels and axles can be found in a pizza cutter, a rolling pin, a water tap and a bicycle wheel. Pizza cutter Rolling pin Water tap Bicycle wheel Axle Wheel 80
• The input force is applied to turn the wheel and the output force is exerted by the axle. • Let’s consider the wheel and axle on a doorknob. When force is applied to the knob (wheel), the central shaft (axle) rotates in a smaller distance and retracts the latch with greater output force causing the door to be opened. • Themechanical advantageis calculated by dividing the radius of the wheel that is the applied force (input force) by the radius of the axle that applies the force (output force). The mechanical advantage is greater than one. • A doorknob, a steering wheel and a screwdriver are examples of wheel and axle where force is applied to the wheel. There are two ways the wheel and axle work together: force is applied to the wheel or force is applied to the axle. Force is applied to the wheel • The input force is applied to turn the axle and the output force is exerted by the wheel. • The mechanical advantage is calculated by dividing the radius of the axle that is the applied force (input force) by the radius of the wheel that applies the force (output force). The mechanical advantage is less than one, that means there is no increase in output force applied by the wheel when the input force is applied to the axle. However, the rotation of the axle over a short distance can cause the wheel to rotate at a greater speed over a longer distance. • Let’s consider the wheel and axle on a Ferris wheel. When force is applied to the axle, the wheel moves a greater distance and turns faster with less output force. • An electric fan and ferris wheel are examples of wheel and axle where force is applied to the axle. Force is applied to the axle Screwdriver Ferris wheel Doorknob Steering wheel Electric fan Chapter 3 Work, Energy and Simple Machines 81
Pulleys A pulley is a wheel on an axle. It is designed to rotate with a rope moving along a groove at its circumference. The groove helps to keep the rope in place. The object to be lifted is tied to one end of the rope and force is exerted to the other end by pulling the rope downwards. The downward force turns the wheel with the rope and lifts the object at the other end. There are three types of pulleys: fixed pulley, movable pulley and combined pulley. The mechanical advantage of a pulley is equal to the number of rope segments pulling up on an object. Effort (input force) Load • It consists of a single pulley. • The rope moves along the groove of the pulley, but the wheel is fixed to a spot. • This type of pulley will not reduce the effort (input force) needed to lift the load; it only reverses the direction of the effort. • As a downward force is applied to one side of the pulley, the other side of the pulley is pulled upwards. • In this single fixed pulley, only one rope segment pulls up on the load, so the mechanical advantage is 1. Moreover, the effort (input force) needed to lift the load is equal to the weight of the load. To lift the load, the length of rope pulled downwards must be equal to the height the load is to be lifted. • Pulleys on a sail, a window blind and a flagpole are fixed pulleys. Movable pulley on a crane Effort (input force) Load • This pulley is attached to the object that is to be lifted. • Both the pulley and load attached to it can move from one place to another. • One end of the rope is attached to a fixed point that does not move. • In this movable pulley, two rope segments pull up on the load, so the mechanical advantage is 2. The pulley multiplies the input force by a factor of 2, in other words the load is twice the input force. For example, if the load weighs 30 kg, the input force required to pull up the load would be 15 kg, but twice as much rope must be pulled to lift the load. • Pulleys on cranes, elevators and weightlifting machines are examples of movable pulleys. Movable Pulley Fixed Pulley Fixed pulley on a window blind 82
