Changes of State

Depending on the temperature, a substance can exist as a solid, liquid or gas.

For example, at room temperature, water is a liquid. However, when heated, it turns into water vapour, which is a gas. When water is put in a freezer, the temperature decreases, so it turns into ice, which is a solid.

Physical changes do not alter the nature of the particles themselves; rather, they affect the particles’ arrangement and movement.

Matter can change between states, and there are five different types of state changes, which you can see below.

Melting: This is the process that changes a solid to a liquid. When a solid is heated, the particles begin to vibrate as they gain more energy. This causes the forces between the particles to become weak.

Boiling: Otherwise known as vaporisation, this process changes a liquid to a gas. As mentioned, adding heat to a liquid will cause the particles to vibrate and move around even more. This also causes the bonds between the particles to break, forming a gas.

Condensation: This process changes a gas to a liquid. To go from a gas to a liquid, the gas needs to be cooled. This causes bonds to form between the particles, which leads to liquid formation.

Freezing: This process changes a liquid to a solid. To freeze a liquid, you’ll need to cool it down, which leads to the formation of bonds between particles.

Sublimation: Under certain conditions, a solid can turn into a gas. An example of this is the sublimation of iodine.

When iodine crystals are heated, they turn into purple iodine gas. This is an example of sublimation because iodine changes directly from a solid to a gas.

Melting and Boiling Points

Pure substances, which are substances made out of only one element, have exact melting and boiling points. The table below shows the melting and boiling points for some pure substances.

ElementMelting Point (°C)Boiling Point (°C)
Oxygen-219-183
Sodium98883
Nitrogen-210-196
Fluorine-220-188
Copper10852560

When you look at tables of melting and boiling points online, you’ll notice that freezing and condensation points are not listed. This is because the melting point for a substance is the same as the freezing point and the boiling point for a substance is the same as the condensation point.

For example, look at oxygen in the table above. If you cool oxygen to -183°C (the boiling point), oxygen becomes a liquid and if you cool oxygen further to -219°C then oxygen becomes solid.

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