Questions & Answers

The metal which reacts with cold water is:
(A) Mercury
(B) Sodium
(C) Zinc
(D) Tungsten

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Hint: The metal which reacts with cold water is highly reactive. It is silvery- white and soft metal. It is also an alkali metal.

Complete step by step solution:
Let’s understand what reactivity means, reactivity refers to the chemical reaction of one or more than one substances that interact with each other and form products.
The reactivity series in chemistry in which metals are arranged by their reactivity from highest to lowest is used to provide information about the reactions of metals with water or acids or some displacement reactions. The reactivity series is mentioned below-
Potassium – k (most reactive)
Sodium – Na
Calcium - Ca
Magnesium - Mg
Aluminium - Al
Zinc - Zn
Iron - Fe
Tin - Sn
Lead – Pb
Copper – Cu
Silver – Ag
Gold – Au
Platinum – Pt (least reactive)
Now we can say that Sodium is highly reactive with water or cold water. In this reaction metal hydroxide is produced and hydrogen gas is released. The reaction involved is mentioned below
$$Na+{{H}_{2}}O\to NaOH+{{H}_{2}}$$
Balanced chemical reaction- $$2Na+2{{H}_{2}}O\to 2NaOH+{{H}_{2}}$$
While Mercury, zinc and Tungsten are found in the bottom of the reactivity series hence they are least reactive with cold water. They are not highly reactive as compared to Na i.e. sodium.

Correct option is option (B) sodium

Note: All the alkali metals i.e. lithium, sodium, potassium, rubidium, cesium and francium react vigorously with water. Each of them on reaction with water produces metal Hydroxide and hydrogen gas is removed. Sodium is stored under kerosene due to its reactivity.
The most interesting fact is as we move down the group in alkali metals the reactivity increases.

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