Chemical Reactions and Equations

Magnesium burns in oxygen to form white magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\)। MgO is a basic oxide and can react with water to form basic magnesium hydroxide: \(MgO + H_2O \rightarrow Mg(OH)_2\)। In contrast, an amphoteric oxide reacts with both acids and bases, whereas MgO is not generally classified as amphoteric.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released to the surroundings during this process, so it is an exothermic reaction. In contrast, an endothermic reaction absorbs heat from the surroundings.

In a decomposition reaction, one compound breaks down into two or more simpler substances. Energy, often in the form of heat, light, or electricity, is supplied to break bonds; therefore, such reactions are generally endothermic. In contrast, exothermic reactions release energy to the surroundings.

Combustion is a chemical reaction in which a fuel reacts with oxygen and usually releases heat and sometimes light. Thus, oxygen is generally necessary for burning in air, making option A correct. Carbon dioxide and water vapour do not support ordinary combustion, while nitrogen is largely unreactive under these conditions and mainly dilutes the oxygen in air.

Bubbles appearing during a reaction usually provide evidence that a gas is being produced or released, so option A is correct. For example, an acid reacting with a carbonate can form carbon dioxide, which escapes as bubbles. A precipitate is an insoluble solid seen as particles or cloudiness, whereas melting and evaporation are physical changes and do not generally explain reaction bubbles.

A change in smell can indicate that a new substance with different properties has formed during the reaction. A change of state alone, such as freezing or boiling of water, does not usually form a new substance. Exam tip: Change in colour or smell, temperature change, gas evolution, and precipitate formation are common signs of a chemical reaction.

Rusting is the reaction of iron with oxygen in the presence of water or moisture. A paint coating forms a barrier between the iron surface and air and moisture, thereby preventing rust formation. In contrast, painting does not raise iron's melting point or increase its chemical reactivity.

The process of applying a protective zinc coating to iron is called galvanisation. Zinc blocks air and moisture from reaching the iron and, even if the coating is scratched, zinc oxidises preferentially and protects the iron. Electroplating is a general method of depositing a metal coating, whereas coating iron specifically with zinc to prevent rusting is galvanisation. Exam tip: If you see ‘iron coated with zinc’, choose galvanisation.

Rusting is an oxidation process because iron reacts with oxygen from air in the presence of moisture to form hydrated iron(III) oxide (rust). In reduction, oxygen is removed from a substance or it gains electrons; in rusting, iron undergoes oxidation instead.

Rancidity is the spoilage of food containing oils or fats when these substances react with oxygen from the air. This oxidation produces unpleasant smells and tastes, so option A is correct. Freezing water, dissolving salt, and melting sugar are physical changes; none explains the characteristic oxidation and bad odour associated with rancid food.

Rancidity occurs when oils and fats in food react with oxygen and become oxidised. This can produce an unpleasant smell and taste. An airtight container reduces the food’s contact with air, especially oxygen. Therefore, the rate of oxidation decreases and rancidity is delayed. Unlike option B, increasing the oxygen available to food would promote oxidation of its oils and fats rather than prevent rancidity.

The oils and fats in chips can oxidise in the presence of oxygen in air, producing an unpleasant smell and taste. This process is called rancidity. Filling the packet with nitrogen reduces the oxygen available, so oxidation of the oils and fats slows down. Rusting involves oxidation of iron, whereas oils in chips undergo rancidity.

Direct answer: A, (aq), is correct. When a substance dissolves in water, it is present in an aqueous solution. The abbreviation aq is written in parentheses after the formula to show this state. For example, NaCl(aq) means sodium chloride dissolved in water. Option B, (s), means the substance is solid, such as NaCl(s) for solid sodium chloride. Option C, (g), means gas, such as H₂(g). Option D, (l), means that the substance itself is a liquid, such as water written H₂O(l); it does not specifically indicate that another substance is dissolved in water. The symbols identify physical state and should not be confused with coefficients, which show relative amounts in a balanced equation. A useful memory cue is “aq sounds like aqua, associated with water.” The distinction between (l) and (aq) is important: a liquid substance and a substance dissolved in water are not described by the same state symbol. Therefore, only A precisely answers the question.

