An object is placed in front of a concave mirror of focal length 18 cm, at a distance of 12 cm. The magnification produced by the mirror is ________.

The correct answer is D, 11.36%. Mass by mass percentage of a solution is the mass of the solute divided by the mass of the whole solution, multiplied by 100. The solute here is 41 g of common salt and the solvent is 320 g of water, so the mass of the solution is 41 plus 320, that is 361 g. The concentration is therefore 41 divided by 361, multiplied by 100, which gives 11.36 per cent. The common mistake is to divide by the mass of the solvent alone instead of the solution; 41 divided by 320 into 100 gives 12.81 per cent, which is not even an option. A is wrong because 12.36 per cent would need about 45 g of salt in the same solution. B is wrong because 9.35 per cent would need about 34 g of salt. C is wrong because 10.36 per cent would need about 37 g of salt. Exam tip: mass percentage always uses solute divided by solute plus solvent, never solute divided by solvent.

The correct answer is C, neutralisation reaction. In the given equation sodium hydroxide, which is a base, reacts with sulphuric acid to give sodium sulphate and water, and a reaction in which an acid and a base cancel each other to form a salt and water is called neutralisation. Its general form is acid plus base gives salt plus water, and heat is given out, so the reaction is exothermic. Two molecules of sodium hydroxide are needed here because sulphuric acid is dibasic and supplies two hydrogen ions. Everyday examples are an antacid taken for acidity in the stomach, lime spread on an acidic soil and baking soda rubbed on a bee sting. A is wrong because a precipitation reaction forms an insoluble solid, while both products here stay in solution. B is wrong because in a decomposition reaction a single compound splits into two or more simpler substances. D is wrong because a redox reaction involves a change in oxidation numbers, and here they do not change. Exam tip: acid plus base giving salt and water is always neutralisation.

The correct answer is B, 540 cal. The heat produced by a current is given by Joule's law of heating, H equals I squared into R into t. Here the current is 0.3 A, the resistance is 418 ohms and the time is 1 minute, which is 60 seconds. So H = 0.09 into 418 into 60 = 2257.2 joule. One calorie equals 4.18 joule, so 2257.2 divided by 4.18 gives 540 calorie. Joule's law says the heat produced is proportional to the square of the current, to the resistance and to the time, which is why doubling the current gives four times the heat. The same law explains the electric iron, the heater, the geyser, the filament of a bulb and the fuse that melts to protect a circuit. A is wrong because 360 calorie would need a current of about 0.245 A for the same resistance and time. C is wrong because it is one tenth of the correct value. D is wrong because 36 calorie would need a current of only about 0.077 A. Exam tip: use H equals I squared Rt, then divide the joule figure by 4.18 to get calories.

The correct answer is C, Momentum. A quantity that has both magnitude and direction is a vector, and momentum, which is the product of mass and velocity, is a vector because velocity has a direction. Its SI unit is kilogram metre per second and its direction is the same as that of the velocity. Quantities that have magnitude alone are called scalars. Displacement, velocity, acceleration, force and weight are other common vectors, while mass, distance, speed, time, work, energy and temperature are scalars. Vectors are added by the triangle or parallelogram law and not by simple arithmetic, which is why two equal forces acting in opposite directions cancel each other. A is wrong because mass is a scalar, the quantity of matter in a body. B is wrong because distance is the total length of the path covered and has no direction; its vector counterpart is displacement. D is wrong because speed is a scalar and its vector counterpart is velocity. Exam tip: distance and speed are scalars, while displacement, velocity and momentum are vectors.

The correct answer is C, Ammonium chloride. Ammonium chloride is the salt of a strong acid, hydrochloric acid, and a weak base, ammonium hydroxide, so its aqueous solution is acidic and its pH is less than 7. When such a salt dissolves, the ion of the weak base hydrolyses in water and releases hydrogen ions, which is why the solution turns blue litmus red. The rule is simple: a strong acid with a weak base gives an acidic salt, a weak acid with a strong base gives a basic salt, and a strong acid with a strong base gives a neutral salt. A is wrong because sodium carbonate comes from a strong base and the weak carbonic acid, so its solution is basic with a pH above 7. B is wrong because sodium chloride comes from a strong acid and a strong base, so its solution is neutral with a pH of 7. D is wrong because sodium bicarbonate is also basic, which is why it serves as baking soda and as an antacid. Exam tip: the stronger parent decides, so strong acid plus weak base means an acidic salt.