Q1.
Let us recall the burning of a magnesium ribbon (Fig. 5.5) learnt in the chapter ‘The World of Metals and Non-metals’. Can you predict if this is a physical or a chemical change?
Answer
It is a chemical change.
Magnesium + Oxygen → Magnesium oxide + Heat + Light
(Ribbon) (Air) (White powder)
(Ribbon) (Air) (White powder)
Three observations prove it:
- The shiny, silvery, flexible ribbon is gone and a white powder is left behind — a completely different substance, magnesium oxide.
- A dazzling white light and a lot of heat are given out.
- The change cannot be reversed — the white ash can never be turned back into a magnesium ribbon in the laboratory.
Why it happens: When magnesium is heated enough, it combines with the oxygen of the air. The new substance, magnesium oxide, has none of the properties of magnesium — it does not shine, it cannot be bent, and it does not burn. A reaction in which a substance combines with oxygen and gives out heat and/or light is called combustion.
Caution: Never look straight at the burning ribbon. The light is bright enough to hurt your eyes. Also, rusting of iron (Fig. 5.4) is a chemical change for the very same reason — a new brown substance, iron oxide, is formed.