Question
How can you identify whether a change is chemical or physical? State five signs that indicate a chemical change has occurred.
Solution — Step by Step
A physical change alters the form or appearance of a substance but not its chemical composition. No new substance is formed. Examples: melting ice, cutting paper, dissolving salt in water.
A chemical change produces one or more new substances with different properties. The change is generally irreversible. Examples: burning wood, rusting of iron, cooking food.
The question is: how do you tell which type has occurred?
Sign 1: Change in colour A new colour appears that wasn’t present before, indicating formation of a new substance. Example: Iron nail turns reddish-brown (rust = iron oxide). Apple flesh turning brown when cut (oxidation). Burning magnesium: silvery magnesium → white magnesium oxide.
Sign 2: Evolution of gas Bubbles form and gas is given off. Example: Adding vinegar to baking soda produces bubbles. Zinc reacting with HCl produces hydrogen gas. Decomposition of releases .
Sign 3: Formation of precipitate A solid forms in a clear solution — a new insoluble substance has been created. Example: Mixing with produces a white precipitate of . Mixing lead nitrate with potassium iodide produces yellow precipitate.
Sign 4: Change in temperature Heat is absorbed or released because chemical bond energies are changing. Example: Burning (exothermic — heat released). Photosynthesis (endothermic — heat absorbed). Dissolving in water (endothermic — feels cold).
Sign 5: Change in smell / emission of light New gases with distinctive odors are produced, or light is emitted. Example: Burning sulfur produces pungent . Combustion reactions emit light and heat. Food spoiling produces foul odors (chemical decomposition).
An important additional indicator: chemical changes are generally irreversible by simple physical means. You cannot un-burn wood or un-rust iron by just reversing the conditions. (Some chemical changes are reversible, like the reaction of and in the Haber process, but this requires a catalyst and specific conditions — not “simple” reversal.)
Physical changes are generally reversible: melted ice refreezes, dissolved salt can be recovered by evaporation.
Note: presence of a sign is strongly suggestive but not proof by itself.
- Dissolving salt in water changes nothing visible, yet it’s physical. But dissolving copper sulfate turns water blue — that’s also physical (the isn’t chemically changed).
- Temperature change during dissolving (like in water) is a physical change despite heat generation.
The true test: is a new substance formed with different properties?
Why This Works
Chemical changes involve breaking and forming chemical bonds, which always involves energy. This energy exchange is what causes the observable signs: heat (bond energy), light (electronic transitions), gas (new volatile compounds), precipitate (new insoluble compounds), colour change (new chromophore groups).
Physical changes only rearrange the physical state or form without affecting bonds between atoms in molecules.
Alternative Method — The “Can I Get It Back?” Test
Ask: “Can I get the original substance back by simple physical means?”
- Can I get ice back from water? Yes (freeze it) → Physical change.
- Can I get wood back from ash? No → Chemical change.
- Can I get dissolved salt back from saltwater? Yes (evaporate) → Physical change.
- Can I get iron back from rust? Not easily → Chemical change.
This isn’t foolproof, but it’s a quick intuitive check for Class 6–8 level problems.
Common Mistake
Saying dissolving is always a physical change. This is generally true (e.g., salt in water), but some dissolutions involve chemical reactions. Dissolving an acid in water involves partial dissociation (chemical change). Dissolving in HCl involves a chemical reaction ( is released). Read the specific context rather than applying a blanket rule.