Page 26 - Curriculum Visions Dynamic Book

Limewater
In the demonstration on pages 6 and 7, you will have noticed that, as the calcium pellet becomes smaller, the water becomes more cloudy. This is because the reaction with water produces a new solution, known as limewater – calcium hydroxide.
When limewater is saturated, calcium hydroxide is precipitated. A precipitate can be made to appear and disappear at will,
as the following demonstration shows.
 As the calcium pellet reacts, it forms calcium hydroxide from the water. Calcium hydroxide is not very soluble in water. If the calcium pellet
is large, the solution will contain as much calcium hydroxide as possible. This solution is known as
a saturated solution. As more calcium reacts, it will not be possible for more calcium hydroxide
to be kept in solution and so the surplus is made into tiny solid particles. This change from solution to solid is called precipitation. The suspension of these tiny particles in the liquid is what causes
it to become cloudy. The particles eventually settle out on the bottom of the beaker, leaving the solution clear. They can be filtered from the solution leaving a clear liquid.
 Blowing a stream of bubbles into limewater using a straw produces a startling effect. When one breathes through the straw, the liquid appears milky.
Breath contains carbon dioxide gas. This reacts with the calcium hydroxide solution, producing calcium carbonate, which is much less soluble
in water. The calcium carbonate is seen as tiny white particles and so the liquid becomes cloudy.
EQUATION: Limewater to calcium carbonate
Calcium hydroxide + carbon dioxide ➪ calcium carbonate + water Ca(OH)2(aq) + CO2(g) ➪ Ca(CO)3(s) + H2O(l)
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 Solution created
by reacting a calcium pellet with water (see page 7) is passed down a glass rod through a filter paper in a funnel. A clear solution of calcium hydroxide passes out of the funnel and is collected in the beaker.