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Answer - 1.  Usually grills of doors and windows are made of iron. 2.    Iron objects get rusted on exposure to moist air. 3.  Since air always contains moisture, iron grills get rusted. 4.   Painting iron objects prevents rusting, hence iron grills of doors and windows are painted before they are used.

Answer - 1.  To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. 2.   The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s) respectively. 3.   The word aqueous (aq) is written if the reactant or product is present as a solution in water

Answer - 1.   Potassium Ferrocyanide gets decomposed when exposed to bright light. 2.   The dark colours of the bottles absorb a lot of energy from the light passing through it. 3.   If stored in dark – coloured bottles, it does not get enough light energy to decompose. 4.     Hence, potassium Ferrocyanide is stored in dark – coloured bottles and kept away from sunlight to prevent its decomposition.

Answer -  1.  One of the properties of iron is that it reacts with oxygen in the presence of moisture and gets corroded by forming rust. 2.  Air contains oxygen and moisture. Hence, iron readily rust when exposed to air. 3.   Steel (stainless steel, not mild steel) is an alloy of iron having the property to resist corrosion including rusting. 4.   Hence, stainless steel, though it contains iron, does not undergo corrosion.

Answer - 1.   If edible oil is allowed to stand for a long time in an iron or tin container the fatty acids in the oil react with rust flakes or powder to form salts. 2.  These salts contaminate the oil and hence the oil becomes rancid. 3.   Rancid oils have a foul odour and unpleasant taste. 4.   Thus, rancid oil is of no use. Hence, Edible oil is not allowed to stand for a long time in an iron or tin container to avoid rancidity.

Answer -  1.   When edible oils are left exposed to air for long period of time, they become rancid (i.e. the oils get oxidized.) 2.   Rancid oils have a foul odour and unpleasant taste. 3.   Thus, rancid oil is of no use. 4.  Edible oils is not allowed to stand exposed to air for long to avoid rancidity.

Answer -  1.   The atomic numbers of the elements increase as we go down the group. Thus, the elements placed lower have more electrons. 2.   To accommodate these electrons new shells are added to the atom. 3.   These new shells take the outermost electrons farther from the nucleus causing atomic size (radius) to increase as we go down the group . 3.