Q1.
When electric current is passed through water, it breaks down into hydrogen and oxygen. Is this a chemical change or a physical change? Recall Curiosity, Grade 7, chapter ‘Changes Around Us: Physical and Chemical’.
Answer
It is a chemical change.
| Test of a chemical change | Does it apply here? |
|---|---|
| New substances are formed | Yes — hydrogen and oxygen, neither of which was there before |
| The new substances have different properties | Yes — one pops when lit, the other makes a flame glow brighter; water does neither |
| The change is not easily reversed | Yes — you cannot get water back simply by cooling the gases |
Why it happens: Compare it with melting ice or boiling water. There, only the state changes and the particles of water remain the same, so the change is physical and reversible by heating or cooling. Here the particles themselves are pulled apart into particles of two different elements — the identity of the substance has changed, and that is what makes a change chemical.