Page 38 - NCERT Science Class 10 English Medium

Paris, the substance which doctors use as plaster for supporting
fractured bones in the right position. Plaster of Paris is a white powder
and on mixing with water, it changes to gypsum once again giving a
hard solid mass.
1 1
CaSO . H O +1 H O→ CaSO .2H O
2
4
2
4
2
2 2
(Plaster of Paris) ( Gypsum)
Note that only half a water molecule is shown to be attached as water
of crystallisation. How can you get half a water molecule? It is written in
this form because two formula units of CaSO share one molecule of
4
water. Plaster of Paris is used for making toys, materials for decoration
and for making surfaces smooth. Try to find out why is calcium sulphate
hemihydrate called ‘Plaster of Paris’ ?

Q U E S T I O N S

1. What is the common name of the compound CaOCl ?
2
2. Name the substance which on treatment with chlorine yields bleaching
powder. ?
3. Name the sodium compound which is used for softening hard water.

4. What will happen if a solution of sodium hydrocarbonate is heated?
Give the equation of the reaction involved.
5. Write an equation to show the reaction between Plaster of Paris and
water.

What you have learnt

n Acid-base indicators are dyes or mixtures of dyes which are used to indicate the
presence of acids and bases.
+
n Acidic nature of a substance is due to the formation of H (aq) ions in solution.
Formation of OH (aq) ions in solution is responsible for the basic nature of a
–
substance.
n When an acid reacts with a metal, hydrogen gas is evolved and a corresponding
salt is formed.
n When a base reacts with a metal, along with the evolution of hydrogen gas a salt is
formed which has a negative ion composed of the metal and oxygen.

n When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives
the corresponding salt, carbon dioxide gas and water.
n Acidic and basic solutions in water conduct electricity because they produce
hydrogen and hydroxide ions respectively.

Acids, Bases and Salts 33

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