Simple Machines in the Human Body Simple machines such as levers, wedges and inclined planes can be found in a human body. The simple machines help humans accomplish various tasks throughout the day. Block and tackle pulley system on sailing ships • This type of pulley is a combination of fixed and movable pulleys that work together. • The mechanical advantage depends on the number of rope segments pulling up on the load, usually 2 or greater than 2. • In this type of pulley, the amount of force required to lift an object is greatly reduced. • An example of combined pulley is the block and tackle pulley system. A fixed pulley block and a movable pulley block are used in the block and tackle pulley system to reduce the amount of force required to lift a heavy object by however many pulleys incorporated into the system. Effort (input force) Load Load Fulcrum Effort F E L Nodding is an example of a first-class lever. The joint where the skull meets the top of the spine is the fulcrum, the muscles at the back of the neck contract to provide the effort force to tilt the head up, and the weight of the head is the load. Combined Pulley / Compound Pulley First-class Lever Chapter 3 Work, Energy and Simple Machines 83
Load Fulcrum Effort F L E Standing on tiptoes is an example of a second-class lever. The ball of the foot is the fulcrum, the calf muscles contract to provide the effort to lift the body, and the weight of the body is the load. Second-class lever is rarely found in the human body. Load Fulcrum Effort F L E Bending of the arm is an example of a third-class lever. The elbow joint is the fulcrum, the biceps contract to provide the effort to lift the arm, and the weight of the forearm or any weight that it is holding is the load. Third-class lever is the most common type of lever in the human body. Can you give other examples? The wedge-shaped front teeth in the mouth, called the incisors, are used to bite food. When we eat, the amount of force produced by both the top and bottom front teeth will break the food apart Second-class Lever Third-class Lever Wedges 84
Activity 5 1 Work in groups. 2 Research on other examples of simple machines in the human body and how each one works (other than those mentioned in the text). 3 Each group prepare a poster to present their findings. Identifying the simple machines in human body Activity 6 1 Work in groups. 2 Browse the Internet to get more information about Rube Goldberg machines. 3 Build your own Rube Goldberg machine based on a simple task to be solved such as switching on a lamp. 4 Collect items such as cardboard, string, cardboard tubes, straw, dominoes, marbles, spoons, needles, toy cars and boxes. 5 By using the concept of the chain of action and reaction, build your machine. 6 Test out your machine to see whether it is working before presenting to the class. Designing a Rube Goldberg machine Rube Goldberg who is an American inventor, engineer and cartoonist, was well known for his creation of the caricature scientist, Professor Butts. The cartoon character he illustrated used complicated machines to perform very simple tasks in the most hilarious ways and eventually the inventions were called the Rube Goldberg Machines. One of his most popular illustrations was the self-operating napkin as shown here. It involves a continuous chain of actions and reactions that uses simple machines like pulleys and levers. Can you decipher all the steps involved from A to M? Up to today, a global Rube Goldberg Machine contest is held annually to challenge students to use their creativity to build their own Rube Goldberg machine that solves a simple task using the most complex process. Science Facts A B C D E G H I J K L M F Chapter 3 Work, Energy and Simple Machines 85
1 is done when a force is used to move an object through a distance in the direction of the force. 2 The S.I. unit of work is . 3 is defined as the rate at which work is done. 4 Coal, , nuclear energy and natural gas are the types of fossil fuels currently in use; they are non-renewable energy sources. 5 Energy sources such as wind, running water, biomass, waves and heat from inside the Earth are energy sources that can be replaced after being used. 6 potential energy is the energy stored in an object due to its vertical position or height from ground while potential energy is the energy stored due to its stretching or compressing condition. 7 Kinetic energy is the energy of a moving object that depends on its and velocity 8 The sum of the potential energy and the kinetic energy is called energy. 9 A simple is a device or tool that helps us to do work easier. 10 The force applied on a machine is called the force. 11 The force that a machine applies to move an object over some distance is called the force. 12 The mechanical advantage of a machine is the measure of its . 13 The mechanical advantage is the ratio of the force to the force of the machine. 14 A lever is a bar that rotates around a fixed point called the . 15 In the class lever, the fulcrum is positioned between the load and the effort. 