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Key points

  • Substances can exist in three states of matter - solid, liquid and gas.

  • Substances can change states. To change the state of a substance energy must be transferred to, or from, the substance.

  • and can be used to determine the temperature at which a state change takes place.

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Video - Changes of state

Can you answer these questions based on the video?

  1. What are the three states of matter?

  2. All substances will change state at specific temperatures. What are these known as?

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Changes of state

Many substances can exist as solids, liquids or gases, which are all different

By heating or cooling a substance, its state can be changed.

There are four main :

  • melting - the process of a solid turning into a

  • freezing - the process of a liquid turning into a

  • evaporating - the process of a liquid turning into a

  • condensing - the process of a gas turning into a liquid

An infographic showing a cycle in which water changes into different states of matter by melting, evaporating, condensing and freezing. Solid, liquid and gas are labelled.
Figure caption,
There are four main changes of state: melting, freezing, evaporating and condensing
Three question marks

Did you know?

Liquids do not have to be heated to their boiling point to evaporate - evaporation can take place at lower temperatures too.

However, evaporation happens very rapidly once the liquid has reached its boiling point.

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Energy and changes of state - melting and freezing

Heating a solid substance, like ice or copper, will initially increase its .

The energy (heat) transferred to the substance causes the to vibrate more rapidly although the particles remain in their fixed positions, in neat, ordered rows.

If the solid is heated more, after reaching its , the energy gained by the particles allows them to partly overcome the strong forces holding them in place.

The particles begin to move from their fixed positions but are still closely spaced together. The substance has melted to become a liquid.

is a reversible, physical change. By cooling the liquid sufficiently, it can turn back into a solid - a process known as .

  • The liquid must first be cooled to its freezing point (the same temperature as its melting point).

  • Continuing to cool the liquid decreases the energy of the particles and they return to fixed positions - in neat, ordered rows.

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Energy and changes of state - evaporating and condensing

Heating a liquid, like water or oil, also increases its at first.

The heat transferred to the substance increases the energy of the . This causes the particles to vibrate more rapidly and move more freely, but they remain closely spaced and randomly arranged.

If the liquid is heated further, after reaching its , some of the particles gain enough energy to completely overcome the forces holding them together.

These particles escape from the liquid altogether and move freely, because they are no longer bound to the other particles in the liquid. These particles have evaporated.

When all the particles have escaped, the liquid has completely evaporated to become a gas.

is a reversible, physical change. By cooling the gas sufficiently, it can turn back into a liquid, a process known as condensing.

  • The gas must first be cooled to its boiling point, after which continuing to cool the gas decreases the energy of the particles.

  • This causes the substance to return to the liquid state, with the particles closely spaced and in a random arrangement.

Dry ice

Did you know?

Solid carbon dioxide is often called dry ice. This is because heating it causes it to change directly into a gas, without becoming a liquid first 鈥 a process called sublimation.

If the gaseous carbon dioxide is cooled, it changes directly from a gas to a solid, without becoming a liquid first 鈥 a process called deposition.

Dry ice
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Heating curves

A can be produced by heating a substance at a constant rate and measuring its .

The diagram shows a heating curve for water.

An infographic of a graph showing the amount of heat absorbed increasing in stages as water changes from solid to liquid to gas.
Figure caption,
A heating curve for water. The water is initially a solid at -40 掳C, melts to become a liquid, and then evaporates to become a gas.

Look through the slide show below to find out more about each stage of the heating curve.

Image gallerySkip image gallerySlide 1 of 5, An infographic of a graph showing the amount of heat absorbed increasing in stages as water changes from solid to liquid to gas - focus on points A and B, Points A to B The sloped section at the start of the line, between A and B, shows that the water is initially a solid (ice) at -40 掳C. As the solid ice is heated, the temperature increases until it reaches its melting point of 0 掳C.

Heating curve for sodium chloride

Not all substances melt (or freeze) at 0 掳C and boil (or condense) at 100 掳C, like water does. Different substances have different melting points and boiling points.

For example, sodium chloride (table salt) has a melting point of 801 掳C and a boiling point of 1465 掳C, as this heating curve shows:

An infographic of a graph showing the heating curve of sodium chloride, with plateaus at temperatures of 801 and 1465 degrees Celsius.

In which section of the graph would the sodium chloride be a liquid?

Three question marks

Did you know?

The boiling point of water is 100 掳C at normal atmospheric pressure, at sea level. At higher altitudes, the air pressure is lower, and water boils at a lower temperature. At the top of Mount Everest, water boils at around 70 掳C.

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Cooling curves

A can be used to determine the at which changes of state occur.

A cooling curve is produced by measuring the temperature of a substance as it cools and then plotting a graph of temperature against the amount of energy transferred.

This diagram shows a cooling curve for water.

An infographic of a graph showing the cooling curve, showing the points at which changes of states occur.
A sign saying remember

Look for the flat sections of the heating or cooling curve to find the temperatures at which the substance changes state.

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Test your knowledge

Quiz

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Atomic Labs game. game

Try out practical experiments in this KS3 science game

Atomic Labs game
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