Tag: changes around us

The option (A) represents correct statements.
Salt can be obtained by the evaporation of sea water.
The salt obtained during crystallisation is not pure and its crystals are small.
After crystallisation, the shape of the crystals cannot be seen clearly.
Large crystals of pure substances can be formed from their solutions.

On evaporation of seawater in shallow pools, salt along with impurities remain behind. Impured salt is taken in a tank and its saturated solution is prepared. In a saturated solution of salt, HCl vapours are passed. Chloride ions being common to both, it's concentration increases, and NaCl gets precipitated which is free from impurities.

The process of forming large crystals of pure substances from their solutions is known as Crystallization. The principle of crystallization is concerned with the thermodynamic conditions of crystallization and the kinetics of crystal nucleation and growth.
Crystallization is the physical transformation of a liquid solution or gas to a crystal which is a solid with an ordered internal arrangement of molecules, ions or atoms.

The process of formation of solid crystals from a homogeneous solution is called crystallization. It is a physical change. Crystallization is the solidification of atoms or molecules into a highly structured form called a crystal. It can also refer to the solid-liquid separation and purification technique in which mass transfer occurs from a liquid solution to a pure solid crystalline phase.

Phenolphthalein [3,3-Bis (4- hydroxyphenyl)- 2 benzofuran- 1-one] is often used as an indicator in acid-base titrations. It turns colourless in acidic solutions and pink in basic solutions. Phenolphthalein is slightly soluble in water and usually dissolved in alcohols for use in experiments. It is a weak acid which can loose $H^{+}$ ions in solution.

Iron reacts with air to form iron oxide(III) which is known by the name rust or rusting of iron. Rust is an iron oxide or red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Rust consist of hydrated iron oxide.

Rusting of iron takes place only when both air and moisture are present.

Damp areas alone supplies both the things air and water required for the rusting of iron.

An action of air and water gradually eats up the metal. This eating up of metals by the action of air and moisture on their surface is called corrosion.

We can prevent iron getting rusted by applying a coat of zinc or a thin coat of plastic helps to prevent rust. The process for coating metal like iron or steel with a thin zinc layer is known as galvanization.