16 For the second-class lever, the mechanical advantage is always than 1. 17 The third-class levers do not give a mechanical advantage, but they are used to increase the of the load. 18 The longer an inclined plane, the the force needed to move an object from a lower to a higher level. 19 A is a movable inclined plane used to split things apart. 20 It is to turn a screw when the threads on the screw are more widely spaced. 21 A pulley is a wheel on an axle designed to rotate with a rope moving along a groove at its . 22 In a wheel and axle, the axle allows the wheel to rotate around it and they together. 23 In the bending of the arm, the joint is the fulcrum. RECALL Fill in the missing words. 86
THINKING CAP Put on your 1 Explain how energy is transferred and transformed when an arrow is shot using a bow? 2 What can be done to increase the mechanical advantage of an inclined plane without changing its height? Give a reason. 3 A wheel and axle system has a mechanical advantage of 3 and the radius of the axle is 15 cm. What is the radius of the wheel? 4 Why are pulleys used to lift objects during the construction of buildings? Chapter 3 Work, Energy and Simple Machines 87
Project Making a Waterwheel Activity objective: To design and create a working waterwheel Problem statement: Waterwheels use the energy of flowing water to turn a wheel and the turning wheel can then power other machines to do work. During ancient times, waterwheels were used to grind grains into flour, drive pumps and run farm equipment. Today, the modern equivalents of waterwheels are the huge turbines of hydroelectric power plants, which generate electricity. Waterwheels do not pollute the environment. We can demonstrate the mechanics of a waterwheel by making our own waterwheel. Fact finding: • Pretend that you are an engineer who needs to build a model of a waterwheel to demonstrate its power and mechanics. • Refer to various sources on the Internet and books on the concept of obtaining power from flowing water. Concept applied: Generate power from flowing water Action plan: (a) According to the design of the model, prepare a variety of building materials. (Suggestion of materials: paper plates, paper cups or plastic spoons, tape, straw, wood skewer) (b) Sketch a few designs of waterwheels before building the model. (c) Build the model that you have sketched. (d) Test your model by pouring water onto it to make it spin. (e) Think about improvements that you can make to your model to make it spin faster. Make the necessary changes to your design and test it again. Solution: Design of the model of waterwheel and its features Presentation: Compare your own model of waterwheel with your friends. Share the information with the class about what you learned about how to obtain power from flowing water. 88
Frangipanis are flowering plants seen almost everywhere in Bali Island, Indonesia. The plants grow well in slightly acidic soil. Do you know any other plants that grow well in acidic soil? There are also plants that grow well in alkaline soil. Can you name some of the plants? What do acidic and alkaline mean? Acids and Alkalis CHAPTER 6 What will you learn? Understand what acids and alkalis are List some common acids and alkalis Recognise hazard symbols of acids and alkalis Compare and contrast between acidic and alkaline solutions Explain the function of pH indicators Able to make pH indicators from plant materials Understand the advantage of using a universal indicator Use the pH scale to compare the acidity or alkalinity of solutions Explain the uses of acids and alkalis in daily life Explain neutralisation reaction and its application
6.1 Acids and Alkalis Have you ever tasted an unripe mango? Do you know why it tastes sour? In our daily life, we can find acids and alkalis in the various substances around us. The word ‘acid’ is derived from the Latin word, acidus which means sour, whereas the word ‘alkali’ is derived from the Arabic word, alqali which means ashes from the burning of saltwort plants. Acids Acids are substances that dissolve in water to form acidic solutions. Many common acids are found around us such as in the food and beverages that we consume. Acids are used in the laboratories in many experiments. Types of acids around us Citric acid is found in citrus fruits such as oranges, limes and lemons. Lactic acid is found in yoghurt. Carbonic acid is found in fizzy drinks. Malic acid is found in apples. Tannic acid is found in tea. Tartaric acid is found in grapes. Acetic acid is found in vinegar. Oxalic acid is found in tomatoes. Some bacteria react with sugars and carbohydrates from the foods we consume producing lactic acid as a by-product. The lactic acid starts to slowly corrode the teeth, causing tooth decay. Types of acids around us Formic acid is found in most ants. Chapter 6 Acids and Alkalis 157
