Question

Plaster of paris hardens by (1) combining with water (2) giving out water (3) giving of \(\mathrm{CO}_{2}\) (4) changing into \(\mathrm{CaCO}_{3}\)

Short answer

Plaster of Paris hardens by combining with water (Option 1).

Step-by-step solution

Understanding Plaster of Paris

Plaster of Paris (POP) is a white powder that is scientifically known as calcium sulfate hemihydrate \((\text{CaSO}_4 \frac{1}{2} \text{H}_2 \text{O})\). It is commonly used for casting and molding.

Reaction with Water

When Plaster of Paris is mixed with water, it undergoes an exothermic reaction. This means it interacts with water and releases heat. POP reverts to its original form, gypsum \((\text{CaSO}_4 \text{2H}_2 \text{O})\), which is a hardened material.

Eliminating Incorrect Options

Option (2) can be eliminated because POP does not give out water but absorbs it. Option (3) is incorrect as \(\text{CO}_2\) is not involved in the process. Option (4) is incorrect because POP does not change into \( \text{CaCO}_3 \) during hardening.

Correct Option

Based on the above steps, the correct way Plaster of Paris hardens is by combining with water.

Key concepts

Calcium Sulfate Hemihydrate

Calcium sulfate hemihydrate is the chemical name for Plaster of Paris. Its chemical formula is \((\text{CaSO}_4 \frac{1}{2} \text{H}_2 \text{O})\).
This compound is known for its ability to form a paste when mixed with water, and subsequently harden into a solid form.
The term 'hemihydrate' indicates that for every molecule of calcium sulfate, there is half a molecule of water.
The white powder of Plaster of Paris is easy to work with, making it popular for creating casts, molds, and sculptures. It serves a variety of industrial needs as well.

Gypsum Formation

When Plaster of Paris is combined with water, it undergoes a chemical reaction.
The compound absorbs water and transforms back into its original form, known as gypsum. Gypsum is chemically termed \((\text{CaSO}_4 2\text{H}_2 \text{O})\).
This absorption of water and change to gypsum causes the mixture to harden.
Understanding this transformation helps explain why plaster becomes solid after mixing with water. The reaction can be represented as:
\( \text{CaSO}_4 \frac{1}{2} \text{H}_2 \text{O} + 1.5\text{H}_2 \text{O} \rightarrow \text{CaSO}_4 \text{2H}_2 \text{O} \)

Exothermic Reaction

The hardening of Plaster of Paris is an exothermic reaction. This means that during the process, heat is released.
As Plaster of Paris absorbs water and changes into gypsum, energy is given off in the form of heat.
This is why you can feel warmth coming from the plaster as it sets.
Exothermic reactions are characterized by:

• Release of heat.
• Temperature increase of the surroundings.
• Common in many everyday processes like rusting iron and burning wood.

Recognizing that the hardening of Plaster of Paris is exothermic helps understand why it releases heat while setting.