Water Changes of State & Water Cycle PDF

2 Water 2.1 Changes in states of water (Book 1A, p. 80–91) 1 Water exists in three physical states: ice is in the solid state; water is in the liquid state; water vapour is in the gas state. 2 Melting is t...

2 Water 2.1 Changes in states of water (Book 1A, p. 80–91) 1 Water exists in three physical states: ice is in the solid state; water is in the liquid state; water vapour is in the gas state. 2 Melting is the change in state from a solid to a liquid. When ice is heated to 0 °C, it melts and changes into water. This temperature is called the melting point of ice. 3 Freezing is the change in state from a liquid to a solid. When water is cooled to 0 °C, it freezes and changes into ice. This temperature is called the freezing point of water. 4 Boiling is the change in state from a liquid to a gas. When water is heated to 100 °C, it boils and changes into steam. This temperature is called the boiling point of water. 5 Water can also change from a liquid to a gas through evaporation. This takes place at a temperature below its boiling point. When water evaporates, it changes into water vapour. 6 Condensation is the change in state from a gas to a liquid. When steam or water vapour cools to or below its boiling point, it condenses and changes into water. 7 At the moment when water changes from one state to another, its temperature remains unchanged; it either absorbs energy from the surroundings, or releases energy to the surroundings. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 1  2 Water 8 Summary of the change in state of water: water absorbs energy from the surroundings Ice (solid) Water (liquid) Steam or water vapour (gas) melting boiling / evaporation freezing condensation water releases energy to the surroundings 2.2 Water cycle (Book 1A, p. 92–103) 1 In Nature, water keeps changing between liquid water and water vapour. The way water circulates in Nature is called the water cycle. 2 The water cycle involves the following processes: 4 Precipitation Precipitation forms in the clouds when water vapour condenses and forms bigger and bigger water droplets. When the droplets are heavy enough, they fall as rain. 2 Condensation Water vapour cools down and condenses to form small sun rain (fresh water) water droplets, which then join together to form clouds. 3 Transportation clouds (fresh water) Clouds may be carried to snow other places by the wind. (fresh water) 1 Evaporation Evaporation occurs when water sunlight warms the surface vapour of the water. Water particles near the water surface absorb energy and river move more quickly. They (fresh water) finally escape from the sea water surface to become (salty water) lake (fresh water) water vapour. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 2  2 Water E 3 The rate of evaporation of water is higher when the surface area exposed to air is larger; the humidity is lower; the temperature is higher; the air speed is higher. 2.3 Dissolving (Book 1A, p. 104–115) 1 When a substance dissolves in a liquid, a solution is formed. sugar water sugar solution (solute) (solvent) (solution) water particle + sugar grain dissolves in sugar particle water into very sugar grain tiny sugar particles 2 The substance that is dissolved is called a solute. The liquid that is used to dissolve it is called a solvent. 3 Substances that can dissolve in water are described as soluble in water. Substances that cannot dissolve in water are described as insoluble in water. 4 The rate of dissolving of a solute in water is higher when the water is stirred; the temperature of water is higher; the size of solute is smaller. E 5 A saturated solution is formed when we keep adding solute to the solution until some solute added remains undissolved. E 6 Solubility of a substance refers to the amount of the substance that can be dissolved in a fixed amount of solvent at a given temperature. E 7 Different substances have different solubilities in water. E 8 Solubility of a substance in water changes with temperature. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 3  2 Water 2.4 Water purification (Book 1A, p. 116–132) 1 Natural water contains soluble impurities, insoluble impurities and microorganisms. 2 If we drink or use natural water directly, the impurities and microorganisms in the water may harm our health. 3 Methods of water purification: a Sedimentation During sedimentation, some insoluble impurities in the water sink to the bottom of the water and form a layer of sediment. Adding alum helps remove some fine solid impurities suspended in water. sediment A beaker of muddy water after standing b Filtration Filtration can help remove solid impurities from water by passing the water through a filter. A filter is made up of materials that consists of many small pores. Only substances smaller than the pores can pass through the filter. Filtration column and filter paper are filters commonly used in school laboratories. The liquid which passes through the filter is called filtrate. The solid which cannot pass through the filter is called residue. Microorganisms and soluble impurities in water are not removed by filtration. residue (solid impurities) filter funnel fine sand coarse sand filter paper rock chips small stones cotton wool filtrate Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 4  2 Water solid impurities filter paper filter funnel filtrate Key: direction of water flow  Filtration of muddy water using filter paper c Distillation Distillation removes all the soluble impurities, insoluble impurities and microorganisms in water. Process of distillation: 2 The steam goes into the condenser. alcohol 3 The steam cools down thermometer and condenses to become liquid water. water out condenser 2 round-bottom 3 flask 1 sea water water in 4 distillate 1 During boiling, only water in the 4 Pure water is collected. The sea water sample turns into steam. water obtained is called Salts and other impurities are left distillate or distilled water. behind in the sea water sample. