Science Notes: States of Matter (PDF)

Air Air is a mixture of different gases. A gas called nitrogen makes up about four-fifths of the air. Air also contains a gas called oxygen. We need oxygen to live. We breathe in oxygen. We breathe out a gas called carbon dioxide, which is also found in air. Air also contains a gas called water va...

Air Air is a mixture of different gases. A gas called nitrogen makes up about four-fifths of the air. Air also contains a gas called oxygen. We need oxygen to live. We breathe in oxygen. We breathe out a gas called carbon dioxide, which is also found in air. Air also contains a gas called water vapour. The layer of air that surrounds the Earth is called the atmosphere. There are many other gases that are not found in air, for example, hydrogen and helium. Air pollution It is also important for us to have clean air to breathe. In many places in the world, the air is not clean. People put smoke, dust and gases from factories, power stations, cars, trucks and aeroplanes into the air. These substances make the air dirty and harmful to living things. We say that they cause air pollution. Air pollution is not only outside. Homes, schools and buildings can also have air pollution. Many things that people use every day can pollute the air. Smoke from cigarettes and cooking, and fumes from spray cans and paints also pollute the air. Air pollution can make us cough, feel dizzy and give us headaches. People who breathe in polluted air also often suffer from breathing problems. The smoke from cigarettes can cause serious diseases that harm our lungs. Water changes state Solids, liquids and gases are different states of matter. Water exists in all three states of matter. Ice is solid water. The water we drink is a liquid. Water vapour in the air is a gas. Water changes state when it is heated or cooled. For example, when we heat ice, it melts to form liquid water. When solids, like ice, are heated, their particles gain energy and start to move more quickly and pull further apart. If the particles gain enough energy, they move further apart from each other. The solid becomes a liquid. When a solid changes into a liquid we say the solid melts. We can show how melting happens like this: solid+heat → liquid If we heat water enough, it will boil. When water boils it forms bubbles of a gas called steam. Steam is heated water vapour. The particles of the liquid gain heat and move much more quickly. Some of the heated particles move far apart and become a gas. This happens throughout the liquid, not just on the surface. We can show how boiling happens like this: liquid + heat→ gas Melting and boiling points All substances melt and boil. Different substances take different times to melt or boil because it takes different amounts of heat for them to change state. The amount of heat in a substance is called the temperature. We measure temperature with a thermometer. The unit we use is degrees centigrade (°C). Like other substances, water needs to reach a certain temperature in order for its physical state to change. The temperature at which a substance melts is its melting point. This is when it changes from a solid to a liquid. The melting point of ice is 0°C. Water freezes at 0°C. This means that the melting point of ice is the same as the freezing point of water. The temperature at which a substance boils is its boiling point. This is when it changes from a liquid to a gas. Melting point and boiling point are properties of substances. They do not change unless the substance itself changes. The boiling point of pure water is 100°C, but most of our water has small amounts of other substances mixed with it. This makes the water boil at a slightly different temperature. Water also boils at a lower temperature the higher we are above sea level. Water expands We know from the particle model of matter that when a substance gains heat its particles move faster and spread out. This makes the substance take up more space. We say that the substance expands. When the substance cools down, its particles move more slowly because they have less energy and move closer together. The cooled substance takes up less space. It gets smaller, or contracts. Heated particles move faster and further apart Cooled particles move more slowly and closer together Evaporation When water or other liquids become warm enough, they change into a gas. Water changes into a gas called water vapour. When particles on the surface of a liquid change into a gas, we say the liquid evaporates. Evaporation happens all around us without us noticing it much, like a puddle drying up or washing getting dry. Evaporation happens because some particles in the liquid gain heat energy. The heated particles start to move faster and move further apart. Eventually the heated particles move so far apart that they escape from the surface of the liquid into the surrounding air. When a liquid evaporates it changes from the liquid State to the gas state. liquid+heat→ gas Condensation The air you breathe out is warm. It contains water vapour gas. When the warm air touches a cooler surface like the mirror it cools down. If the surface is cold enough, the water vapour gas changes to drops of liquid water. The process is called condensation. It is the reverse of evaporation. Condensation happens because the particles of a gas lose energy when they get cooler. This makes them slow down and move closer together to become a liquid. The water cycle Water keeps going around and around in what we call the water cycle. In the water cycle, water moves from the land and sea to the air and back again. The water cycle happens mainly because of evaporation and condensation. Freezing and melting can also happen in the water cycle. The rain and snow that fall from clouds onto Earth are called precipitation. Sometimes balls of ice called hail also fall from the clouds. Dissolving and solutions Tea is a liquid. Sugar is a solid. When you put sugar in your tea and stir it, the sugar seems to disappear. You know the sugar is there because you can taste it in the tea. What happened to the sugar? The sugar has dissolved in the tea. This means that the particles of sugar have spread out into the spaces between the particles of the tea. Solids that can dissolve in a liquid are called soluble solids. Being able to dissolve is a property of the solid. Not all solids can dissolve. These solids are insoluble. Mixtures are made of particles of different substances mixed together. Dissolving is a type of mixing because when the solid dissolves it mixes with the liquid. We call the mixture a solution. The solid in the solution is called the solute, for example sugar. The liquid is called the solvent, for example tea. Being able to dissolve a solid is a property of the liquid. A solution will always contain a solute and a solvent. Sea water is another solution. The salt dissolves in the water to make the solution. The salt is the solute and the water is the solvent. Many substances dissolve in water. This is another one of water\'s properties. Water is sometimes called the universal solvent because so many substances can dissolve in water. Water pollution Some solutions can be harmful. Some soluble substances can pollute water. We cannot see these substances because they are dissolved in the water, so we don\'t know that the water is polluted. Polluted water can harm plants, animals and people. Dissolved chemicals, such as acids, from factories can pollute rivers and lakes. Farmers put chemical fertilisers in the soil to make their crops grow better. These fertilisers sometimes wash into rivers and pollute the water. Many farmers also spray chemicals on their crops to kill pests like insects. These chemicals are called pesticides. They can also pollute natural water sources when they dissolve in rainwater and get washed into rivers and the sea. Insoluble substances in water can also cause pollution. Examples include human body waste, oil and plastics. The particles of the solute move between the solvent particles when they dissolve. The solute particles spread evenly in the solvent. Because of this you cannot see the solute in a solution after it has dissolved. We say that a solution has a uniform appearance - it is the same throughout. This means that it looks the same throughout. The picture shows how the solute particles spread when they dissolve in water. Separating a solution A solution is a mixture of the solute and the solvent. We can separate most mixtures because the particles of the substances in the mixture are not chemically joined together. Can we separate a solution? We can separate the dissolved solute from a solution by evaporation. We call this a reverse process because we can get back the dissolved solute from the solution.