Sour Acid tastes sour. Reacts with an alkali to produce salt and water When an acid and an alkali react, salt and water are produced. Reacts with certain metals to produce hydrogen gas The hydrogen gas produced gives a pop sound when tested with a lighted wooden splinter. Conducts electricity Acids are good electrical conductors. Acids such as sulphuric acid are used in car batteries because they conduct electricity well. Corrosive Acids react with certain materials causing corrosion. Due to its corrosive property, the acid in the car battery reacts with the metal terminals of the battery causing corrosion where flaky substances that is blue, white or green can be observed at the terminals. Reacts with a carbonate to produce carbon dioxide gas The carbon dioxide gas produced turns the limewater milky. Changes the colour of blue litmus paper to red When a blue litmus paper is dipped into an acidic solution, it turns red. Calcium carbonate + Hydrochloric acid Calcium chloride+ Water + Carbon dioxide Dilute hydrochloric acid + Magnesium (metal) Magnesium chloride + hydrogen (gas) Hydrochloric acid + Sodium hydroxide Sodium chloride (salt)+ Water Some fruits such as lemons taste sour due to the presence of acid. Acid Marble chips (Calcium carbonate) Carbon dioxide gas Limewater Blue litmus paper Hydrochloric acid (acid) Sodium hydroxide (alkali) Sodium chloride (salt) + water Lighted wooden splinter Hydrogen gas Magnesium (metal) Dilute hydrochloric acid (acid) Properties of an acid 158
Predict what happens when a raw egg is submerged completely in a container filled with vinegar and left aside for one day. Give a reason for your prediction. Think About It Hydrochloric acid, nitric acid, sulphuric acid and ethanoic acid are common acids found in school laboratories. They are corrosive to the skin and eyes. Acids such as nitric acid and hydrochloric acid may release corrosive vapours at room temperature when in a concentrated form. Concentrated acids can be diluted with water to make them less corrosive. Think About It When you are travelling along the highway, have you ever seen this hazard symbol at the back of a road tanker? What does the symbol indicate? Hazard symbol of corrosive substance printed on the label of a bottle of concentrated hydrochloric acid Hazard symbols are usually used to label the containers storing acids to warn users of the dangers when handling them. The hazard symbol is universal, and it can quickly warn users of the dangers involved compared to if it was written in a sentence. Some of the precautions that need to be taken when handling acids are as follows: • Do not touch or taste any of the acids without the teacher’s permission. • If the acid accidentally enters your mouth, quickly spit it out and rinse your mouth with lots of water. Inform your teacher immediately. • If the acid comes into contact with the skin or clothing, wash them with lots of water. Inform your teacher immediately. • Wear safety goggles and gloves when handling acids. • When mixing acid and water, always add the acid to the water, not water to acid. Chapter 6 Chapter 6 Acids and Alkalis Acids and Alkalis 159
Alkalis Alkalis are substances that dissolve in water to form alkaline solutions. Alkalis can be found in our homes in many household products. They are also used in school laboratories. Sodium hydroxide with the chemical formula NaOH, is found in these household products. Magnesium hydroxide is used as an antacid to relieve symptoms of indigestion, stomach acid, heartburn and also as a laxative to treat constipation. Ammonia is used in the production of glass cleaners. Do you know the reason? Potassium hydroxide with the chemical formula KOH, is found in these products. Nail polish remover Bar soaps Alkaline batteries Bleach Liquid soap Sodium hydroxide Magnesium hydroxide Ammonia Potassium hydroxide Types of alkalis around us Drain cleaner 160
