Acids, Bases and Salts: questions with answers

Phenolphthalein is colourless in an acidic solution and pink in a basic solution. Each indicator has its own colours, so it does not copy litmus.

Vanilla is an olfactory indicator, whose smell responds to acids and bases. Its characteristic smell remains detectable in hydrochloric acid but is not detected in sodium hydroxide solution.

Acidity increases as pH falls, so the hydronium ion concentration falls as pH rises. The highest pH here is 13, so the pH 13 solution has the fewest hydronium ions. The pH 1 solution has the most. The pH 7 solution is neutral, but it still holds hydronium and hydroxide ions in equal amounts.

Hydrochloric acid is a strong acid with more hydronium ions and a lower pH than acetic acid of the same concentration. So it reacts more vigorously with magnesium.

An acid with a hydrogencarbonate gives a salt, water and carbon dioxide. The gas that bubbles off is carbon dioxide, not hydrogen or oxygen.

Carbon dioxide first forms insoluble calcium carbonate, which makes lime-water milky. Excess carbon dioxide turns it into calcium hydrogencarbonate, which dissolves, so the liquid clears.

Hydrogen burns with a pop sound when a burning candle is brought near it. A glowing splint relighting is the test for oxygen, and lime-water turning milky is the test for carbon dioxide.

The metal gives the positive part of the salt and the acid gives the negative part. Magnesium with hydrochloric acid gives magnesium chloride and hydrogen.

Carbon dioxide, a non-metallic oxide, reacts with a base to give a salt and water, so it is acidic. Copper oxide, a metallic oxide, reacts with an acid to give a salt and water, so it is basic.

A hydrogen ion cannot exist alone in water. It combines with a water molecule to form the hydronium ion, $H_3O^+$. So an acid in water gives hydronium ions.

Adding water spreads the same hydroxide ions through a larger volume, so their concentration per unit volume falls. No acid is added, so nothing reacts with the base and the solution stays basic.

Hydrochloric acid in the stomach helps to digest food, so the acid itself is useful. Pain comes from too much acid. The mild base reacts with this extra acid and neutralises it, giving salt and water. The stomach acid is not taken away altogether, and the base does not work by dilution alone.

An acid and a base react to give a salt and water. Potassium sulphate is the salt formed here. Sodium chloride is only one of many salts.

Baking soda is sodium hydrogencarbonate, $NaHCO_3$. Washing soda is a different compound, sodium carbonate decahydrate, $Na_2CO_3.10H_2O$.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Copper sulphate crystals are written as $CuSO_4.5H_2O$, so each formula unit holds five water molecules. On heating, the blue crystals lose this water and turn white. Adding water to the white powder gives back the blue crystals. So this water is part of the crystal itself, not dampness lying on its surface.