Which of the following contains citric 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.