Bitter Alkali tastes bitter. Conducts electricity Alkalis are good electrical conductors. Corrosive Changes the colour of red litmus paper to blue When a red litmus paper is dipped into an alkaline solution, it turns blue. Soapy and slippery Due to the alkali in shower gel, it feels slippery and soapy to the touch. Reacts with a dilute acid to produce salt and water When an alkali and an acid react, salt and water are produced. Potassium hydroxide + Nitric acid Potassium nitrate(salt) + Water Red litmus paper Properties of an alkali Potassium hydroxide (alkali) Nitric acid (acid) Potassium nitrate (salt) + water Alkalis such as potassium hydroxide are used in alkaline batteries because they conduct electricity. Strong alkalis such as sodium hydroxide and potassium hydroxide are extremely corrosive. Due to the alkali in soap, the soap bubbles taste bitter if they enter the mouth unintentionally. Chapter 6 Acids and Alkalis 161
Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia solution are common alkalis found in school laboratories. They are corrosive to the skin and eyes. Concentrated alkalis can be diluted with water to make them less corrosive. Hazard symbols are used to label the containers storing alkalis in laboratories. Properties of Acids and Alkalis We can find out the properties of acids and alkalis by conducting some tests. Activity 1 Aim: To study the properties of acid and alkali Materials and apparatus: Lime juice, bitter gourd juice, dilute and concentrated hydrochloric acid, dilute and concentrated sodium hydroxide solutions, universal indicator solution, soap water, magnesium strip, white tile and filter paper, test tubes, droppers, sandpaper, blue litmus paper, red litmus paper, wooden splinter and safety goggles Procedure: A Taste 1 Taste the lime juice. 2 Rinse your mouth with water and repeat step 1 by using bitter gourd juice to replace lime juice. 3 Record your observations. B Feel to the touch 1 Touch the lime juice with your fingers, then rub the fingers together. 2 Wash your fingers and repeat step 1 with soap water. What do you feel? 3 Record your observations. Studying the properties of acids and alkalis • Do not touch or taste any of the alkalis without the teacher’s permission. • If the alkali accidentally enters your mouth, quickly spit it out and rinse your mouth with lots of water. Inform your teacher immediately. • If the alkali comes into contact with the skin or clothing, wash them with lots of water. Inform your teacher immediately. • Wear safety goggles and gloves when handling alkalis. Some of the precautions that need to be taken when handling alkalis are as follows: Resource 162
C Effect on litmus paper 1 Place a blue litmus paper and a red litmus paper on a white tile. 2 Put two drops of dilute hydrochloric acid on both the litmus papers. Observe if there are any colour changes. 3 Repeat steps 1 and 2 using dilute sodium hydroxide solution. Observe if there are any colour changes. D pH value 1 Fill two test tubes with dilute hydrochloric acid and dilute sodium hydroxide solution respectively. 2 Add three drops of universal indicator solution into the test tube filled with dilute hydrochloric acid. 3 Repeat step 2 with the test tube filled with dilute sodium hydroxide solution. 4 Determine the pH value by using the universal indicator chart. E Reaction with metal 1 Clean a magnesium ribbon with sandpaper and put it into a test tube containing dilute hydrochloric acid. 2 Invert another test tube on top of it to collect any gas that is released. 3 If gas is released, test it with a lighted wooden splinter and record your observation. 4 Repeat steps 1 to 3 with a test tube containing dilute sodium hydroxide solution. F Corrosiveness 1 Place a piece of filter paper on a white tile. 2 Add a drop of concentrated hydrochloric acid in the middle of the filter paper and record your observation. 3 Repeat steps 1 and 2 using concentrated sodium hydroxide solution and record your observation. Observation: Write down what you observe. Caution • Conduct this activity in a fume chamber. • Wear safety goggles. • Use only a small amount of acid and alkali. White tile Dropper Dilute hydrochloric acid Drops of universal indicator Dropper Universal indicator solution Dilute hydrochloric acid White tile Dropper Concentrated hydrochloric acid Discussion: 1 What is the pH range of acid and alkali? 2 Magnesium ribbon is cleaned with sandpaper before using it. Give a reason for it. 3 State the operational definition of an acid and an alkali. Conclusion: Write down your conclusion. Magnesium ribbon Test tube Gas collected Test tube Dilute hydrochloric acid Lighted wooden splinter Chapter 6 Acids and Alkalis 163
