Four more changes
The remaining three parts give a result quite unlike the camphor experiment.
Activity 16.1 (ii) — burning magnesium
Hold a cleaned piece of magnesium ribbon in a pair of tongs and burn it.
The observation
The magnesium ribbon burned with a brilliant white flame and left a white powder behind.
Activity 16.1 (iii) — NaOH into acid
Measure the starting temperature of a dilute sulphuric (H2SO4) acid solution. Add a few pellets of solid sodium hydroxide (NaOH) to it and stir. Measure the temperature again.
The observation
The sodium hydroxide dissolved. The beaker became hot. The thermometer reading had risen.
Activity 16.1 (iv) — magnesium into acid
Drop a piece of magnesium ribbon into the beaker of dilute sulphuric acid.
The observation
The magnesium ribbon dissolved and gas bubbles were given off. The beaker became hot.
What all three share
In all three, the substances present have changed and new substances have formed. The magnesium ribbon became a white powder; the acid and the alkali reacted with one another; the magnesium and the acid between them produced a new gas.
Chemical change
Changes in which the substances present change and new substances form are called chemical changes, or chemical reactions.
Unlike the camphor experiment, not one of these can be undone by simply cooling. Burnt magnesium is not a ribbon of magnesium again.