Chemical Reactions and Equations

A precipitate is a solid substance formed when two aqueous solutions react and the product is insoluble in the liquid. It settles or remains suspended separately from the solution, so a downward arrow (↓) is placed after its formula in a chemical equation. A gas is represented by an upward arrow (↑), while heat release and light emission require separate symbols or words and are not the meaning of the downward arrow.

Black copper(II) oxide reacts with hydrogen on heating: CuO + H₂ → Cu + H₂O. The black CuO loses oxygen and forms reddish-brown copper metal. Removal of oxygen from a compound is defined as reduction, while hydrogen is oxidised to water. Thus the colour change and reaction demonstrate reduction of copper oxide, not decomposition or freezing of the other substances.

In the reaction \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), hydrogen takes oxygen from copper oxide to form water. Gain of oxygen is oxidation; therefore, hydrogen is oxidised. In terms of electrons, hydrogen’s oxidation number changes from 0 to +1, meaning that it loses electrons. Option C is incorrect because gaining electrons is reduction. Exam tip: Identify oxidation by oxygen gain or electron loss.

The governing concept is oxidation–reduction. Gaining oxygen is oxidation, not reduction. In elemental magnesium, Mg has oxidation number 0; in MgO it becomes +2, so magnesium loses electrons and is oxidised. Oxygen changes from 0 to −2 and is reduced. Therefore option B correctly evaluates the claim. Option A reverses the definitions, C gives the wrong oxidation-number change, and D wrongly calls oxygen oxidation.

A redox reaction involves transfer of electrons between reacting species. When one substance loses electrons, it is oxidised; another substance must accept those electrons and is reduced. Thus the two processes occur simultaneously and are understood together. Option A expresses this conservation and transfer of electrons. Options B, C and D are incorrect because redox reactions involve chemical changes, not merely colour or physical changes.

Iron rusts only when both oxygen and water (moisture) are available. Moisture acts as a medium for the electrochemical reaction, while oxygen participates in the oxidation of iron. The product is hydrated iron(III) oxide, called rust. Option C contains oxygen but lacks the necessary moisture in dry air, so rusting does not occur. Exam tip: Painting, oiling, or greasing prevents rust by blocking air and moisture from reaching iron.

Paint forms a protective barrier between iron and air and moisture. When the layer is scratched, the exposed iron can contact oxygen and water, so oxidation begins and rust may form. The scratch itself does not produce rust; it removes the protective coating and allows the required conditions. Exam tip: Remember that rusting needs both oxygen and moisture.

Rusting is the oxidation of iron in the presence of oxygen and water or moisture. An oil layer forms a protective barrier on the iron surface, reducing the access of air and moisture; therefore, rusting decreases. Option B is incorrect because a catalyst increases the reaction rate, whereas oil prevents the required reactants from reaching the iron surface. Exam tip: Paint, grease and oil prevent rusting by forming barrier coatings.

Oils and fats, especially unsaturated fats, undergo oxidation on exposure to oxygen in air. This produces compounds with an unpleasant smell and taste, causing rancidity. Hydrolysis may break down fats, but oxidation is the main cause of usual rancidity in oils and fats. Exam tip: Nitrogen is filled in chip packets to reduce oxidation.

Oils and fats react with oxygen in air and undergo oxidation. This produces an unpleasant smell and taste, called rancidity. A higher temperature increases the reaction rate, so oily food spoils faster in warm open air. Nitrogen is relatively unreactive and does not convert oils into sweet substances. Exam tip: Connect spoilage of fats and oils with oxidation and rancidity.

Oils and fats can react with oxygen in air and become oxidised. This produces an unpleasant smell and taste, called rancidity. Nitrogen filling displaces oxygen from the packet, so oxidation of the oils slows down. Converting oils into solid fats is hydrogenation, not the effect of nitrogen packaging. Exam tip: Associate nitrogen packing of oily foods with preventing oxidation and rancidity.

This reaction is a single-displacement reaction because zinc, being more reactive than copper, replaces copper from copper sulphate: Zn + CuSO₄ → ZnSO₄ + Cu. It is also a redox reaction: Zn changes from oxidation state 0 to +2 and is oxidised, while Cu²⁺ changes to Cu and is reduced. Hence A is the most complete classification. It is not combination, thermal decomposition, or double displacement.

Gas evolution may indicate the formation of a gaseous product during a chemical reaction. A cold container suggests that the process may have absorbed heat from its surroundings and may therefore be endothermic. Hence, option A is the most careful conclusion. Option B is incorrect because a heat-releasing reaction would generally tend to make the container or surroundings warmer. Exam tip: When evidence is limited, prefer cautious statements such as “may” over statements claiming certainty.

When heat is written on the product side of a chemical equation, the reaction releases heat. The hot container further indicates that energy is being transferred out of the reaction mixture. Therefore, the reaction is exothermic. In an endothermic reaction, heat is shown on the reactant side because the reaction absorbs heat from the surroundings. Exam tip: Heat shown with the products indicates an exothermic reaction.

Iron is more reactive than copper, so it displaces copper from CuSO₄: Fe + CuSO₄ → FeSO₄ + Cu. FeSO₄ causes the green colour, while copper deposits on the nail. A combination reaction forms one product from reactants. Exam tip: use the reactivity series to identify displacement reactions.

During respiration, glucose undergoes controlled, stepwise oxidation with the help of enzymes. Energy is released gradually, and a useful portion is captured in ATP; hence, respiration is an energy-releasing chemical process. Combustion also involves oxidation, but energy is released very rapidly, mainly as heat and light. Option C is incorrect because respiration is exothermic, not endothermic. Exam tip: Remember respiration as “slow, controlled oxidation” and combustion as “rapid oxidation.”

In photosynthesis, plants use light energy to make glucose from carbon dioxide and water. The energy is therefore stored in the chemical bonds of glucose. During cellular respiration, glucose is oxidised and energy is released as ATP and heat. Hence, option A is correct. Option C is a close distractor but is incorrect because respiration does not require an external energy input such as light; it uses the energy stored in glucose. Exam tip: Remember photosynthesis as energy storage and respiration as energy release.

Formation of a new substance is clear evidence of a chemical reaction, and gas evolution is a common observation during such reactions. In a closed container, no matter enters or leaves, so the total mass of reactants and products remains unchanged. Therefore, A is correct. B is incorrect because a physical change does not form a new substance. Exam tip: For a reaction in a closed system, compare total mass before and after the reaction using the law of conservation of mass.

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. In a closed container, the gas remains inside even though it may not be visible, and the precipitate also remains inside. Their physical forms change, but the total amount of matter is conserved. Therefore option A is correct. Gas formation does not remove mass, and precipitate formation does not create extra mass.

The conservation-of-mass law applies to a complete, closed system. In an open container, the gas produced may escape into the surroundings before the final weighing. The balance then measures only the remaining material, not the escaped gas, so the observed mass is lower even though total mass has been conserved. Therefore A is correct. Mass is not destroyed, the reaction need not stop, and a chemical reaction does not become physical merely because gas escapes.