Question

During setting of plaster of paris (1) mass is expanded (2) heat is liberated (3) hydration takes place (4) all of these

Short answer

All of these processes take place.

Step-by-step solution

Identify the Process

During the setting of plaster of Paris, we need to identify what physical or chemical changes occur. Let's examine each option to determine if it applies.

Analyze Option 1: Mass Expansion

Consider if the mass of the plaster of Paris increases during setting. When plaster of Paris (calcium sulfate hemihydrate) sets, it absorbs water and changes to calcium sulfate dihydrate. This reaction does increase the volume slightly.

Analyze Option 2: Heat Liberation

Consider if the process releases heat. The setting of plaster of Paris is an exothermic reaction, meaning it releases heat.

Analyze Option 3: Hydration

Consider if hydration takes place. Hydration is the process where water molecules are added. During the setting, water molecules are integrated into the structure forming calcium sulfate dihydrate.

Combine All Factors

Since setting involves mass expansion, heat liberation, and hydration, all of these processes occur.

Conclusion

Thus, the correct choice is that all of these processes take place during the setting of plaster of Paris.

Key concepts

Mass Expansion

When plaster of Paris sets, it undergoes a slight mass expansion. This happens because plaster of Paris is initially in a powder form called calcium sulfate hemihydrate. During the setting process, it reacts with water and changes into calcium sulfate dihydrate. This chemical transformation incorporates water molecules into the structure. Because water is being absorbed, the overall volume of the material increases. This volume increase is termed mass expansion.

In simpler terms, the plaster of Paris 'grows' a bit as it sets. Though the change is not enormous, this expansion is noticeable. So, if you’re mixing plaster for a project, always account for this slight increase in size.

Exothermic Reaction

The setting of plaster of Paris is an exothermic reaction. This means that as the plaster sets, it gives off heat. An exothermic reaction is one where energy is released into the surroundings, usually in the form of heat.

When plaster of Paris (calcium sulfate hemihydrate) combines with water, it forms calcium sulfate dihydrate. This chemical reaction releases energy, causing the mixture to become warm. If you touch a setting plaster, you’ll notice it heats up. This temperature rise is a clear indication that an exothermic reaction is occurring.

For those unfamiliar with exothermic reactions, think of it like the warmth you feel from a hand warmer. Both processes release heat, making the surroundings warmer.

Hydration Process

The hydration process is a crucial part of setting plaster of Paris. Hydration refers to the addition of water molecules into a substance. In the case of plaster of Paris, this process involves water molecules being integrated into calcium sulfate hemihydrate, converting it to calcium sulfate dihydrate.

During hydration, the water and plaster mix and react together. This reaction results in the plaster changing from a powder to a solid form. Essentially, water acts like a binding agent, allowing the plaster particles to fuse together and harden. This transformation is what enables the plaster to set and maintain its shape.

To make it easy, think of hydration like adding water to dry cement. Just like cement turns into concrete, plaster of Paris turns into a sturdy structure upon setting due to the hydration process.