Mixtures can also be classified into solutions, suspensions and colloids. Solutions A solution is a homogeneous mixture where its solid particles have been mixed and fully dissolved in a liquid. The solid particles are known as the solute and the liquid is known as the solvent. For example, when salt dissolves in water, a solution of salt water is formed. The particles of salt (solute) have completely dissolved in the water (solvent). The salt water appears uniform throughout and no particles settle at the bottom of the beaker if we leave it to stand. Salt (Solute) Water (Solvent) Salt water (Solution) stirred A solution can be either acidic or alkaline. It can be tested using indicators such as methyl orange, phenolphthalein and litmus paper. Indicator Colour in acid Colour in alkali Litmus paper Blue litmus paper turns red Red litmus paper turns blue Methyl orange Red Yellow Phenolphthalein Colourless Red/ pink Indicators that cause colour change in acids and alkalis Suspensions A suspension is a heterogeneous mixture of two or more substances from which particles settle over time when left to stand. It is a heterogeneous mixture because the particles can be clearly identified from the dispersion medium. We use the suspension of flour in water to thicken sauces and gravies When a suspension of water and sand are mixed up, the sand will disperse throughout the water. If left alone, the sand will settle to the bottom. Chapter 5 Elements, Compounds and Mixtures 145 ©Praxis Publishing_Focus On Science
The constituents of a mixture can be separated by physical processes. The constituents of a mixture can be mixed in any proportion by mass. An aerosol is a type of colloid where a liquid that is released spreads throughout the air. The addition of an egg yolk to a mixture of oil and vinegar produces mayonnaise which is a colloidal dispersion of a liquid in a liquid. 1. Is dirt a heterogeneous mixture? 2. What are the two key differences between suspensions and colloids? Think About It Mixtures are not formed bychemical reactions. All constituents of a mixture retain their original physical properties. General properties of mixtures Colloids A colloid is a heterogeneous mixture with the size of particles in between those found in suspensions (big particles) and in solutions (very small particles). Colloids appear similar to solutions, where they look almost clear when they are dilute. The particles stay suspended in the dispersing medium that contains them rather than completely dissolving. Colloids appear cloudy or milky like suspensions when they are concentrated, but the particles do not settle to the bottom after a period of time. Although there are many types of mixtures with different properties, they have some general properties. 146 ©Praxis Publishing_Focus On Science
Separating Mixtures Very often, we need to separate different components of a mixture, to separate the useful components from the non-useful components. The methods used to separate the mixtures are filtration, distillation, chromatography, centrifugation, sublimation and evaporation. However, the method used depends on the physical properties of the components that make up the mixture and the components to be obtained from the mixture. Sometimes, a combination of two or more methods are needed to separate a mixture. Filtration Filtration is a method for separating insoluble solid particles from a solid-liquid mixture or a solid-gas mixture. This process of separating insoluble solid particles from a solid-liquid mixture involves utilising a filter funnel and filter paper. The filter paper functions as a sieve and has tiny pores that only allow smaller liquid particles to pass through while retaining larger insoluble solid particles. The insoluble solid particles left on the filter paper are known as residue, while the liquid that goes through it is filtrate. How filtration separates insoluble solid particles from a solid-liquid mixture Insoluble solid particles that are larger than the pores of the filter paper cannot pass through it, so they remain on the filter paper. Smaller liquid particles can pass through the small pores of the filter paper Filtrate Residue Solid-liquid mixture Filter paper Filter funnel Filter paper The table below shows the differences between a compound and a mixture. Compound Mixture It has a fixed composition. It has no fixed composition. The elements are combined chemically where there are chemical bonds between atoms of different elements. The components are not combined chemically, but physically where there are no chemical bonds between atoms of different components. The elements cannot be separated by physical means. A chemical reaction is needed to separate them. The components can be separated by physical means. A chemical reaction is not needed to separate them. It has a fixed melting and boiling points. It has variable melting and boiling points. Chapter 5 Elements, Compounds and Mixtures 147 ©Praxis Publishing_Focus On Science
