Siyapath
Sign up

Lesson: Chapter 16 — Changes of Matter

16. Physical and chemical change 4 of 5

Telling the two apart

Now we can set the two kinds side by side. That is what table 16.1 does.

Observations relating to a physical change Observations relating to a chemical change
Only the arrangement of the particles making up the substance changes. No new substance appears. New substances with different physical and chemical properties arise from the substances present.

Examples

  1. Breaking stone (lumps → powder)
  2. Melting wax (solid → liquid)
  3. Water evaporating (liquid → gas)
  4. Water vapour becoming droplets (gas → liquid)

Examples

  1. Burning firewood (ash forms, gases given off)
  2. Heating limestone (calcium oxide forms, gas given off)
  3. Heating condy's crystals (oxygen given off)
  4. Iron rusting (rust forms)

The deciding question

Read the first row of the table and you see that one question alone separates the two kinds: has a new substance appeared?

A common mistake

"Can it be reversed?" is a useful hint but it is not the definition. Some chemical reactions can be reversed — they are called reversible reactions, and we meet one in section 16.3. The deciding question is always whether a new substance formed.

Dissolving salt in water, for instance, is a physical change — the salt is still salt, and evaporating the water brings it back.