A small piece of calcium metal is put into a small trough containing water. There is effervescence and white turbidity is formed.

(a) Name the gas formed in the reaction. How would you test the gas ?

(b) Write an equation for the reaction.

(c) What do you observe when a few drops of red litmus solution are added to the turbid solution ?

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Solution

a) The gas liberated by reaction of calcium metal with water is hydrogen and that can be tested by bringing a matich stick near the gas which gives a pop sound.

b)
$C a + 2 H_{2} O ⟶ C a (O H)_{2} + H_{2}$

c). The turbidity is due to the formation of $C a (O H)_{2}$
As a base is formed, red litmus solution turns to blue colour in basic solution.

d) When HCl solution is added to turbid solution, neutralisation reaction occur, whichclears the turbid solution forming precipitate of Calcium chloride and water.
$C a (O H)_{2} + 2 H C l ⟶ C a C l_{2} + 2 H_{2} O$

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Preparation of Hydrogen from Cold Water and Metals

Standard IX Chemistry

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