Tea bags made from filter paper prevents the tea leaves from escaping by allowing only water and other small particles to pass through them. Some of the examples where filtration of solid-liquid mixture or solid-gas mixture are used in our daily life are shown below. Filtrated water out Raw water in Fine sand Gravel Perforated drain pipe Filters in certain vacuum cleaners trap dust and other particles to enable clean air to pass through. Oil filters in vehicles trap particles in the engine oil. Sand filters are used in water treatment plants to remove solid particles from raw water. 148 ©Praxis Publishing_Focus On Science
Distillation The process of distillation uses two processes, boiling and condensation, to extract a pure liquid from a mixture of solid-liquid where the solid is soluble, or liquid-liquid with different boiling points. For example, pure water can be separated from salt or sugar that dissolved in the water by distillation. Also, the distillation method is useful to separate alcohol from a solution of alcohol and water. The figure below shows how pure water can be obtained from a salt solution by distillation. Thermometer Distillation flask Water out Liebig condenser Water in Salt solution Bunsen burner Water vapour Boiling of salt solution Salt solution is heated in a distillation flask until the mixture boils.When the water boils, it changes into watervapour that rises and enters the Liebig condenser. Fractional distillation is a type of distillation that is used to separate a mixture of more than two liquids with different boiling points. In fractional distillation, the various components are separated into parts using a fractionating tower. In industry, the various components of crude oil are separated using fractional distillation because it is a mixture of oils with different boiling points. Residue left behind The salt solution becomes more concentrated as distillation continues. At the end, a solid salt will be left in the distillation flask as a residue. Collection of distillate (pure water) Pure water that flows out from the Liebig condenser is collected as a distillate in a beaker. Condensation of water vapour When the watervapour touches the cold surface of the Liebig condenser, it cools down,condenses and changes into pure water. 1 2 3 4 Chapter 5 Elements, Compounds and Mixtures 149 ©Praxis Publishing_Focus On Science
Centrifugation A common method for separating solid particles scattered in a liquid medium, such as blood cells and plasma, is centrifugation. This is done by spinning the mixture around an axis at a high speed. Red blood cells, white blood cells, platelets and plasma are separated using this method. During this process, the mixture’s denser component (blood cells) moves farther from the axis while its lighter component (blood plasma) moves closer to the axis. Finally, the blood cells settle to the bottom with the blood plasma at the top. Plasma (55%) White blood cells and platelets (4%) Red blood cells (41%) Centrifugation of blood Chromatography A drop of ink or dye can be separated into its coloured components by chromatography. This is possible because different coloured components travel at different speeds and get separated. Different coloured components of ink can be separated as follows. Pencil line Spot of black ink Glass jar Water rises up on the filter paper Water Spots of different colours separated from black ink Chromatograph Strip of filter paper Glass rod Cellotape The separation of ink into its coloured components via chromatography. Different coloured components of ink are observed at different heights on the filter paper. This is because when water travels up the filter paper, the ink dissolves in the water and moves up with it. The coloured components travel at different speeds, therefore they are located at different levels on the filter paper. Food colouring is used in the food industry to colour foods and beverages. Chromatography can be used to analyse the food colourant samples to determine the ingredients that are present. This helps in detecting illegal compounds and ensure that the colouring used are safe for consumption. Chromatography is also used to identify the constituents of blood, urine and inks. Athletes who use illegal drugs to boost their performance can be tested positive via their urine and blood samples using this method. 150 ©Praxis Publishing_Focus On Science
Activity 3 Aim:To separate a mixture by sublimation Materials and apparatus: Ammonium chloride, sand, inverted funnel, evaporating dish, Bunsen burner and cotton Procedure: 1 Place a mixture of sand and ammonium chloride in the evaporating dish. 2 Put the inverted funnel over the evaporating dish. 3 Put a cotton at the end of the funnel. 4 Heat the mixture slowly and observe what happens. Observation: Write down what you observe. Conclusion: Write down your conclusion. Separating a mixture Cotton Funnel Sand + ammonium chloride A centrifuge machine used to separate the components in blood Sublimation When heated, some solids instantly transform into gases. These solids are known as sublime solids which undergo the transformation process called sublimation. A mixture with one sublimable component and one non-sublimable component (commonly referred to as the impurity) can be separated using the sublimation process. Chapter 5 Elements, Compounds and Mixtures 151 ©Praxis Publishing_Focus On Science