Similarities and Differences between Acids and Alkalis Similarities Not slippery The feel when touched Slippery Sour The taste Bitter Turns red Effect on blue litmus paper No changes No changes Effect on red litmus paper Turns blue pH 0 − pH 6 pH value pH 8 − pH 14 Reacts and releases hydrogen gas Reaction with metal No reaction with most metals Reacts with an alkali to produce salt and water Produce salt and water Reacts with an acid to produce salt and water Acid solutions are composed of hydrogen (H+) ions. Ions present Alkali solutions are composed of hydroxide (OH– ) ions. Depends on the concentration of the hydrogen ions Strength Depends on the concentration of the hydroxide ions • Most strong acids and alkalis are corrosive. • Both acids and alkalis change the colour of litmus paper. • Both acids and alkalis are electrolytes which means that they are good conductors of electricity. Aspects Acid Alkali Differences There are similarities and differences between the properties of an acid and an alkali as shown below. 164
Role of Water to Show the Properties of Acids and Alkalis The presence of water is essential in acids for the formation of hydrogen ions that cause acidity. Without the presence of water, an acid does not show its properties. Likewise, the presence of water in alkalis is essential for the formation of hydroxide ions that cause alkalinity. Without the presence of water, an alkali does not show its properties. pH Indicators If you have three beakers of solution, A, B and C, how do you know which is an acidic solution, which is an alkaline solution and which is a neutral solution without tasting or feeling it? Other than litmus paper, we can use a suitable pH indicator to find out which solution is acidic, alkali or neutral. The anhydrous tartaric acid dissolved in water turns blue litmus paper red whereas the anhydrous tartaric acid without the presence of water does not change the colour of the litmus paper. The barium hydroxide dissolved in water turns red litmus paper blue whereas the solid barium hydroxide without the presence of water does not change the colour of the litmus paper. Blue litmus paper does not change colour Anhydrous tartatic acid Anhydrous tartaric acid dissolved in water Blue litmus paper turns red Red litmus paper does not change colour Solid barium hydroxide Barium hydroxide dissolved in water Red litmus paper turns blue A B C Without water Without water With water With water Chapter 6 Acids and Alkalis 165
pH indicators are substances that change colour according to the acidity and alkalinity of the solutions. They are used to quickly determine whether a solution is acidic or alkaline. The diagram below shows the colours of various pH indicators in acidic, neutral and alkaline solutions. The natural colour of an indicator is shown in a neutral solution. pH indicator Acidic solution Neutral solution Alkaline solution Phenolphthalein Methyl orange Litmus solution Phenol red Universal indicator Bromocresol green Do you know the difference between bases and alkalis? A base is a substance that reacts with an acid and neutralise it. An alkali is also a substance that reacts with an acid and neutralise it. However, there are bases that do not dissolve in water and there are bases that dissolve in water. Bases that dissolve in water are the ones known as alkalis. All alkalis are bases, but not all bases are alkalis, only soluble bases are alkalis. Science Facts Colour key Colourless Pink Red Yellow Orange Blue Green Purple 166
Activity 2 Aim: To determine acidic and alkaline substances in daily life Materials and apparatus: Lemon juice, dishwashing liquid, vinegar, toothpaste, baking powder, distilled water, pineapple juice, yoghurt, carbonated drink, milk of magnesia, olive oil, pH meter, red and blue litmus papers, universal indicator, methyl orange, phenolphthalein, glass containers and glass rod Procedure: 1 Place a small amount of each substance to be tested into separate glass containers. Dissolve it with a little distilled water if it is a solid. 2 Determine the pH value of the substance by placing a pH meter into the solution. 3 Repeat step 2 using litmus paper, universal indicator, methyl orange and phenolphthalein. Record your observations in a table. As for the substances tested with universal indicator, the colour should be compared with the colours on the universal indicator chart. pH ≤ 3 pH 4 pH 5 pH 6 pH 7 pH 8 pH 9 pH ≥ 10 Universal Indicator Chart Result: Substance Indicator Acid or pH Alkali meter Red and blue litmus papers Universal indicator Methyl orange Phenolphthalein Lemon juice Dishwashing liquid Vinegar Toothpaste Baking powder Distilled water Pineapple juice Yoghurt Carbonated drink Milk of magnesia Olive oil Discussion: 1 In step 1, why is it necessary to dissolve a solid substance in water? 2 What is the advantage of using pH meter compared to litmus paper? 3 Make an inference on a substance with a pH value of 7. Conclusion: Write down your conclusion. Determining acidic and alkaline substances in our daily life Solution pH meter Chapter 6 Acids and Alkalis 167
