2. Changes of State

When you heat or cool matter, it can change from one state to another. Each change has a name, and the same substance stays the same throughout — only its state changes.

The changes of state are:

• Melting — solid → liquid (ice → water).
• Freezing — liquid → solid (water → ice).
• Evaporation / boiling — liquid → gas (water → water vapor).
• Condensation — gas → liquid (water vapor → water).
• Sublimation — solid → gas directly (dry ice → CO₂ gas).
• Deposition — gas → solid directly (water vapor → frost).

Adding energy (heat) pushes matter toward gas; removing energy pushes it toward solid. Because it's still the same substance, changes of state are physical changes — melt the water again and it's still water.

⚠️ Common misconception: melting or boiling does not make a new substance. Ice, liquid water, and steam are all water — just in different states.

💡 Tip: add heat → toward gas (melt, evaporate); remove heat → toward solid (condense, freeze).