Evaporating dish Salt water Salt water becomes concentrated Salt Evaporation Evaporation is the process in which a substance is converted from liquid state to gaseous state due to an increase in temperature. Evaporation is used to separate a soluble solid from a liquid in a solid-liquid mixture, also known as a solution. When the solution is heated, the liquid evaporates, thus the solid is left behind as residue. We can obtain salt from salt water by heating up the water as shown below. When water evaporates, the salt which is the solid is left behind as residue. Heaps of sea salt in an evaporation pond obtained through evaporation of the seawater. Salt obtained from salt water through evaporation 152 ©Praxis Publishing_Focus On Science
1 are the basic building blocks of all matter. 2 There are three types of subatomic particles in an atom, namely protons, electrons and . 3 Protons and neutrons make up the of an atom. 4 A molecule is made up of two or more chemically combined. 5 A molecule of an consists of a fixed number of the same type of atom chemically combined. 6 The molecule of ozone is an example of a molecule of an . 7 The chemical of a molecule shows the number and types of atoms the molecule contains. 8 The chemical formula of an molecule is NH3 . 9 are pure substances that cannot be broken down into other simpler substances by physical or chemical means. 10 The most abundant element in our Earth is . 11 All the elements are classified in a table called the Table of Elements to enable the study of elements systematically. 12 The elements in Group 1 are known as alkali . 13 Chlorine is placed in Group 17 and known as a . 14 The property of elements in the Periodic Table of Elements change from being metallic to non-metallic from to . 15 Germanium which has properties of both metals and non-metals is a . 16 Generally, non-metals are conductors of electricity and heat. 17 Copper can be drawn into wire, thus it is said to be . 18 are formed when two or more elements combined chemically in a chemical reaction while have two or more components that are physically combined. 19 Sulphates consist of the elements oxygen and . 20 The properties of a compound and its constituent elements are . 21 A mixture is a mixture with uniform composition throughout the mixture while a mixture is a non-uniform mixture with visible, individual components. 22 The methods used to separate mixtures are filtration, sublimation, distillation, chromatography and centrifugation. The method used depends on the properties of the components that make up the mixture and the components to be obtained from the mixture. 23 A common method for separating solid particles scattered in liquid medium, such as blood cells and plasma, is . RECALL Fill in the missing words. Chapter 5 Elements, Compounds and Mixtures 153 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 Is air considered as a solution? Give a reason. 2 Can we separate sugar from sugar solution through filtration? Explain your answer. 3 How do you separate a mixture of water, sand and iron filings? 4 The presence of harmful dyes X and Y is determined using the chromatography method on a sample of food colouring Z. The chromatogram shows the result as below. Z X Y Is food Z safe to be consumed by the public? Explain your answer. 154 ©Praxis Publishing_Focus On Science
Project Pure Water Supply Activity objective: To obtain pure water from salt water Problem statement: Currently, freshwater supply from resources such as rivers, ponds and underground water is decreasing due to uncertain weather conditions (drought) and water pollution. Therefore, sea water should also be used as a source of water to overcome the water crisis that has occurred over the last few years. However, the cost is high to build a desalination plant that produces pure water from sea water. Procedure: • Divide the students in the class into several groups. • Each group should identify suitable natural methods for obtaining pure water from salt water easily. Students in each group need to discuss and search the information on the Internet. Concept applied: Method of separating a mixture (evaporation) Action plan: (a) Form a mixture of water and salt. (b) Identify the methods to be used to separate the mixture of salt solution. (c) Determine the materials to be used and calculate the total cost of the material. Solution: Obtain pure water from a mixture of salt solution using the natural evaporation method (the Sun). Explain the advantages of the method used. Presentation: Present the findings using Microsoft PowerPoint. Example of a method to obtain pure water from salt water: Fill a bowl with salt water. Put a cup in the middle of the bowl. Cover the bowl with plastic wrap and place a weight (rock) in the centre of the plastic wrap. Place the bowl under direct sunlight. Water will evaporate and condensation occurs on the plastic wrap. Chapter 5 Elements, Compounds and Mixtures 155 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
