Question

Give reasons to show that following are chemical changes: Adding calcium to water.
[A] A new substance is formed with different properties
[B] New substance is formed and has the same properties
[C] The reaction is reversible
[D] The reaction is irreversible

Answer 1

Hint: A chemical change changes the chemical properties of a substance. If we can restore the original form of the substance after the change then the change is considered reversible otherwise it is irreversible.

Complete answer:
To answer this, let us discuss what chemical changes are and their properties.
We can define a chemical change as the change which causes a substance to change its composition and result in the formation of a totally new substance. For example we can take rusting of iron or curdling milk.
In simple words we can say that chemical change is the change in the chemical structure of the substance and forming a totally new substance with a different chemical formula.
Changes can either be reversible or irreversible. After a change, if we get a substance which cannot be reverted back to the original substance then the change is irreversible. For example if we stretch a rubber band and then leave it, it reverts back to its original form thus it is a reversible change whereas curdling of milk on adding a drop of lemon is an irreversible process.
Generally, a chemical is irreversible. However, in some chemical changes we can get back the original substance but this also includes further chemical changes.
In the question, we are given adding water to calcium. Let’s write the reaction first-
     \[Ca+2{{H}_{2}}O\to Ca{{\left( OH \right)}_{2}}+{{H}_{2}}\]
We can see that calcium and water gives us calcium hydroxide and hydrogen gas is liberated.
Here, a new substance is formed and its property is different from the reactant i.e. calcium. Also, the reaction is irreversible. So, we can say that this is a chemical change.

Therefore, the correct answer is option- [A] A new substance is formed with different properties and [D] The reaction is irreversible.

Note:
Generally, chemical changes are irreversible but a reversible chemical change is a change where the reactants and the products both are at equilibrium. Once equilibrium is reached, the reaction can move either way. The reactants can change into products and the product can also change back into the reactants.