Question

A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such behaviour? Give the reaction involved.

Answer 1

Calcium$\left({\mathrm{Ca}}^{+2}\right)$ belongs to group 2. Calcium sulfate salt is a white soft substance and is a naturally occurring calcium salt. It is commonly known for its dihydrate form gypsum$\left({\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\right)$.

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula ${\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}$. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall.

Gypsum$\left({\mathrm{CaSO}}_4.2\mathrm{H}2\mathrm{O}\right)$ heated at 373 K to prepare Plaster of paris. On heating, Gypsum loses water molecules and becomes Calcium Sulphate Hemihydrate$\left({\mathrm{CaSO}}_4.\frac{1}{2}\mathrm{H}2\mathrm{O}\right)$.

:$\underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}}\left(\mathrm{s}\right)\to \underset{\mathrm{Plaster}\mathrm{of}\mathrm{Paris}}{{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}\mathrm{Crystal}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Plaster of Paris is a popular chemical substance that is utilized most commonly for sculpting materials and in gauze bandages. From the above properties of plaster of Paris, we can conclude that the given white powder is plaster of Paris. Some of the important properties of the Plaster of Paris are as follows:

1. Usually white in color and available in powder form.
2. When water is added Gypsum crystals are formed leading it to achieve a solid-state.
3. The exothermic setting process can be catalyzed by Sodium chloride. The plaster of Paris is retarded by alum or borax.
4. Plaster of Paris forms Anhydrous calcium Sulfate at 473K. This is sometimes known as the dead burnt Plaster of Paris.
5. When the Plaster of Paris is left for some time in the open, it crystallizes into a solid mass because of a reaction with moisture present in the atmosphere.
6. The solid mass so formed is known as gypsum and cannot be further used for molding. The reaction involved in the process is given below: $\underset{\mathrm{Plaster}\mathrm{of}\mathrm{paris}}{{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}+\underset{\mathrm{water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}}\underset{}{\to }\underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}}\left(\mathrm{s}\right)$
7. Plaster of Paris shows such behavior because of the presence of half water molecule as water of crystallization. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard solid mass.

Final answer:

The Sulphate salt is Plaster of Paris and shows such behavior because of the presence of half water molecule as water of crystallization. When it is left open for some time, it absorbs moisture from the atmosphere and forms Gypsum, which is a hard solid mass.