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 ©Praxis Publishing_Focus On Science
6.2 Neutralisation Do you know what happens to the pH of an acid when an alkali is gradually added to it? The pH changes as shown in the diagram below. pH = 1 pH = 3 pH = 7 pH = 9 Acid + universal indicator Alkali When we add alkali to acid, the solution becomes less acidic and the pH of the solution increases. When we add more alkali, at one point, the alkali completely cancels out the effects of the acid. A neutral solution with a pH of 7 is produced. When we add more alkali to the neutral solution, it becomes more alkaline and the pH of the solution increases. The reaction between the right amounts of an acid and an alkali produces a neutral solution of salt and water, with a pH of 7. This reaction is known as neutralisation. Neutralisation takes place when the right amounts of an acid and an alkali react together. The reaction of different acids and alkalis produce different salts. For example: Acid + Alkali Salt + Water Hydrochloric acid + Sodium hydroxide Sodium chloride (salt) + Water Nitric acid + Potassium hydroxide Potassium nitrate (salt) + Water Sulphuric acid + Potassium hydroxide Potassium sulphate (salt) + Water Titration is the method used to carry out neutralisation. 176 ©Praxis Publishing_Focus On Science
Activity 6 Aim: To study acid-alkali neutralisation reaction through titration method Materials and apparatus: Phenolphthalein, hydrochloric acid (0.5 M), sodium hydroxide solution (0.5 M), burette, 25 ml pipette, 250 ml conical flask, retort stand with clamp, white tile, small filter funnel, evaporating dish, tripod stand and Bunsen burner Procedure: Pipette Conical flask Sodium hydroxide solution Phenolphthalein (indicator) Burette Hydrochloric acid Retort stand with clamp Sodium hydroxide solution + phenolphthalein White tile Conical flask 1 Add 40 cm3 of hydrochloric acid into a burette using a filter funnel. 2 Record the initial reading of the burette. 3 Add 25 cm3 of sodium hydroxide solution into a conical flask using a pipette. 4 Add three drops of phenolphthalein into the sodium hydroxide solution and shake the conical flask slowly. 5 Add hydrochloric acid drop by drop slowly from the burette into the sodium hydroxide solution while softly swirling the conical flask. Take note of any changes in colour of the content of the flask. 6 Stop adding the acid when the colour of the sodium hydroxide solution turns from pink to colourless. 7 Record the final reading of the burette. Calculate the volume of acid used. 8 Pour the solution in the conical flask into an evaporating dish and heat it until all the water has evaporated. What is the end product formed in the evaporating dish? Observation: Write down your observations. Discussion: 1 Name the reaction that occurs and write a word equation to represent the reaction. 2 Why should we stop adding the acid in step 6 when the colour of the sodium hydroxide solution turned from pink to colourless? 3 Name the method used in this activity. Conclusion: Write down your conclusion. Studying neutralisation reaction Solution Water evaporates Evaporating dish Heat End product Chapter 6 Acids and Alkalis 177 ©Praxis Publishing_Focus On Science
Applications of Neutralisation in Daily Life Some of our daily life activities make use of the neutralisation reaction. Here are some examples. Hydrochloric acid is produced in the stomach to aid in the digestion process and to kill germs found in food. However, when there is too much acid in the stomach, indigestion occurs, and antacids or indigestion tablets are generally recommended. This is because the antacids are alkaline. Antacids neutralise the excess acid to relieve any discomfort in the stomach. Neutralising excess acid in the stomach A wasp sting is alkaline in nature. Vinegar (ethanoic acid) which is acidic can be used to relieve the pain by neutralising the sting. A bee’s sting is acidic in nature, and it causes inflammation and pain around the infected area due to the release of formic acid. The pain can be eased by treating it with baking soda (bicarbonate of soda). When the bacteria in the mouth feed on the leftover food in between the teeth, acid is produced, and it can cause tooth decay. It is advisable to brush our teeth at least twice a day. This is because the alkali in toothpaste neutralises the acid present in the mouth. Thus, tooth decay can be prevented. Neutralising acidity in the mouth Treating wasp’s sting Treating bee’s sting 178 ©Praxis Publishing_Focus On Science
