Lithium and Sodium both react strongly and quickly with cold water to produce hydroxides and hydrogen gas along with large amount of energy in the form of heat.These have a single electron in the outermost shell.They can readily exchange this electron to produce cation which causes chemical reactions which are sometimes explosive in nature.

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These metals are very active so that they even react vigorously with water.
They release a large amount of energy when they react with water.
So they are called as ACTIVE METALS. 
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