Acids, Bases and Salts – MCQs

Correct answer: B. H₃O⁺.

Acids produce hydrogen ions in water. These ions associate with water molecules to form hydronium ions, H₃O⁺. The presence of these ions gives an aqueous solution its acidic character.

Hydrochloric acid transfers a proton to water, producing hydronium and chloride ions.

Correct answer: A. It turns red.

Litmus is a natural acid-base indicator. Acids turn blue litmus red, while bases turn red litmus blue. Litmus helps identify whether a solution is acidic or basic, but it does not give an exact pH value.

Correct answer: C. Litmus.

Litmus is obtained from lichens and is used as a natural indicator. Phenolphthalein and methyl orange are commonly used synthetic indicators. Indicators change colour depending on the acidity or basicity of a solution.

Correct answer: B. Pink.

Phenolphthalein is colourless in acidic solutions and becomes pink in basic solutions. It is useful for identifying the endpoint of certain acid-base titrations.

Correct answer: C. Hydrogen.

Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas. A lit splint produces a characteristic “pop” when brought near a small sample of hydrogen. This test should only be performed under proper laboratory supervision.

Correct answer: B. Carbon dioxide.

Acids react with carbonates to produce a salt, water and carbon dioxide. The carbon dioxide can be passed through limewater, which turns milky due to the formation of calcium carbonate.

Correct answer: A. pH below 7 is acidic.

At approximately 25°C, an aqueous solution with pH below 7 is acidic, pH 7 is neutral, and pH above 7 is basic. The pH scale is logarithmic, so a difference of one pH unit corresponds to a tenfold difference in hydronium-ion concentration.

Correct answer: A. pH 2.

A lower pH indicates a higher hydronium-ion concentration in the usual aqueous-solution context. Therefore, pH 2 is more acidic than pH 5 or pH 7. A pH of 10 indicates a basic solution.

Correct answer: B. Water is needed for HCl to ionise and produce hydronium ions.

Hydrogen chloride shows acidic behaviour in aqueous solution because it forms hydronium ions. Without moisture, the ions needed to produce the usual litmus colour change are not generated in the same way.

Correct answer: B. It generally lowers hydronium-ion concentration per unit volume.

Adding water to an acid reduces its concentration. For a typical acidic solution, its pH rises as it is diluted, although it remains acidic unless sufficiently diluted towards neutrality. Dilution can release heat, so always add acid slowly to water, never water to concentrated acid.

Correct answer: A. Salt and water.

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. In a typical acid-base neutralisation, hydrogen ions from the acid combine with hydroxide ions from the base to form water.

Correct answer: B. Magnesium hydroxide.

Magnesium hydroxide is a base used in some antacid preparations. It reacts with excess stomach acid and helps relieve acidity.

Antacids should be used according to appropriate medical guidance.

Correct answer: A. Sodium hydroxide.

Sodium hydroxide (NaOH) is a strong base. It reacts with fats or oils during saponification to produce soap and glycerol. Potassium hydroxide is also used to manufacture certain soaps.

Correct answer: B. Carbon dioxide gas is released.

Acids react with hydrogen carbonates to form a salt, water and carbon dioxide. The bubbles observed during this reaction are caused by the escaping gas.

Correct answer: B. Sodium hydroxide.

Sodium hydroxide dissolves readily in water and produces hydroxide ions. A base that dissolves in water is called an alkali. Not all bases are soluble, so the terms “base” and “alkali” are not interchangeable in every situation.

Correct answer: B. Hydrogen.

In the chlor-alkali process, electricity is passed through concentrated aqueous sodium chloride (brine). Hydrogen is produced at the cathode, chlorine at the anode, and sodium hydroxide remains in solution.

Correct answer: A. Hydrogen and chlorine.

The electrolysis of brine produces sodium hydroxide, hydrogen gas and chlorine gas. Sodium hydroxide is used in industries such as soap and paper manufacture. Chlorine is used in water treatment and chemical production, while hydrogen has various industrial uses.

Correct answer: B. Citric acid.

Lemon juice contains citric acid, a naturally occurring organic acid. Many citrus fruits contain citric acid, which contributes to their sour taste. Never identify an unknown laboratory chemical by tasting it.

Correct answer: B.

Diluting a concentrated acid can release considerable heat. Adding acid slowly to water while stirring helps distribute the heat and reduces the risk of sudden boiling or splashing. Always follow laboratory safety procedures and wear appropriate protective equipment.

Correct answer: B. Neutral.

At approximately 25°C, a neutral aqueous solution has pH 7, with equal concentrations of hydronium and hydroxide ions. The neutral pH can differ with temperature, so pH 7 is not universally neutral at every temperature.

Correct answer: B. NaCl.

Common salt is sodium chloride. It is obtained from sources such as seawater and underground deposits. It is an important raw material for making sodium hydroxide, chlorine, hydrogen and other useful chemicals.

Correct answer: B. Sodium hydrogen carbonate.

Baking soda is sodium hydrogen carbonate, also known as sodium bicarbonate. It is used in baking powder formulations and some antacid preparations. When heated, it releases carbon dioxide, which can help baked goods rise.

Correct answer: C. Na₂CO₃·10H₂O.

Washing soda is sodium carbonate decahydrate. The ten water molecules shown in its formula are water of crystallisation. Washing soda is used in cleaning and in water softening because it can help remove permanent hardness.

Correct answer: A. CaOCl₂.

Bleaching powder is commonly represented as CaOCl₂ in school-level chemistry. It is prepared by passing chlorine over dry slaked lime under suitable conditions. It is used for bleaching and disinfecting water.

Correct answer: B. CaSO₄·½H₂O.

Plaster of Paris is calcium sulphate hemihydrate. It is prepared by heating gypsum under controlled conditions to remove part of its water of crystallisation.

When mixed with water, plaster of Paris sets into a hard mass of gypsum, making it useful for casts, moulds and decorative work.

Correct answer: A. CaSO₄·2H₂O.

Gypsum is calcium sulphate dihydrate, containing two water molecules per formula unit. Controlled heating of gypsum produces plaster of Paris. The difference in their water content explains why they behave differently when mixed with water.

Correct answer: B. A fixed number of water molecules associated with a crystalline salt.

Water of crystallisation is the definite amount of water incorporated into the crystal structure of certain salts. For example, copper sulphate crystals contain five water molecules per formula unit.

Heating hydrated copper sulphate removes this water, changing its blue crystals into white anhydrous copper sulphate.

Correct answer: A. They turn white.

Blue hydrated copper sulphate loses its water of crystallisation on heating and forms white anhydrous copper sulphate. Adding water again can restore the blue colour. This is a useful demonstration of hydration and dehydration.

Correct answer: A. It releases carbon dioxide under suitable conditions.

On heating, sodium hydrogen carbonate decomposes to form sodium carbonate, water and carbon dioxide. The gas creates bubbles that help baked foods rise and develop a lighter texture.

Correct answer: B. It reacts with water to form gypsum.

When water is added, plaster of Paris rehydrates and forms gypsum crystals. The formation and interlocking of these crystals cause the material to set into a hard mass. This property makes it useful for moulds, decorative objects and medical casts.