Neutralisation plays a major role in farming. For crops to grow well, the soil should not be too acidic. However, the excessive use of pesticides and fertilisers makes the soil too acidic. Quick lime (calcium oxide) or slaked lime (calcium hydroxide) can be added to the soil to neutralise the acid. Soil treatment Most of the waste generated by factories are toxic and acidic in nature. The waste are normally discharged into the nearby flowing water bodies such as the rivers. This is harmful to the aquatic plants and animals. To minimise the harmful effect of the waste, it must be neutralised to minimize its toxicity before it enters the water. Some of the substances that are used for neutralisation are sodium bicarbonate, magnesium hydroxide, calcium oxide and calcium carbonate. Industrial waste treatment When fossil fuels in power stations are burnt, the acidic gas sulphur dioxide is produced. This gas can dissolve in rainwater to form acid rain. However, in some power stations, this gas is removed through a process known as scrubbing. In this process, an alkaline substance or solution such as limestone or sodium hydroxide, is used to neutralise the acidic sulphur dioxide gas. Smoke without sulphur dioxide is then released through the chimneys. Reducing air pollution Chapter 6 Chapter 6 Acids and Alkalis Acids and Alkalis 179 ©Praxis Publishing_Focus On Science
The laundry detergent that we use to wash our clothes is alkaline in nature. Fabric softeners which are mildly acidic, are used to neutralise the detergent to bring the texture and feel of the fabrics back to its original condition. Fabric softeners also reduces the risk of skin irritation which may be caused by alkaline residues. Fabric softeners Face cleansers which are alkaline can make the skin on the face becomes dry. Therefore, acidic toners are used to neutralize the slightly alkaline skin after cleansing. Toners After washing with shampoo, usually the hair feels rough and becomes tangled because the shampoo is alkaline in nature. A hair conditioner which is a mild acid can be applied to the hair to neutralise the alkali in the shampoo and make the hair smoother. Shampoos and conditioners 180 ©Praxis Publishing_Focus On Science
1 Acids are substances that dissolve in water to form solutions. 2 Alkalis are substances that dissolve in water to form solutions. 3 Acid solutions are composed of ions while alkali solutions are composed of ions. 4 acid is found in tea. 5 Some bacteria react with sugars and carbohydrates from the foods we consume to produce acid. 6 with the chemical formula KOH, is found in alkaline batteries. 7 Acids and alkalis are electrical conductors. 8 change the colour of blue litmus paper to red whereas change the colour of red litmus paper to blue. 9 Acids taste while alkalis taste . 10 Without the presence of , an acid or an alkali does not show its properties. 11 are substances that change colour according to the acidity and alkalinity of the solutions. 12 Red cabbage extract changes its colour to red in solutions. 13 A solution is acidic if the pH value is 7. 14 A solution is alkaline if the pH value is 7. 15 A solution is neutral if the pH value is . 16 A solution with a pH of 2 is acidic than a solution with a pH of 6. 17 A solution with a pH of 9 is alkaline than a solution with a pH of 13. 18 When milk is kept too long at room temperature, it becomes sour because the bacteria found in milk convert lactose in the milk to acid. 19 takes place when the right amounts of an acid and an alkali react to produce salt and water. 20 A wasp sting which is can be treated with vinegar. 21 can be added to the soil to treat soil which is too acidic. RECALL Fill in the missing words. Chapter 6 Acids and Alkalis 181 ©Praxis Publishing_Focus On Science
THINKING CAP Put on your 1 The diagram shows the inner part of an alkaline electric cell. Which property of the potassium hydroxide used in the cell makes it suitable as a component of the cell? 2 What happens to the pH of a solution with a pH of 13 if you keep adding a lot of water to it? Give a reason for your answer. Positive terminal Zinc case Potassium hydroxide (alkali) Carbon rod Negative terminal 3 You are given 100 ml of a solution of pH 5. Explain how you could prepare a solution of pH 6 from the solution given. 182 ©Praxis Publishing_Focus On Science
Project Investigating pH of Soils Activity objective: To determine the pH of different types of soil and the plants that grow well in them. Problem statement: Soil pH is important for the healthy growth and yield of plants. There are many factors that affect the pH of soil. Not all plants need the same soil conditions. The type of plant which suits a certain soil condition must be determined for maximum growth. Concept applied: Method of testing the pH of soil Procedure: 1 Divide the students in the class into several groups. 2 Each group should carry out the following tasks: (a) Collect different soil samples in the surroundings. (b) Identify the methods used to test the pH of the soil samples. (c) Determine the apparatus and materials to use. (d) Test the pH of the soil samples. Solution: Identify the pH of the soil and conclude which plants will grow well in each type of soil. Explain the advantages of the method used to test the pH of the soils. Presentation: Present the finding using Microsoft PowerPoint. Chapter 6 Acids and Alkalis 183 ©Praxis Publishing_Focus On Science
