If you tried to stir hot cooking oil with a plastic spoon, the plastic would melt. Plastic is solid. This means that it will remain solid at temperatures and pressures normally found on Earth. But change the circumstances, and you can change the normal state of a substance. In the same way, if you put orange juice, normally a liquid, into a freezer, it will go solid. And if you breathe onto a cold windowpane, the water vapour in your breath (normally a gas) will condense into drops of liquid. If the sun shines on them, the heat turns them back into a gas and they evaporate into the air again. Even the hardest rocks melt at the very high temperatures and pressures found underneath the Earth’s crust. Most substances that we know will change state when the temperature or pressure changes enough.
From Solid to Gas
If you heat a solid to a special temperature called the melting point, it will melt into a liquid. If you heat it even more, it gets to a point when the liquid changes into a gas. This is the boiling point. At the boiling pint, all particles in a liquid get enough energy from the heat to break free from each other. Then bubbles of gas form in the liquid. Liquids are always slowly changing to gas, even below boiling point. This is called evaporation.
Cold glasses get little droplets of water on them because water vapour from the air turns back into water on the cold glass. Cold glass removes the energy from the particles and so turns them into liquid.
Why does wet ink dry? Because the water in it turns into water vapour and evaporates into the air. Some of the water particles get enough energy to escape and form the gas.
Sometimes a solid turns straight into a gas. This is called sublimination. It is what happens with dry ice, used to make dramatic looking clouds on stage in a theatre. Dry ice is really frozen carbon dioxide. It is called dry because it does not become liquid before turning into a gas.
Particles in a solid are packed tightly together. But when heated, they vibrate more and more until they can break free of fixed positions, and move freely over each other. The solid turns into a liquid.
A dripping candle will soon freeze into a solid if you blow it out. This is because the particles which are sped up by the heat of the flame, slow down again and move closer together when the heat is removed. When they slow down enough, they get locked into position again, forming a solid.
The temperature at which liquid boils (boiling point) depends on the pressure around it. When the pressure goes down, the boiling point goes down because the molecules can escape as a gas more easily. When the pressure goes up, the boiling point goes up because it is more difficult for the molecules to escape. In pressure cookers, the increased pressure raises the water’s boiling point. At the higher temperature, the food cooks more quickly.
States of Water
Water is unusual because it can be found in all three states of matter in everyday life. In solid form it is ice, in liquid form, water, and in gaseous form, water vapour. The properties of water in these three states are important to everything on Earth. For example, plants and animals need water regularly to survive.
The never ending cycle between the different states of water is essential to everything on earth. Liquid water evaporates and solid snow sublimes into the air. Water vapour condenses into droplets to form clouds. Water droplets fall back to earth as rain or snow.
The never ending water
When the temperature is high, water evaporates quickly. In the warm, tropical forests of South America, there is plenty of rain and because the temperature is high, water evaporates all the time. The water vapour in the air makes it very humid, and means that special types of plants, such as certain orchids, can thrive. They take all the moisture they need straight from the air, not the ground.
When water boils, it turns into steam. Steam is water vapour that is hot. Being a gas, it takes up much more space than the liquid it came from. It is full of energy and can be used to drive heat engines such as the steam turbine. It enters the turbine at high temperature and pressure, and drives the turbine wheels around.
Changes with Pressure
Pressure can bring about a change of state. It is possible to ice skate because the skates move over a thin layer of water. The weight of the skater is concentrated into the blade which makes the ice melt as the skate moves over it.
Have you noticed how pipes often burst in freezing weather? This is because the water inside them expands as it freezes into a solid.
From the Dorling Kindersley Science Encyclopaedia.