An element is soft and can be cut easily with a knife. It is very reactive and cannot be kept open in the air. It reacts vigorously with water. This element is most likely to be:

1. magnesium
2. potassium
3. phosphorous
4. aluminium

Correct Answer: B. potassium

Explanation: Potassium soft and can be cut easily with a knife. It reacts with water to potassium hydroxide and hydrogen gas. The reaction potassium with water is highly exothermic in nature. It produces a lot of heat energy and this heat energy can cause the hydrogen gas produced during the reaction, to catch fire. In order to keep it safe potassium is stored in kerosene.

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Updated on: 10-Oct-2022

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