Energy and changes in state Melting - solid to liquid Freezing - liquid to solid Boiling - liquid to gas Condensation - gas to liquid Evaporation Sublimation

Whenever a substance changes its state, like when ice melts to form liquid water, it is said to undergo a "change in state". Changes in state can occur when a substance is heated, cooled or even if an external pressure change occurs. An example of this is when propane gas is pumped into a sealed tank and it turns into a liquid, LPG. Liquid carbon dioxide fire extingushers are also a good example of a change in state brought on by pressure.

Primary Connections has many free downloadable units you can use to teach different year levels about changes in state. In Melting Moments, Year 3 students learn about how adding and removing heat can cause a change of state between solid and liquid. In What's the Matter?, Year 5 students explore the different properties of solids, liquids, and gases and their behaviours under varied conditions. Change Detectives provides Year 6 students with hands-on opportunities to identify and explain physical and chemical changes to everyday materials. 

This page will consider changes of state between solid, liquid and gas brought about by changes in temperature only, as these are the most commonly experienced changes. Remember changes in state are physical changes, not chemical. The substance itself remains chemically the same, i.e. water is always H₂O whether it is ice, water or steam. See The water cycle for more on the changes of the state of water.

Energy and changes in state
Changes in state occur because energy is either added or removed from a substance, affecting the way the particles interact with each other. If a substance is heated, energy is added and the particles will become more active; vibrating, rotating and even moving about faster. If the substance has enough energy, it can overcome the bonding forces holding the particles together and, in doing so, undergo a change in state. Cooling, on the other hand, removes energy thus making the particles less active and allowing the bonding forces to take hold within the substance.

Melting - solid to liquid
Melting occurs when a solid is heated and turns to liquid. The particles in a solid gain enough energy to overcome the bonding forces holding them firmly in place. Typically, during melting, the particles start to move about, staying close to their neighbouring particles, then move more freely. For pure substances, the temperature at which this change occurs is quite precise and is called the melting point of the substance.

Freezing - liquid to solid
Freezing occurs when a liquid is cooled and turns to a solid. Eventually the particles in a liquid stop moving about and settle into a stable arrangement, forming a solid. This is called freezing and occurs at the same temperature as melting. Hence, the melting point and freezing point of a substance are the same temperature. The melting and freezing point of a substance are defined as the temperature above which, the substance is liquid and below which, it is solid.

Boiling - liquid to gas
Boiling occurs when the particles in a liquid gain enough energy to overcome the bonding forces holding them loosely in place in the liquid and they become free, fast moving, individual particles in a gas. For pure substances, the temperature at which this change occurs is quite precise and is called the boiling point of the substance.

Condensation - gas to liquid
If a gas is cooled, its particles will eventually stop moving about so fast and form a liquid. This is called condensation and occurs at the same temperature as boiling. Hence, the boiling point and condensation point of a substance are the same temperature. The boiling and condensation point of a substance is defined as the temperature above which, the substance is gas and below which, it is liquid.

Evaporation
Evaporation occurs when particles in a liquid pass directly into the gas state, at a temperature below the boiling point of the liquid. When a puddle "disappears" after rain, the water has evaporated. Evaporation is dependent on individual particles gaining enough energy to escape the surface of the liquid and become gas particles.

Sublimation
The change in state which occurs when a solid goes directly to a gas is called "sublimation'' Sublimation is like evaporation; it does not occur at any particular temperature, but varies in rate with temperature. Mothballs sublime and so does dry ice. Watch carefully and you will see the solid slowly disappearing and forming a gas or odour without first melting. For a solid to sublime, individual particles on the surface of the solid obtain enough energy from their surroundings to jump off the surface of the solid and become individual gas particles.

Sublimation

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