Change of State

What Are Changes of State?

Changes of state are physical changes in matter. They are reversible changes that do not involve changes in matter’s chemical makeup or chemical properties. Common changes of state include melting, freezing, sublimation, deposition, condensation, and vaporisation.

Changes of state are a result of changes in temperature - heating and cooling.

Heating

Heat can change solids into liquids or gases.

• Most solids melt into liquid when they are heated.
• A liquid evaporates into a gas when it is heated.

Cooling

When we cool something we take heat away from it. Cooling changes a gas into a liquid, and a liquid into a solid.

• A gas condenses into a liquid when it is cooled.
• A liquid freezes into a solid when it is cooled.

Sublimation

A much less common change is when a solid absorbs heat and changes directly to a gas without melting and going through the liquid phase. This is called sublimation . An example of sublimation is when dry ice (frozen carbon dioxide) sublimes to form carbon dioxide gas. Dry ice is used on stage and in movies to produce the effect of fog. The word ‘sublimation’ may also be used to describe the action of a gas changing directly into a solid.

Changes of State - Particle Model

The particle model can be used to explain changes in state. The amount of energy that particles absorb will determine whether the bonds that keep them close together or not will be broken.

• Solid to Liquid

To change a solid (e.g. ice) to a liquid (e.g. water), heat energy must be added to make the particles vibrate more. This causes solids to expand. Adding more heat energy eventually ‘loosens’ the bonds between the particles and a liquid is formed. This change is called melting.

• Liquid to Solid

The reverse of melting is solidification or freezing. When a liquid loses energy, the vibration of particles lessens and the bonds between particles are again strong enough to keep them in fixed positions.

• Liquid to Gas

To change a liquid into a gas, heat must be added to completely break the bonds between particles. When heat is added to a liquid, small bubbles of gas will form within the liquid. When enough heat is added, these gas bubbles become large enough to float to the surface and boiling occurs.

A liquid does not have to boil in order for evaporation to occur—boiling just speeds up the process.

• Gas to liquid

The opposite of evaporation is condensation. Condensation occurs when gas particles lose energy and turn into liquid.