Page 47 - NCERT Science Class 10 English Medium

We have observed in Activity 3.9 that all metals do not react with
oxygen at the same rate. Different metals show different reactivities
towards oxygen. Metals such as potassium and sodium react so
vigorously that they catch fire if kept in the open. Hence, to protect them
and to prevent accidental fires, they are kept immersed in kerosene oil.
At ordinary temperature, the surfaces of metals such as magnesium,
aluminium, zinc, lead, etc., are covered with a thin layer of oxide. The
protective oxide layer prevents the metal from further oxidation. Iron
does not burn on heating but iron filings burn vigorously when sprinkled
in the flame of the burner. Copper does not burn, but the hot metal is
coated with a black coloured layer of copper(II) oxide. Silver and gold do
not react with oxygen even at high temperatures.

Do You Know? Anodising is a process of forming a thick oxide layer of aluminium. Aluminium

develops a thin oxide layer when exposed to air. This aluminium oxide coat makes it
resistant to further corrosion. The resistance can be improved further by making the
oxide layer thicker. During anodising, a clean aluminium article is made the anode
and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode
reacts with aluminium to make a thicker protective oxide layer. This oxide layer can
be dyed easily to give aluminium articles an attractive finish.

After performing Activity 3.9, you must have observed that sodium
is the most reactive of the samples of metals taken here. The reaction of
magnesium is less vigorous implying that it is not as reactive as sodium.
But burning in oxygen does not help us to decide about the reactivity of
zinc, iron, copper or lead. Let us see some more reactions to arrive at a
conclusion about the order of reactivity of these metals.

3.2.2 What happens when Metals react with Water?

Activity 3.103.10
3.10
Activity 3.10
Activity
Activity
Activity 3.10
CAUTION: This Activity needs the teacher’s assistance.
n Collect the samples of the same metals as in Activity 3.9.
n Put small pieces of the samples separately in beakers half-filled
with cold water.
n Which metals reacted with cold water? Arrange them in the
increasing order of their reactivity with cold water.
n Did any metal produce fire on water?
n Does any metal start floating after some time?
n Put the metals that did not react with cold water in beakers
half-filled with hot water.
n For the metals that did not react with hot water, arrange the
apparatus as shown in Fig. 3.3 and observe their reaction with steam.
n Which metals did not react even with steam?
n Arrange the metals in the decreasing order of reactivity with water.

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