NCERT Solutions Curiosity Chapter 9 Chapter opener — Probe and ponder

People are collecting natural salt from the seashore. Behind them is the sea; in front of them the wet sand is crusted with white salt, and a man in a dhoti is bending down to pick up a handful of it.

The science of the picture is the science of this whole chapter. Sea water is not pure water — it is a solution in which common salt and other salts are the solutes and water is the solvent. Sunshine and wind evaporate the water, but the salt cannot evaporate. As water leaves, the solution left behind becomes more and more concentrated, until it is saturated. Beyond that point every further litre of water that evaporates leaves solid salt behind, and the salt appears as white crystals on the sand. The people are simply picking up what the sea left.

The tea has become a saturated solution of sugar at that temperature. It cannot hold any more, so the extra sugar sinks and stays as grains at the bottom of the cup.

Two things will fix it, and one will not:

• Heat the tea (or add hot tea). Solubility of a solid rises with temperature, so a saturated solution behaves as an unsaturated one once it is warmer, and the settled sugar dissolves.
• Add more tea — that is, more solvent. The same sugar now has more water to spread through, so the solution is no longer saturated.
• Stirring alone will not help. Stirring only makes dissolving faster; it does not raise the limit. Once the limit is reached, you can stir all evening and the grains will stay.

Because dissolving is a tug-of-war between attractions, and water wins it while oil does not.

In a grain of salt or sugar the particles are held tightly to one another in a regular arrangement. For the grain to dissolve, the particles of the liquid must pull those particles away one by one and then keep them apart and evenly spread. Water particles can do this: each water particle has a slightly positive side and a slightly negative side, so it grips the particles at the surface of the crystal, tugs them loose and surrounds them. Oil particles have no such pull. They attract one another well enough, but they hardly attract salt or sugar particles at all — so the crystal is never pulled apart and simply lies in the oil unchanged.

Why water is called a good solvent:

• It dissolves a very wide range of substances — salts, sugars, ORS powder, many medicines, and even gases such as oxygen and carbon dioxide.
• It is liquid over the whole range of everyday temperatures, is cheap, is not poisonous and does not catch fire.
• It carries dissolved substances through living things — sap in a plant, blood in your body, digested food in your intestine.

Because a tall cylinder is easy to stand, easy to hold and easy to pack, and a sphere is none of these.