Acids, Bases and Salts: pH Scale and Common Salts

The correct answer is B, orange. Citrus fruits such as orange, lemon, lime and grapefruit owe their sharp sour taste to citric acid, which is the acid textbooks name for this family and the one examiners expect here. Option A is wrong because tomato is listed with oxalic acid in the standard table, the same acid found in spinach. Option C is wrong because tamarind, like grapes, contains tartaric acid, which is also the acid mixed with baking soda to make baking powder. Option D is wrong because sour milk and curd contain lactic acid, produced when bacteria act on the sugar of milk. Learn the food and acid pairs as a single block, since one of them appears in almost every science paper: lemon and orange with citric acid, vinegar with acetic acid, apple with malic acid and ant sting with formic acid.

The correct answer is D, decreases 1000 times. The pH is the negative logarithm of the hydrogen ion concentration, so each rise of one unit means the concentration has fallen to one tenth. A change from pH 3 to pH 6 is a rise of three units, which is ten multiplied by ten multiplied by ten, that is a thousandfold fall in hydrogen ions, and the solution becomes much less acidic. Options A and B are wrong for two reasons: a rise in pH always means fewer hydrogen ions, not more, and the change is never a simple doubling or tripling because the scale is logarithmic. Option C is wrong because a hundredfold fall would correspond to a change of only two units, say from pH 3 to pH 5. Keep in mind that the same rule works downward, so moving from pH 6 to pH 3 multiplies the hydrogen ion concentration a thousand times.

The correct answer is D, blue. Borax, sodium tetraborate, is the salt of a strong base, sodium hydroxide, and a weak acid, boric acid, so its solution in water is basic. A basic solution turns litmus blue, which is the standard test. Option A is wrong because pink is the colour phenolphthalein takes in a base, not the colour of litmus; mixing up the two indicators is the usual mistake in this question. Option B is wrong because orange belongs to methyl orange, which is red in acid and yellow in base, and never gives a plain orange reading as an answer here. Option C is wrong because litmus turns red in an acidic solution, the opposite of what borax gives. Remember the pairing: salts of a strong base with a weak acid, such as borax, sodium carbonate and sodium acetate, all give a basic solution.

The correct answer is B, NaHCO3. Baking soda is sodium hydrogen carbonate, a mild, non corrosive base used as an antacid, as an ingredient of baking powder along with a mild edible acid such as tartaric acid, and in soda acid fire extinguishers, where it releases carbon dioxide. Option A is wrong because Na2CO3 is sodium carbonate, and its decahydrate is washing soda, used for cleaning and for softening hard water. Option C is wrong because NaOH is sodium hydroxide, caustic soda, a strong alkali obtained from the chlor alkali process. Option D is wrong because CaOCl2 is bleaching powder, made by passing chlorine over dry slaked lime. The pair that catches students out is sodium carbonate against sodium hydrogen carbonate, so remember that the one with hydrogen in the name is the baking soda used in the kitchen.

The correct answer is A, formic acid. The sting of an ant and the hairs of the nettle plant inject formic acid, also called methanoic acid, which is why the skin burns; rubbing on a mild base such as baking soda or the leaf of the dock plant relieves it by neutralising the acid. Option B is wrong because citric acid belongs to lemon, orange and other citrus fruits. Option C is wrong because lactic acid is formed in curd and sour milk, and also in muscles during heavy exercise. Option D is wrong because tartaric acid is found in tamarind and grapes and is used with baking soda in baking powder. This question is a favourite because it also carries a neutralisation idea with it: the treatment for an acidic ant sting is a base, while a wasp sting, which is alkaline, is treated with a mild acid such as vinegar.

The correct answer is B, gypsum. Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum, which is calcium sulphate dihydrate, to about 373 kelvin so that it loses three fourths of its water of crystallisation. When water is added again it sets into a hard mass of gypsum, which is why it is used for plaster casts, moulds, statues and false ceilings, and why it must be stored in a moisture proof container. Option A is wrong because limestone is calcium carbonate and heating it gives quick lime and carbon dioxide. Option C is wrong because quick lime is calcium oxide, the product of that reaction, and adding water to it gives slaked lime. Option D is wrong because washing soda is sodium carbonate decahydrate and heating it simply drives off water of crystallisation. Note also that heating gypsum too strongly destroys the setting property.

The correct answer is C, hydrochloric acid. The stomach wall secretes hydrochloric acid, which makes the gastric juice strongly acidic, helps the enzyme pepsin digest proteins and kills many germs swallowed with food. The stomach protects itself with a layer of mucus, and when the acid is produced in excess the burning sensation called acidity follows, treated with an antacid such as milk of magnesia. Option A is wrong because sulphuric acid is an industrial acid used in fertilisers, batteries and detergents and would destroy living tissue. Option B is wrong because nitric acid is likewise a strong laboratory and industrial acid, used in explosives and fertilisers. Option D is wrong because acetic acid is the weak acid of vinegar, which is edible but is not the acid the stomach itself produces. Note that an antacid works by neutralisation, not by stopping the secretion.

The correct answer is A, acidic. A salt takes its character from the strength of the acid and the base that formed it. Ammonium chloride comes from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base, so the strong partner dominates and the solution has a pH below seven. Option B is wrong because a basic solution is given by the opposite combination, a weak acid with a strong base, as in sodium carbonate or sodium acetate. Option C is wrong because a neutral solution is given when both partners are strong, the standard example being sodium chloride from hydrochloric acid and sodium hydroxide. Option D is wrong because amphoteric describes a substance such as aluminium oxide or water that can act as both acid and base, which is not what a salt solution is being asked about here. Fix the three combinations and this whole family of questions is settled.

The correct answer is C, 5.5. Tooth enamel is made of calcium phosphate, the hardest substance in the body, and it begins to corrode when the pH in the mouth drops below about 5.5. Bacteria in the mouth break down leftover sugar into acid, which is why sweets left on the teeth cause decay and why cleaning the teeth with a mildly basic toothpaste neutralises the excess acid. Option A is wrong because 7.0 is the neutral point, at which enamel is perfectly safe. Option B is wrong because at 6.5 the mouth is only slightly acidic and the enamel still holds. Option D is wrong because 3.5 is far more acidic than the threshold; damage would already have begun well before that value is reached. Note the neighbouring figure often confused with this one: rain is called acid rain when its pH falls below about 5.6.

The correct answer is A, dry slaked lime. Bleaching powder, calcium oxychloride, is made in industry by passing chlorine gas over dry slaked lime, that is calcium hydroxide. It is used for bleaching cotton, linen and wood pulp, for disinfecting drinking water, and as an oxidising agent in chemical industries. Option B is wrong because limestone is calcium carbonate, and heating it gives quick lime and carbon dioxide rather than a bleaching agent. Option C is wrong because gypsum is calcium sulphate dihydrate, the raw material of plaster of Paris. Option D is wrong because sodium carbonate is washing soda, used for cleaning and for softening hard water. The chain worth memorising runs from limestone to quick lime on heating, from quick lime to slaked lime on adding water, and from dry slaked lime to bleaching powder on passing chlorine.