Changing states

We can describe the state of an object as being a solid, a liquid or a gas. Some solids, liquids and gases can change and become a different state when they are heated or cooled.

Processes such as evaporation, boiling or freezing can change the state of substances.

For example:

• When an ice cube is heated it melts and changes from a solid (ice) to a liquid (water).

• When water is cooled to a very low temperature it freezes and turns into ice. It changes from a liquid back in to a solid.

• When the water is heated, it will eventually boil and evaporate, becoming a gas. Melting and boiling are changes of state, and they can occur in reverse too if a substance is cooled down.

• When you cool gases, through condensation it turns back into a liquid.

Fascinating facts about changing states!

• Water expands by 9% when it freezes. This is why pipes carrying water can burst in very cold weather.

• In order to turn helium into a solid from a liquid, you must cool it to below -272°C.

• 97% of the water on Earth is salt water and 2% is stored in ice and glaciers.

• Lava is molten or liquid rock that erupts from under the Earth in volcanoes. It can sometimes travel as fast as 30 mph on the slopes of steep mountains.

• Glass is made from heating up sand particles to over 1000°C to make it into a liquid using a furnace.

• Our water cycle can change water from a gas (water vapour) to liquid (rain) and into a solid (snow).

• Most gases are invisible to the human eye, but for safety we add a smell to the gas that heats our homes.

• The polar ice caps have melted by over 9% in mass in the last decade.

When some solids are heated, they melt and turn into a liquid.

For example, wax and ice melt when they are heated. The temperature this happens at is called the melting point, and it is measured in degrees Celsius (°C).

The melting point of ice (solid water) is 0 degrees Celsius (0°C). Different materials have different melting points. Chocolate melts at around 45°C.

Did you know?

The metal tungsten has the highest melting point of any elements.

Tungsten needs to be heated to over 3400 °C to reach its melting point.

When a liquid is heated it boils and turns into a gas. For example, if water is heated in a pan or in a kettle, it boils and turns into steam.

The temperature this happens at is called the boiling point. The boiling point of water is 100°C.

If you heat water slowly at a lower temperature than boiling point, then the water on the surface will evaporate. For example the Sun may heat up water in a puddle and it will slowly evaporate. The puddle may evaporate quicker on a very hot day, if it was windy or the water was spread out thinly across the surface.

When a gas is cooled, it condenses, and changes into a liquid. For example, the steam from a boiling pan turns into condensation on a cold window in the kitchen.

We also see this in the water cycle, when water vapour cools as it rises in the air and condenses to form clouds.

Did you know?

Matter changes state when its particles gain or lose energy.

When the particles in a liquid gain energy, it becomes a gas.

When the particles in a liquid lose energy, it becomes a solid.

Boiling – When a liquid turns into a gas. Water boils to become steam.

Boiling point – The temperature at which a liquid boils and turns into gas.

Condensation – When gas turns into a liquid. When you take a cold bottle out of the fridge, water vapour will cool on the bottle and form drops of water.

Evaporation – When a liquid turns into a gas. Water evaporates from puddles after rain.

Freezing – When a liquid turns into a solid. Water freezes to become ice.

Melting – When a solid turns into a liquid. Ice melts to become water.