Chemical Reactions and Equations – MCQs

Correct answer: B. A process in which new substances form.

During a chemical reaction, reactants are converted into products with different chemical properties. For example, magnesium reacts with oxygen to form magnesium oxide, a new substance.

Correct answer: C. Gas formation or a colour change.

Gas evolution, colour change, temperature change and precipitate formation can indicate a reaction. These observations are clues; the defining feature is the formation of new substances.

Zinc reacts with dilute hydrochloric acid and releases hydrogen gas.

Correct answer: C. 2H₂ + O₂ → 2H₂O.

The left side has four hydrogen atoms and two oxygen atoms. The right side also has four hydrogen atoms and two oxygen atoms. Therefore, the equation is balanced.

Remember: balance equations by changing coefficients, not the small-number subscripts in chemical formulas.

Correct answer: C. To obey conservation of mass.

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Atoms are rearranged, so each element must have the same number of atoms on both sides of the equation.

Correct answer: C. (g).

The state symbols are (s) for solid, (l) for liquid, (g) for gas and (aq) for a substance dissolved in water. For example, hydrogen gas is written as H₂(g).

Correct answer: A. Gas evolution.

The upward arrow is traditionally used to show that a gas is released. A downward arrow (↓) may be used in school-level equations to indicate a precipitate, which is an insoluble solid.

Correct answer: C. 4Fe + 3O₂ → 2Fe₂O₃.

There are four iron atoms and six oxygen atoms on each side. Option C therefore satisfies the requirement that each element has the same number of atoms among the reactants and products.

Correct answer: B. Relative particle or mole ratios.

In N₂ + 3H₂ → 2NH₃, one nitrogen molecule reacts with three hydrogen molecules to form two ammonia molecules. The same coefficients give the relative mole ratio in a chemical reaction.

Correct answer: C. Adjust coefficients only.

Subscripts define the composition of a compound. Changing H₂O to H₂O₂ changes water into hydrogen peroxide, a different substance. Balance an equation by adjusting the numbers placed before the formulas.

Correct answer: B. Zn + 2HCl → ZnCl₂ + H₂.

Two HCl molecules provide two hydrogen atoms and two chlorine atoms. These form one H₂ molecule and one ZnCl₂ formula unit. Zinc displaces hydrogen from the acid, releasing hydrogen gas.

Correct answer: B. Combination reaction.

Two reactants combine to produce one main product. Calcium oxide is quicklime and calcium hydroxide is slaked lime. The reaction releases heat, so it is also exothermic.

Correct answer: C. CaCO₃ → CaO + CO₂.

A single compound breaks into two simpler substances. Because heat supplies the energy, this is called thermal decomposition. The symbol Δ above an arrow commonly indicates heating.

Correct answer: B. Iron(III) oxide, sulphur dioxide and sulphur trioxide.

On strong heating, ferrous sulphate decomposes. The solid residue becomes brown because iron(III) oxide forms. Sulphur dioxide and sulphur trioxide are also produced.

Correct answer: B. Photochemical decomposition.

Light supplies the energy needed to break silver chloride down into silver and chlorine. The white solid turns grey because metallic silver is formed. Silver halides have been used in traditional photographic materials because of this light sensitivity.

Correct answer: C. Fe + CuSO₄ → FeSO₄ + Cu.

Iron is more reactive than copper and displaces it from copper sulphate solution. The blue solution changes towards green as iron(II) sulphate forms, while reddish-brown copper is deposited.

Correct answer: C. Double displacement reaction.

The ions of the two compounds exchange partners. Insoluble barium sulphate forms a white precipitate, so this is also a precipitation reaction.

Correct answer: C. Reaction of quicklime with water.

An exothermic reaction releases energy to the surroundings, often as heat. Quicklime reacting with water releases considerable heat. Decomposition by heating and electrolysis generally require an energy input.

Correct answer: B. A reaction that absorbs energy.

An endothermic reaction takes in energy from its surroundings. For example, thermal decomposition of calcium carbonate requires heating to produce calcium oxide and carbon dioxide.

Correct answer: A.

A combination reaction follows the general pattern A + B → AB. A decomposition reaction follows AB → A + B. For example, calcium oxide and water combine, whereas calcium carbonate decomposes on heating.

Correct answer: C. Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃.

When the solutions are mixed, insoluble lead iodide forms as a bright yellow precipitate. Potassium nitrate remains dissolved in water. The reaction is also a double displacement reaction.

Correct answer: B. Addition of oxygen.

Copper combines with oxygen to form black copper oxide. Copper is therefore oxidised. In electron-transfer terms, oxidation is the loss of electrons.

Correct answer: B. Removal of oxygen.

Copper oxide loses oxygen and forms copper. Therefore, copper oxide is reduced. In electron-transfer terms, reduction is the gain of electrons.

Correct answer: C. Hydrogen.

Hydrogen combines with oxygen to form water, so hydrogen is oxidised. Copper oxide loses oxygen and is reduced to copper. Both processes happen together, making this a redox reaction.

Correct answer: C. Oxidation and reduction occur together.

In a redox reaction, oxidation and reduction are linked. For example, hydrogen gains oxygen from copper oxide to form water, while copper oxide loses oxygen to form copper.

Correct answer: B. Gradual deterioration of a metal through environmental reactions.

Corrosion occurs when metals react with substances in their surroundings. Rusting is a familiar example in which iron reacts in the presence of oxygen and moisture to form hydrated iron(III) oxide. Corrosion can weaken structures and damage equipment.

Correct answer: C. Both oxygen and moisture.

Ordinary rusting requires oxygen and water. Keeping iron dry or covering it with paint, oil or grease reduces exposure to these conditions. Galvanisation protects iron by applying a zinc coating.

Correct answer: B. Iron is coated with zinc.

Zinc forms a protective barrier that reduces contact between iron and moisture or oxygen. Zinc is also more reactive than iron and can provide sacrificial protection if the coating is damaged under suitable conditions.

Correct answer: B. Spoilage of fats and oils, often due to oxidation.

Oxidation of fats and oils can produce unpleasant smells and tastes. Airtight packaging, suitable refrigeration, and protection from excessive heat and light can help slow rancidity.

Correct answer: C. To reduce oxidation of oils and fats.

Nitrogen is relatively unreactive under ordinary packaging conditions. It displaces much of the oxygen in the packet, helping slow oxidation and delay rancidity. This helps preserve flavour and quality during storage.

Correct answer: C. CuO + H₂ → Cu + H₂O.

Hydrogen gains oxygen to form water and is oxidised. Copper oxide loses oxygen to form copper and is reduced. Since oxidation and reduction occur simultaneously, the reaction is a redox reaction.