Page number 75, 76 and 77.
What are changes of state?
A change of state is a physical change in a matter. They are reversible changes and do not involve any changes in the chemical makeup of the matter. Common changes of the state include melting, freezing, sublimation, deposition, condensation, and vaporization. These changes are shown in the figure given below.
1. Melting (Solid → Liquid)
• Example: Ice turning into water
• What happens:
Heat energy is added to the solid. This energy makes the particles vibrate faster until they break free from their fixed positions. The solid becomes a liquid because the particles can now move around each other, though they’re still close together.
2. Freezing (Liquid → Solid)
• Example: Water turning into ice
• What happens:
Heat is removed from the liquid. The particles lose energy, move more slowly, and eventually settle into fixed positions, forming a solid. This process releases heat (exothermic).
3. Evaporation/Boiling (Liquid → Gas)
• Example: Water boiling into steam
• What happens:
Heat energy is added to the liquid. Particles gain enough energy to completely overcome the forces holding them together. They spread out and move freely, forming a gas.
• Evaporation happens slowly at the surface.
• Boiling happens quickly throughout the liquid at the boiling point.
4. Condensation (Gas → Liquid)
• Example: Steam turning into water droplets
• What happens:
Gas particles lose energy as heat is removed. They slow down and come closer together, forming a liquid. This is also an exothermic process.
5. Sublimation (Solid → Gas)
• Example: Dry ice (solid CO₂) turning directly into gas
• What happens:
Some solids can gain enough energy for their particles to go directly from a solid to a gas without becoming liquid. This requires a lot of energy.
6. Deposition (Gas → Solid)
• Example: Frost forming from water vapor
• What happens:
Gas particles lose energy so quickly that they change straight into a solid, skipping the liquid phase.