Have you ever wondered what is beneath the soil that we step on daily? Do you know how many layers are there in our Earth? CHAPTER 7 What will you learn? Describe the structure of Earth and Earth’s surface processes Explain the movement of tectonic plates Describe the different plate movements Describe what an earthquake is Describe the types of volcanoes Respond appropriately during natural disasters such as earthquakes and volcanic eruption Structure of the Earth and Natural Disasters ©Praxis Publishing_Focus On Science
7.1 Structure of Earth Earth’s surface is uneven because of a variety of landforms found on it. Landforms are the natural physical features found on Earth’s surface which were formed due to various forces of nature such as wind, water and ice. Some landforms are created in a very short period, while others take millions of years. There are many types of landforms on the Earth’s surface such as mountains, valleys, deserts, dunes, islands, plains, rivers, oceans and deltas. Some of the landforms found on Earth’s surface around the world Mount Agung in Bali, Indonesia Flores Island in Indonesia Nile River in Egypt Sahara Dessert in North Africa River valley in Sumbawa, Indonesia Atlantic Ocean Chapter 7 Structure of the Earth and Natural Disasters 185 ©Praxis Publishing_Focus On Science
Characteristics of the Earth’s Layers The Earth can be divided into layers based on chemical composition and physical characteristics. Based on chemical composition, our Earth is made up of three main layers, namely the crust, mantle and core. Based on physical characteristics, the Earth can be divided into five distinct layers, namely the lithosphere, asthenosphere, mesosphere, outer core and inner core. Crust Mantle Outer core Inner core Lithosphere Asthenosphere Mesosphere Earth’s layers based on chemical composition and physical characteristics Chemical Layers Crust The Earth’s crust is the outermost layer of the Earth which we live on. It is 5 to 70 km in thickness. Earth’s crust is divided into two types: oceanic crust and continental crust. The thickness of the continental crust which makes up the land on Earth ranges from 35 km to 70 km while the thickness of the oceanic crust beneath the ocean floor ranges from 5 km to 7 km. The average density of the Earth’s layers increases from the crust to the core. Thus, the Earth’s crust has the lowest density compared to other layers. The crust makes up less than 1% of the Earth’s mass. It consists of soil, rocks that are easily broken and various elements. There are eight elements that make up 99% of the Earth’s crust. They are oxygen, magnesium, aluminium, silicon, calcium, sodium, potassium and iron. The temperature of Earth’s crust varies considerably based on location and time it is measured. Its average surface temperature depends on whether it is being taken on land or beneath the sea. Since most of the Earth’s crust lies beneath the oceans further from the sun, the temperature ranges from as low as 0°C to 3°C. On the other hand, the deeper you dig into the continental crust, the higher the temperature gets. For example, the temperature at Mponeng gold mine in South Africa, which is 4 km deep from the ground, can reach up to 60°C. Chemical layers Physical layers Resource 186 ©Praxis Publishing_Focus On Science
The crust is mostly covered by water, sand, soil and ice. Underneath the loose layer of soil, sand and rock particles, is the solid rock known as the bedrock as shown here. Organic layer A layer that consists of organic matter such as dried leaves, grasses, dead leaves and twigs, and other decomposed organic matter. 1 2 3 4 5 Topsoil A layer that consists of minerals, decomposed organic matter and organisms such as earthworms and bacteria. Most plant roots grow here. Subsoil A layer that consists of sand, silt and clay with less organic matter. Weathered rock A layer that consists of rock particles. Bedrock A layer that consists of solid rocks. The different layers in the Earth’s crust Mantle The Earth’s mantle is the layer underneath the crust that goes down almost halfway to the centre of Earth. The mantle is divided into two zones, namely the upper mantle and lower mantle. The mantle is mostly solid with a small amount of molten material between the crust and the uppermost part of the mantle. The mantle is the Earth’s thickest layer with a thickness of about 2900 km. It makes up about 68% of the Earth’s mass. It is made up of silicon, magnesium, oxygen, aluminium and iron. Its temperature is between 500°C and 2000°C. The upper mantle consists of huge slabs of rock, called tectonic plates which are brittle and moving constantly. The movement of tectonic plates is extremely slow and when they rub against each other, earthquakes and volcanic activities occur. The lower mantle consists of rocks which are soft but not in the molten form. However, when rocks in the lower mantle are subjected to forces, they flow instead of breaking. Core Upper mantle Lower mantle 2900 km Mantle Cross section of the Earth showing two different zones of the mantle: upper mantle and lower mantle. Chapter 7 Structure of the Earth and Natural Disasters 187 ©Praxis Publishing_Focus On Science