The correct answer is A: (s). In a chemical equation, a state symbol is written in brackets after a substance’s formula to show its physical state. The letter s stands for solid, so NaCl(s) means solid sodium chloride. Option B, (g), is wrong because g means gaseous state. Option C, (l), is wrong because l means liquid state. Option D, (aq), is wrong because it means the substance is dissolved in water and forms an aqueous solution; it is not the general symbol for a solid. A useful memory cue is: s = solid, l = liquid, g = gas, and aq = aqueous. These symbols describe physical state, not the number of atoms or the type of reaction.

The correct answer is A: (l). State symbols are placed after chemical formulas to identify the physical condition of substances. The lowercase l comes from liquid and therefore represents the liquid state; for example, H₂O(l) indicates liquid water. Option B, (s), means solid, so it cannot represent a liquid. Option C, (g), means gas, which is a different state. Option D, (aq), means aqueous: the substance is dissolved in water. An aqueous solution may look liquid, but (aq) gives the special information that water is the solvent; it is not simply the general symbol for every liquid. Remember l for liquid, while carefully distinguishing it from the number 1.

In a chemical equation, an upward arrow shows that a gas is evolved during the reaction. For example, hydrogen gas is produced when zinc reacts with a dilute acid. A downward arrow is generally used for the formation of a precipitate, so option B is not correct. Exam tip: remember—upward arrow for gas, downward arrow for precipitate.

A downward arrow (↓) in a chemical equation indicates that an insoluble solid has formed and settled out of the solution. This solid is called a precipitate, so option A is correct. An upward arrow (↑), where used, indicates gas evolution; heat is shown by reaction conditions, and an aqueous state is represented by (aq), not by a downward arrow.

A skeletal chemical equation contains the correct chemical formulae of both reactants and products, but the number of atoms of each element may be unequal on the two sides. It must therefore be balanced to obey the law of conservation of mass. Options B and C are incorrect because a skeletal equation includes the formulae of both reactants and products.

A word equation gives the names of the reactants and products. The first step is to write their correct chemical formulae; for example, hydrogen is H₂ and oxygen is O₂. The equation is balanced afterwards by changing coefficients to make the number of atoms equal on both sides. Changing subscripts in a formula is not the correct way to balance an equation.

While balancing a chemical equation, only the coefficients written before formulae are changed. This makes the number of atoms of every element equal on the reactant and product sides, satisfying the law of conservation of mass. For example, the 2s in \(2H_2 + O_2 \rightarrow 2H_2O\) are coefficients. Changing a subscript is not allowed because changing \(H_2O\) to \(H_2O_2\) changes water into a different substance.

According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction. Therefore, in a closed system, the total mass of the reactants equals the total mass of the products. Option B is incorrect because the actual mass does not decrease; in an open container, the measured mass may appear lower if a gas escapes.

A rise in temperature during a reaction usually indicates that heat is being released by the reaction. Such a reaction is called an exothermic reaction. In an endothermic reaction, heat is absorbed, so the temperature generally tends to decrease. Exam tip: Associate release of heat or a temperature rise with an exothermic reaction.

A fall in the temperature of the reaction mixture generally means that the reaction is absorbing heat from its surroundings. Such reactions are called endothermic reactions. In contrast, exothermic reactions release heat and generally cause a temperature rise, so option B is incorrect. Formation of a gas or a metal is not a necessary indication of a temperature decrease.

A colour change is only one possible sign of a chemical reaction; it is not compulsory. Many gases, such as hydrogen and carbon dioxide, are colourless, yet bubbling or gas evolution can indicate that a reaction has occurred. Therefore, the absence of a visible colour change does not prove that no reaction took place. Option B is incorrect because not all gases are coloured, while option C is incorrect because temperature change and precipitate formation can also indicate a reaction.

During a chemical reaction, new substances are formed, and observable evidence may include a colour change, gas evolution, a temperature change, or precipitate formation. Therefore, option A is an appropriate indication of a reaction. In contrast, merely knowing the names of the reactants does not show that a reaction has occurred.