2.4.4. Are the Crystals of Salts really Dry?

n Heat a few crystals of copper sulphate in a dry boiling tube.

n What is the colour of the copper sulphate after heating?

n Do you notice water droplets in the boiling tube? Where have these come from?

When we heat copper sulphate crystals in a dry boiling tube, we observe that the crystals change color. Specifically, the blue color of the copper sulphate disappears, and the salt becomes white. This color change occurs because the heating process removes water molecules that are bound to the salt. These water molecules are known as water of crystallization, and their removal causes a change in the physical appearance of the crystals.

Imagine a sponge soaked in water. When you heat it or leave it out to dry, it loses that moisture and appears shriveled and different from when it is wet. Similarly, heating the copper sulphate crystals removes the water, making them look different.

n Add 2-3 drops of water on the sample of copper sulphate obtained after heating.

n What do you observe? Is the blue colour of copper sulphate restored?

After heating, when we add a few drops of water to the white copper sulphate, we notice that the blue color returns. This happens because the water we added rehydrates the copper sulphate crystals, restoring them to their original hydrated form known as hydrated copper sulphate (CuSO4·5H2O). The rehydration indicates that the original salt was not completely dry; it contained specific water molecules necessary for its crystalline structure.

This is similar to when you add water to a dried-out plant; it starts to regain its vibrant color and becomes healthy again. The same process occurs with the copper sulphate crystals, as they come back to life with the addition of water.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO . 5H O.

Water of crystallisation refers to the specific number of water molecules that are incorporated into the structure of a salt. In the case of copper sulphate (CuSO4), each formula unit includes five molecules of water (indicated as CuSO4·5H2O). These water molecules play a crucial role in stabilizing the structure of the salt and contribute to its color and physical properties. If these water molecules are removed, as observed during the heating process, the salt loses its ability to exhibit its original characteristics.

Think about how ice cubes are solid because they contain water, and when they melt, they change shape and properties. Similarly, the water molecules in salts are essential for their structure; without them, the salts lose their identity.

One other salt, which possesses water of crystallisation is gypsum. It has two water molecules as water of crystallisation. It has the chemical formula CaSO .2H O.

Gypsum is another example of a salt that contains water of crystallisation. Its chemical formula is CaSO4·2H2O, indicating that each formula unit consists of two water molecules. The presence of water molecules affects not only the appearance but also how gypsum behaves in various applications, such as in construction (as Plaster of Paris) or decoration. Like other salts, when gypsum is heated, it can lose the water of crystallisation and change its properties.

Consider how a cake batter transforms into a cake in an oven. The water content in the batter is crucial for the cake's structure. Once baked and cooled, the texture and properties change completely. Gypsum reacts similarly when it interacts with water in construction.

On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO . H O). This is called Plaster of Paris.

When gypsum is heated to around 373 K, it loses water molecules to become a powder known as calcium sulphate hemihydrate (CaSO4·0.5H2O), commonly known as Plaster of Paris. Upon mixing it with water, it rehydrates back into gypsum and sets hard. This process is significant in construction and medicine, as Plaster of Paris is used to create molds, casts for broken bones, and intricate architectural designs.

Picture a fine sand mixture that, when water is added, solidifies into a sturdy bucket. Plaster of Paris behaves similarly; it hardens upon contacting water, making it useful for creating durable shapes and supports.

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