Litmus is extracted from lichen that grows on tree branches. Litmus is a water-soluble mixture of different dyes absorbed onto filter paper to produce the pH indicator to test materials for acidity or alkalinity. Science Facts We can use some of the dye extracted from the leaves, flowers and other plant parts as indicators. These indicators are known as natural indicators. Some of them are shown below. The table below shows the colour changes of natural indicators. Red cabbage Red spinach Hibiscus Geranium Turmeric Purple hydrangeas Pink hydrangeas Blue hydrangeas Some flowers like hydrangeas become blue if they are planted in soil that is acidic, purple if the soil is neutral and pink if the soil is alkaline. The intensity of the colour is affected by the amount of acid or alkali present in the soil. Plant Colour of dye extracted from the plant Colour of dye in acidic solution Colour of dye in alkaline solution Red cabbage Purple Ranges from red to pink Ranges from blue to greenish yellow Red spinach Red Pink Yellow Hibiscus Dark red Red Green Geranium Red Orange Yellow Turmeric Yellow Yellow Red 168
Activity 3 Aim: Making a natural indicator using red cabbage extract. Materials and apparatus: Red cabbage, lime juice, milk, vinegar, laundry detergent, bleach, coffee, dish cleaner, water, blender and strainer Procedure: 1 Put some red cabbage in a blender. 2 Add water to it until the blender is half-filled and blend the cabbage until a juice is obtained. 3 Strain this mixture into a glass to obtain a red cabbage extract. 4 Add lime juice to a small amount of the red cabbage extract. There will be a change in the colour according to the acidity or alkalinity of the lime juice. Record your observation. 5 Repeat step 4 by using milk, vinegar, laundry detergent, bleach, coffee and dish cleaner. Observation: Tabulate your observation. Substance Colour change Acid/Alkali/Neutral Lime juice Milk Vinegar Laundry detergent Bleach Coffee Dish cleaner Conclusion: Write down your conclusion. Making indicator from plant extract We can make our own indicator. Dyes that we extract from plants or plant parts make good indicators. Chapter 6 Acids and Alkalis 169
The pH Scale In the previous activity, would you be able to determine which solution is more acidic by just looking at the colour of the red cabbage indicator in all the acidic solutions? Similarly, would you be able to determine which solution is more alkaline by just looking at the colour of the red cabbage indicator in all the alkaline solutions? A universal indicator displays several colour changes to indicate how strong an acidic solution or an alkaline solution is. Besides that, the universal indicator turns green in a neutral solution that is neither acidic or alkaline. The colour of a universal indicator is compared to the pH scale. The pH scale ranges from 0 to 14. • A solution is acidic if the pH value is below 7. • A solution is neutral if the pH value is 7. • A solution is alkaline if the pH value is above 7. The pH scale shows the strength of an acidic or alkaline solution. • The more acidic a solution or the more stronger the acid, the lower the pH value. • The more alkaline a solution or the more stronger the alkali, the higher the pH value. The pH scale is logarithmic. It means that an increase or decrease of an integer value will change the concentration by tenfold. Here are some examples. Science Facts A solution with a pH of 2 is ten times more acidic than a solution with a pH of 3. A solution with a pH of 11 is ten times more alkaline than a solution with a pH of 10. How many times more is the acidity of a solution with a pH of 1 compared to a solution with a pH of 3? Think About It Acidic Neutral Alkaline pH 2 pH 10 pH 3 pH 11 170
The chart below shows the colours of universal indicator for the pH values of some common solutions. Here you can see which is the most acidic solution and the least acidic solution. You can also see which is the most alkaline solution and the least alkaline solution. Universal indicator comes in paper and as a solution. Colours of universal indicator in common solutions of various pH values 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Battery AcidicStomach Acid Tomato Coffee Toothpaste Baking Soda Soap Bleach Lemon Vinegar Milk Water Milk of magnesiaAmmonia Solution Drain Cleaner Neutral Alkaline Milk of Magnesia BAKING Chapter 6 Acids and Alkalis 171
