Question

While heating the sand to dryness, it was not necessary to place a watch glass over the evaporating dish. Suggest a reason why this might be unnecessary but not while drying the salt solution it was necessary.

Answer 1

There are two main reasons behind not to put watch glass.

First

Normal aqueous soluble salts , due to their inbuilt colligative properties , elevate the boiling point of solution . Then , at the elevated temperature , when the water boils , may be portion of salt melt or gone away with the solvent water . So , there is a missing of salt . But , Sand does not dissolve in water , rather just physically  attracts the water molecules due to hygroscopic nature . This does not elevate boiling point . So , no chance of loosing sand during drying .

Second

Sand is extremely hard material made of SiO₂ . So , it has very high melting point (1650⁰C) , that is even greater than not only the melting points , but the boiling points of many common water soluble inorganic salts also . As example , NaCl has melting point 801⁰C and boiling point 1465⁰C that is less than sand's melting point . Thus , evaporation of sand during drying is nearly impossible and thus , no need to put watch glass in order to prevent it .