1. What Is Change of State?
Change of state refers to the transformation of matter from one physical form to another—solid, liquid, or gas—when heat is added or removed. These changes happen because heating or cooling affects how strongly particles are held together.
During a change of state, temperature remains constant even though heat is absorbed or released. This is because the supplied heat is used to change the arrangement of particles, not to raise temperature.
2. Why Substances Change State
Matter changes its state when the energy of its particles changes. Heating increases particle motion and weakens intermolecular bonds, while cooling slows down particles and strengthens these bonds. The balance between particle motion and intermolecular force decides whether a substance is solid, liquid, or gas.
2.1. Effect of Heating
When heat is supplied, particles gain energy and move faster. This extra energy helps them overcome the forces holding them together. As a result:
- solids may melt into liquids
- liquids may boil or evaporate into gases
2.2. Effect of Cooling
Cooling removes energy from particles, making them slow down. With lower energy, particles come closer and intermolecular forces become stronger. This can make:
- liquids freeze into solids
- gases condense into liquids
3. Common Changes of State
Some state changes are familiar in daily life—ice melting, water boiling, steam condensing on a cold surface, or frost forming on cold mornings. These are caused by heat being added or removed.
3.1. Melting (Solid to Liquid)
Melting occurs when a solid absorbs heat and its particles gain enough energy to break out of their fixed positions. The temperature at which this happens is the melting point.
Example: Ice melts into water at 0°C.
3.2. Freezing (Liquid to Solid)
Freezing is the reverse of melting. When a liquid loses heat, its particles slow down until they arrange themselves in fixed positions to form a solid. The temperature at which this happens is the freezing point.
Example: Water freezes into ice at 0°C.
3.3. Boiling (Liquid to Gas)
Boiling occurs when a liquid absorbs enough heat for its particles to escape into the gas state. Boiling happens at a fixed temperature called the boiling point.
Example: Water boils at 100°C under normal pressure.
3.4. Evaporation (Liquid to Gas at Any Temperature)
Evaporation is a slow process that happens at the surface of a liquid. Unlike boiling, it does not require a fixed temperature. Particles with higher energy escape from the surface into the air.
Example: Wet clothes dry on a sunny day due to evaporation.
3.5. Condensation (Gas to Liquid)
Condensation happens when a gas loses heat and its particles slow down enough to come closer and form a liquid.
Example: Water droplets forming on a cold glass.
3.6. Sublimation (Solid to Gas)
Sublimation is when a solid changes directly into a gas without becoming a liquid in between.
Examples: dry ice, camphor, naphthalene.
3.7. Deposition (Gas to Solid)
Deposition is the reverse of sublimation, where a gas becomes a solid directly without becoming a liquid first.
Example: Frost forming on cold surfaces.
4. Why Temperature Stays Constant During Change of State
During a state change, temperature stops rising even though heat is being supplied. This seems strange at first, but the reason is simple: the heat energy does not increase particle speed; instead, it is used to weaken or break intermolecular bonds.
4.1. Latent Heat
The heat absorbed or released during a state change is known as latent heat. The word “latent” means hidden, because this heat does not raise temperature.
- Latent heat of fusion: heat absorbed during melting
- Latent heat of vaporisation: heat absorbed during boiling
5. Energy Involved in State Change
The energy required for state change depends on the mass of the substance and its latent heat.
\( Q = mL \)
Here, L is latent heat (fusion or vaporisation).
5.1. Meaning of the Formula
- Q → Heat absorbed or released
- m → Mass of the substance
- L → Latent heat
This formula is used in calorimetry and other heat calculations.
6. Factors Affecting Change of State
Several factors influence when and how substances change state.
6.1. Pressure
Increasing pressure can raise melting points or lower boiling points depending on the substance. For example, boiling occurs at a lower temperature on mountains because pressure there is lower.
6.2. Impurities
Impurities generally lower the melting point and raise the boiling point. This is why salted ice melts faster and why adding salt to water increases its boiling point.
7. Everyday Examples of State Change
- Ice turning into water in a glass.
- Steam condensing on bathroom mirrors.
- Sugar melting when heated.
- Frost forming on winter mornings.
- Frozen food thawing when left outside.