Activity 4 Aim: To study the relationship between the pH value and the strength of an acid and an alkali. Materials and apparatus: Universal indicator, 1 M hydrochloric acid, 1 M sodium hydroxide solution, fresh milk, lime juice, soap water, baking powder solution, droppers, test tubes and test tube rack Procedure: Fresh milk Lime juice Hydrochloric acid Universal indicator Baking powder solution Soap water Sodium hydroxide solution Universal indicator 1 Pour the fresh milk, lime juice, hydrochloric acid, baking powder solution, soap water and sodium hydroxide solution into separate test tubes until the test tubes are half filled as shown in the diagram. 2 Add 3 to 5 drops of universal indicator into each test tube and shake the test tube slowly. 3 Observe the colour changes. Based on the pH chart, determine the pH value of each substance. 4 Record all the observations. Observation: Tabulate your observations. Substance Fresh milk Lime juice Hydrochloric acid Baking powder solution Soap water Sodium hydroxide solution pH value Discussion: 1 Arrange the substances based on the strength of their acidity in ascending order. 2 Arrange the substances based on the strength of their alkalinity in ascending order. 3 State the relationship between the pH value and the strength of an (a) acid (b) alkali. Conclusion: Write down your conclusion. Understanding the strength of an acid and an alkali 172
The Uses of Acid and Alkali We use acids and alkalis in our daily lives for various purposes. For example, we use laundry detergents to wash clothes, citric acid to preserve food and vinegar for added flavour in certain food. In our body, the stomach secretes hydrochloric acid to aid digestion by creating the optimal pH enviroment for the enzymes to function. Acids and alkalis are used in various sectors such as the medical, industrial and agricultural sectors. Activity 5 1 Work in groups. 2 Gather information on the uses of acids and alkalis in daily life in various sectors. 3 Present the findings of your group creatively in class. Exploring the uses of acids and alkalis in daily life Formic acid is used to coagulate latex to produce rubber sheets. Latex is the white substance extracted from the bark of a mature rubber tree. Ammonia is used in the making of plant fertilisers. Chapter 6 Acids and Alkalis 173
Issues Associated with Acidity Besides being used for various purposes, there are several issues associated to acids. Acid Rain The diagram illustrates the formation of acid rain Compound such as sulphur dioxide (SO2 ) and nitrogen dioxide (NO2 ) are released into the atmosphere. These compounds are from the burning of fossil fuels at power plants and from vehicles and oil refineries. 1 The air pollutants rise very high up into the atmosphere and 2 react with water, oxygen and other chemical substances. 3 The sulphuric acid (H2 SO4 ) and nitric acid (HNO3 ) formed from the reaction mix with rainwater and fall to the Earth as acid rain. Acid rain enters the water system and sinks into the soil. Acid rain corrodes limestone structures causing loss of carved details Do you know what acid rain is? Acid rain is a type of rain that is unusually acidic. Acid rain has detrimental effects on the soil, plants, aquatic organisms and insects. It corrodes steel structures such as bridges and deteriorates limestone buildings and sculptures. Acid rain can also affect human health. 174
Milk Becomes Sour Acidic Soil Soil can become very acidic from excessive use of chemical fertilisers and from acid rain. Acidic soil can restrict plant growth and yield. At a low pH, many major plant nutrients such as phosphorus, nitrogen and molybdenum become less available to plants. The low pH also increases the availability of some elements, particularly aluminium. High aluminium levels are toxic to plants. Aluminium toxicity will restrict root growth which will then restrict plant growth. Acidic soil is also less favourable to the survival of useful bacteria, earthworms, and other soil organisms. When milk is kept too long at room temperature, it becomes sour. This is because Lactobacillus (bacteria) found in milk convert lactose (sugar) in the milk to lactic acid (acid). The amount of lactic acid increases over time, thus lowering the pH value of the milk and causing it to taste sour. Chapter 6 Chapter 6 Acids and Alkalis Acids and Alkalis 175