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 5  2 Water 2.5 Further treatment of drinking water (Book 1A, p. 133–138) 1 Methods to kill microorganisms: Method Description Adding chlorine Chlorine is toxic to microorganisms. This method is commonly used in local water treatment works and swimming pools. Chlorine may leave a pungent smell in water. Too much chlorine in water irritates our eyes and skin. Adding ozone Ozone is added in some swimming pools and water treatment works. It can cause chest pain and shortness of breath when inhaled. This does not leave a bad smell in water. It is more effective than chlorine in killing microorganisms. Using Ultraviolet light is used in water dispensers and fish tanks to kill ultraviolet light microorganisms in water. It is used in some water treatment works and swimming pools. 2 Fluoride can strengthen the enamel of our teeth. Adding it to drinking water can help prevent tooth decay. This addition of fluoride to water is called fluoridation. E 3 In Hong Kong, drinking water is treated at water treatment works before being distributed to us to make the water safe to drink and use. E 4 The water treatment process in Hong Kong involves sedimentation, filtration, adding chlorine and fluoridation. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 6  2 Water E 5 The general water treatment process in Hong Kong is shown below. alum clarifier: sedimentation filtration tank rainwater + Dongjiang water screen rapid mix chamber sludge water pumping clear water fluoride chlorine station tank distribution system 2.6 Water conservation and pollution (Book 1A, p. 139–151) 1 Most fresh water on Earth is trapped in glaciers, ice caps and deep underground. Only a very small part is on the Earth’s surface which is available for use. 2 The limited supply of fresh water is unevenly distributed across the Earth. A large proportion of the world’s population are facing water shortages. 3 Possible problems caused by water shortages: Society: Dehydration of the body may occur. This can cause various health problems. A lack of water may also lead to poor hygiene and inconvenience. Environment: Plants and animals may die from a dehydration. Economy: Without water, the productivity will drop. This lowers economic growth. 4 Fresh water is precious natural resource. It is important to conserve it. Below are some ways we can save water at home: Turn off the tap while brushing your teeth. Take a shower instead of a bath and shorten your showering time. Wash a full load of clothes to reduce the number washes. Wash dishes and vegetables in a basin of water instead of under running water. Choose appliances that use water more efficiently. Reuse the water for rinsing vegetables for watering plants. 5 Water sources may become unsuitable for use because of water pollution, which has harmful effects on our health and the environment Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 7  2 Water 6 Some sources of water pollutants and their harmful effects: Source Harmful effects Waste water Waste water from homes and farms may contain human and animal waste, which may carry harmful microorganisms. If we eat seafood from waters polluted with harmful microorganisms, we may get diseases such as cholera. If we swim in polluted water, we may get eye or skin diseases. Detergents and fertilizers in waste water from homes and farms may contain nutrients that promote the growth of algae. If the population of algae increases rapidly, red tide may result. The algae may use up the oxygen in water. Other living things in the water may die because of a lack of oxygen. The algae may produce toxins that will kill sea animals. If we eat seafood contaminated with toxins, we may get poisoned. Waste water from factories may contain toxic chemicals. If we eat seafood contaminated with these chemicals, the chemicals may build up in our body and may cause health problems. Solid waste Animals may die if they eat the waste or get trapped in it. Plastics in the sea may break down into microplastics which can harm or even kill them by blocking their digestive systems. These plastics will get into our body if we eat contaminated seafood. Oil spills Oil may leak into the sea because of accidents. The spilled oil floats on the surface of the water and blocks air from going into the water. Sea animals may die due to a lack of oxygen. If the feathers of seabirds are covered with oil, the seabirds cannot fly and hunt. They may die of hunger. 7 Ways for individuals to help control water pollution: Use less detergent when washing dishes. Do not throw rubbish into the sea or rivers. Do not pour waste water into the rainwater drains. Do not wash clothes or dishes in rivers or steams. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 8  2 Water E 8 In Hong Kong, waste water is treated in sewage treatment works before it is discharged into the sea: Screening Sedimentation waste water Disinfection Biological treatment Treated waste water is discharged into the sea. E 9 Waste water containing harmful chemicals is sent to the Chemical Waste Treatment Centre for treatment before it is discharged into the sea. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 9