Chemical Reactions and Equations

A balanced chemical equation follows the law of conservation of atoms and mass. During a chemical reaction, atoms are not created or destroyed; they are rearranged to form new substances. Equal numbers of each element on both sides show this rearrangement and also give the correct stoichiometric proportions. Therefore A is correct. Colour, the physical state of every product, and heat release are not the scientific reasons for balancing an equation.

A subscript gives the fixed ratio of atoms in one molecule or formula unit. For example, H₂O and H₂O₂ are different substances because their atomic compositions differ. Changing a subscript while balancing would therefore change the reactant or product itself, not merely its amount. The correct method is to change coefficients placed before formulas, which alter the number of particles without changing identity. Hence A is correct; B, C and D do not describe the consequence.

A skeletal equation gives the correct chemical formulae of reactants and products, but the number of atoms of each element may not be equal on the two sides. In a balanced equation, only the coefficients are adjusted to make the number of atoms of every element equal on both sides. Thus, it obeys the law of conservation of mass. Option C is incorrect because chemical formulae are also written in a skeletal equation. Exam tip: while balancing, do not change subscripts in formulae; change only coefficients.

Writing the correct chemical formulas identifies the substances, but a complete equation must also obey conservation of atoms. The number of atoms of every element must be equal on the reactant and product sides. If even one element has unequal counts, the equation is not balanced and suitable coefficients must be added without changing subscripts. Thus A is correct. The other options confuse an incomplete equation with no reaction or no substances.

The chemical formula of water is \(H_2O\). Each water molecule contains 2 hydrogen atoms and 1 oxygen atom, making 3 atoms altogether. Therefore, three water molecules contain \(3 \times 3 = 9\) atoms. Option A, 6, counts only the hydrogen atoms in three water molecules, not all atoms. Exam tip: Add the atoms shown in the formula and multiply by the number of molecules.

The formula of magnesium oxide is MgO. It contains one magnesium atom and one oxygen atom per formula unit, so the Mg:O ratio is 1:1. Five formula units contain five magnesium atoms and five oxygen atoms; simplifying 5:5 gives 1:1. Therefore option A is correct. The number five increases both quantities equally, so it does not change the ratio. Options B and C count only one element correctly, while D does not follow the formula.

Magnesium reacts with oxygen in air to form a thin, stable layer of magnesium oxide on its surface. This coating prevents fresh magnesium from contacting oxygen easily and can delay or weaken ignition. Rubbing the ribbon with sandpaper removes the oxide layer, exposing clean magnesium so the combustion reaction starts clearly. Oxygen is still needed and remains present; liquid magnesium and white powder are not the obstacle being removed.

The reaction is represented as 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants, magnesium and oxygen, form a single product, magnesium oxide. It is also oxidation because oxygen is added to magnesium during the reaction. The reaction is not dissolution, decomposition, or precipitation, so the alternatives do not describe the observed chemical change.

When quicklime reacts with water, the equation is CaO + H₂O → Ca(OH)₂ + heat. The product is calcium hydroxide, commonly called slaked lime. Because heat is released rather than absorbed, the process is exothermic. Calcium carbonate would require carbon dioxide, while calcium chloride and calcium sulphate require different reactants, so those options do not fit this reaction.

On heating, calcium carbonate decomposes according to CaCO₃ → CaO + CO₂. A single compound breaks into two simpler products: solid calcium oxide and gaseous carbon dioxide. Since heat is supplied to cause the breakdown, the reaction is specifically called thermal decomposition. It is not combination, because one product is not formed, and no exchange or displacement of ions occurs.

The thermal decomposition equation is CaCO₃(s) → CaO(s) + CO₂(g). Calcium oxide remains behind as the solid residue, while carbon dioxide escapes as a gas. The products are determined by the composition of the carbonate: the calcium forms the oxide and the carbonate part releases carbon dioxide. Hydroxide, sulphate, chloride, hydrogen, oxygen, and nitrogen are not products of this reaction.

Carbon dioxide turns lime water milky because it reacts with aqueous calcium hydroxide to form insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O. The suspended calcium carbonate particles produce the milky appearance. Hydrogen, oxygen, and nitrogen do not normally give this characteristic limewater test, so they cannot explain the observed precipitate.

Heating lead nitrate causes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide is the characteristic brown gas, while oxygen is colourless. The order in option A matches the question’s order—brown gas followed by colourless gas. Option B reverses the gases, and the other choices contain gases not produced by lead nitrate decomposition.

Lead nitrate decomposes on strong heating according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide escapes as a brown gas and oxygen as a colourless gas, leaving lead oxide as the solid residue. Lead oxide is yellow when hot and may appear differently on cooling. Chloride, sulphate, and carbonate residues require other anions and are not formed here.

During heating, lead nitrate produces nitrogen dioxide, oxygen, and lead oxide: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide has a characteristic brown colour, whereas oxygen is a colourless gas. Therefore the brown fumes cannot be oxygen; they are nitrogen dioxide. Oxygen may also be produced, but its colourless appearance distinguishes it from the brown gas.

Silver chloride undergoes photochemical decomposition in sunlight: 2AgCl(s) → 2Ag(s) + Cl₂(g). The light supplies energy for the compound to split, producing metallic silver and chlorine gas. Finely divided silver is grey, so its formation causes the original white silver chloride to become grey or dark. Chlorine solidification, oxygen formation, and water formation do not explain this observation.

Silver bromide is a photosensitive salt, so it undergoes a chemical change when exposed to light. In photographic film, this light-induced decomposition helps record an image; the exposed portions later produce metallic silver during development. Therefore, photography is the correct use. Galvanisation protects iron with zinc, water purification removes impurities, and melting metals involves heating rather than photochemical change.

Many silver salts, especially silver halides, are photosensitive. Light supplies energy that can break their chemical bonds and cause photolytic decomposition, sometimes producing silver and another product. A dark or amber bottle absorbs or blocks much of the incident light and therefore improves storage stability. It does not increase heat, solubility, or convert a precipitate into a gas, so option A is correct.

The balanced decomposition equation is 2H₂O → 2H₂ + O₂. Under the same conditions of temperature and pressure, gas volumes follow the coefficient ratio, so hydrogen : oxygen = 2 : 1. If hydrogen occupies 36 mL, oxygen occupies 36 ÷ 2 = 18 mL. Thus option A is correct; 36 mL ignores the ratio, 72 mL reverses it, and 12 mL uses an incorrect factor.

During electrolysis, water decomposes according to 2H₂O → 2H₂ + O₂. Oxide ions or water molecules are oxidised at the anode, producing oxygen gas, while hydrogen is formed at the cathode. The collected hydrogen volume is twice the oxygen volume. Therefore oxygen is the gas at the anode, making option A correct; nitrogen and carbon dioxide are not products of water electrolysis.