Core The core is the centre of the Earth and about 2900 km below Earth’s surface. It makes up about 31% of the Earth’s mass. The ball-shaped core layer is divided into two parts, namely the outer core and inner core. The outer and inner cores make up about half of Earth’s diameter. There is a liquid-solid boundary between the inner and outer cores which is the hottest part of the core, as hot as the surface of the Sun. The outer core is beneath the mantle and mainly composed of molten iron and nickel. Both these metals are magnetic. Its thickness is about 2200 km, and the temperature is between 4000°C and 6000°C. The inner core is the densest part of the Earth. It is mainly composed of solid iron and nickel. The inner core is in solid form because the intense pressure it experiences prevents the iron from melting. Its thickness is about 1270 km and the temperature is between 5000°C and 7000°C. Activity 1 1 Work in groups. 2 Find out more about the Earth’s layer based on physical characteristics. You may use various source such as the books from your library, internet or videos. 3 Each group will present their findings on a poster. Researching the Earth’s physical layers The Kola Superdeep Borehole is the deepest manmade hole ever dug, purely sciencedriven, by the Soviet Union near the Russian border with Norway. Soviet scientists wanted to investigate more about the Earth’s crust, to get a better understanding on how that crust was formed and evolved. They wanted to drill as deep as possible into the Earth’s crust. The hole was finally drilled into a depth of 12 262 m, a depth that took about 20 years to reach. It is deeper than the deepest point in the ocean, the Mariana Trench, that has a depth of 11 035 m. Drilling of the Kola borehole began in May of 1970 but was abandoned in 1992 when drillers encountered extremely high temperatures as they dig deeper. Science Facts Mariana Trench Kola Superdeep Borehole 12 262 m 188 ©Praxis Publishing_Focus On Science
Earth’s Surface Processes Do you know the processes that cause changes to the Earth’s surface? They are known as geomorphic processes, which are important in the creation of landforms on Earth’s surface. There are two geomorphic processes, namely the endogenic process and exogenic process. The endogenic process is caused by forces that originate from within the Earth. Endogenic forces include earthquakes and volcanic activities. Their impact can be observed immediately. Mantle convection process The Earth’s crust is composed of pieces of moving plates, according to the Plate Tectonics theory. Due to the constant movement of these plates, collision and divergence occur. Thus, various landforms are formed on Earth. Movement of Earth’s crust A volcano is a vent on the Earth’s crust. Molten magma flows out through the vent in a strong eruption. The erupted materials deposited around the slope of the vent form the hillshaped landform known as the volcanic cone. Magma activity Mantle Asthenosphere Outer core Inner core Convection currents are produced in the asthenosphere layer due to the high temperatures in the mantle and core of Earth. The convection currents cause the movement of the Earth’s crust. Endogenic Processes Movement of Earth’s crust Lithosphere Vent Lava (molten rock erupted out of the Earth’s surface) Cone Magma (molten rock beneath Earth’s surface) Chapter 7 Structure of the Earth and Natural Disasters 189 ©Praxis Publishing_Focus On Science
The exogenic process is caused by forces that originate from the Earth’s surface or Earth’s atmosphere. Exogenic forces include the tidal force of the moon and erosion. Their impact can still be observed thousands or millions of years later. Exogenic Processes Weathering A process of fragmentation and decomposition of rocks on Earth’s surface caused by various factors such as changes in temperature, rain, wind and organisms. Mass wasting The movement of rocks and soil from the top of a slope downwards due to gravitational force. Erosion A process in which the materials on Earth’s surface are worn away by natural forces such as water, wind and waves. Transportation and sedimentation The movement of weathered or corroded materials by agents such as water, wind and glaciers. Eventually, the materials are sedimented when the agents slow down. 190 ©Praxis Publishing_Focus On Science
Slow Processes that Change the Surface of the Earth Some changes on the Earth’s surface are caused by slow processes that have occurred over billions of years ago. The slow processes involved are weathering, erosion and deposition. Weathering Weathering is the process where rocks on the Earth’s surface break into smaller pieces and gradually wear away. There are three types of weathering, namely physical, biological and chemical weathering. Physical Weathering Physical weathering, also known as mechanical weathering, is the breaking down of rocks without involving any changes to the chemical composition of the rocks. Temperature changes between day and night Cracks at outer layer of rock Thermal stress Physical weathering can occur due to temperature changes in a process called thermal stress. During the day when it is hot, the outer layer of a rock becomes hot while its centre is cold. So, the outer layer of the rock expands slightly. At night, when it is cold, the outer layer cools faster than its centre and contracts slightly. As the expansion and contraction occur repeatedly, over time the rock weakens. Cracks may form at the outer layer and pieces of the rock will fall off. Water enters the rock Water freezes and expands The rock breaks apart The effect of ice wedging can be seen at Bryce Canyon National Park, USA. The rocks go through freeze and thaw cycles repeatedly until they break apart. Freeze and thaw cycles occur hundreds of times each year, thus breaking the rocks into unusual shapes. In very cold areas, water can cause physical weathering through a process known as ice wedging that involves repeated freeze and thaw cycles as shown above. Rainwater or melting snow seeps into the crevices of a rock and when the water or melted snow freezes, it expands. This expansion pushes the crevices further apart. The freeze and thaw cycle occurs repeatedly and over time, the expanding ice widens the crevices, and the rock breaks apart. Ice wedging Chapter 7 Structure of the Earth and Natural Disasters 191 ©Praxis Publishing_Focus On Science
It is very cold on top of mountains, especially in the evenings and at night. The freezing and expansion of water in the rocks can break apart the rocks into fragments causing rockfalls. For this reason, mountain climbers start their journey early in the morning and they try to complete it by midday. The action of wind, gravity, rain and waves can also cause physical weathering. When there is strong wind, it carries sand particles that hit against rocks and breaks off the softer parts. These pieces will eventually fall off due to gravity. Over time, the sand in the wind sculpts large rocks into interesting shapes such as this arch found in Arches National Park, USA. The movement of water is a major force in physical weathering. Coastal rocks will gradually wear down due to the persistent crash of waves against them. 192 ©Praxis Publishing_Focus On Science
Biological Weathering Biological weathering is caused by the movements of living things such as plants, animals and microbes. Animals such as rabbits, chipmunks and badgers can burrow into a crack in a rock, thus widening it and eventually splitting the rock. A plant may grow in the crack of a rock and, as its roots grow, they wedge the rock apart. Sometimes humans contribute to biological weathering. This occurs when they walk over the same patch of rock which will gradually cause the rock to wear down. Humans wearing hiking shoes or boots to walk on rocks can contribute to weathering of the rocks. The pressure created in the rocks by the growing roots of plants can wedge the rock. When burrowing animals such as badgers dig tunnels underground, water may enter the rocks causing them to break apart over time. Chapter 7 Structure of the Earth and Natural Disasters 193 ©Praxis Publishing_Focus On Science
Chemical Weathering Chemical weathering of rocks involves chemical reactions which change the mineral compositions of rocks. Chemical weathering occurs due to agents like rain and organisms. The burning of fossil fuels such as coal, oil and gasoline releases carbon dioxide, nitrogen and sulphur into the atmosphere, which dissolve in rainwater to form carbonic, nitric and sulphuric acids respectively. This is what makes rain slightly acidic. Limestone is a type of rock that reacts with acidic rainwater, also known as acid rain, making the rock weathered. Organisms such as lichens also contribute to chemical weathering of rocks by excreting various organic acids, particularly oxalic acid, which breaks down rocks. Lichens are one of the organisms responsible for the weathering of rocks by causing the break down of rocks over time. Limestone is a type of rock mainly composed of calcium carbonate. When rainwater seeps through cracks in the roofs of limestone caves, the rainwater loses some of its dissolved carbon dioxide and deposits part of its dissolved calcium carbonate. Some of the deposits form stalactites that hang downwards from the roofs and some form stalagmites that grow up from the floor. When limestone reacts with acid rain, a soluble substance formed during the reaction weathers the limestone as the substance washes away. 194 ©Praxis Publishing_Focus On Science