Chemical Reactions and Equations

A colour change during a chemical reaction often indicates the formation of a new substance, because the product may have properties different from those of the reactants. It does not always mean a physical change, nor does it require the temperature to decrease. Exam tip: Remember colour change as a possible sign of a chemical reaction, along with gas evolution, precipitate formation, and temperature change.

When a gas is formed in a chemical reaction, it often escapes from the mixture or solution as bubbles. Therefore, the appearance of bubbles is a common sign of gas evolution. In contrast, filtration is a method of separating a solid from a liquid, not an indication that a gas has formed.

In a chemical equation, the physical state of a substance is indicated by a state symbol in brackets. \((g)\) represents the gaseous state, so option B is correct. In contrast, \((s)\) represents a solid, \((l)\) a liquid, and \((aq)\) a substance dissolved in water as an aqueous solution.

State symbols provide information about the physical form of each substance in a chemical equation. The symbol (l) means liquid, as in H₂O(l), so option A is correct. The symbol (s) denotes a solid, (g) denotes a gas, and (aq) means that the substance is dissolved in water as an aqueous solution. These symbols should not be confused.

The state symbol (aq) means aqueous: the substance is dissolved in water and forms an aqueous solution. Therefore, option D is correct. The symbols (s), (l), and (g) represent solid, liquid, and gaseous states respectively. An aqueous solution may be liquid in appearance, but (l) only identifies a pure liquid state and does not specifically indicate dissolution in water.

When balancing an equation, chemical formulae and subscripts must not be changed because doing so changes the substance. \(H_2O\) is water, whereas \(H_2O_2\) is hydrogen peroxide. The equation is balanced by using coefficients only: \(2H_2 + O_2 \rightarrow 2H_2O\). It has 4 hydrogen atoms and 2 oxygen atoms on each side. Changing \(O_2\) to \(O\) is also incorrect because it changes the chemical formula of oxygen.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The ferrous sulphate formed makes the blue solution light green, and displaced copper deposits on the nail. Option B is incorrect because a displacement reaction occurs between iron and copper sulphate.

During digestion, enzymes convert complex food components such as carbohydrates, proteins, and fats into simpler new substances. Digestion is a chemical change because new substances are formed and their properties differ from those of the original food. Option B describes a physical change, such as breaking food into smaller pieces, in which no new substance is formed.

Milk changes into curd through bacterial fermentation, mainly involving the conversion of lactose into lactic acid. New substances form, the composition changes, and the properties of curd differ from those of milk; therefore, it is a chemical change and option B is correct. It is not merely a change of state, shape, or physical appearance, and the process is not readily reversible.

In a chemical reaction, new substances are formed. Evidence may include a colour change, gas evolution, a temperature change, or formation of a precipitate. Therefore, option A is correct. For example, a precipitate indicates formation of a new insoluble substance, whereas the shape of the container does not prove that a reaction has occurred.

When two solutions are mixed and an insoluble solid appears, the solid is called a precipitate. Its formation usually means that ions from the original solutions have rearranged to produce a new, insoluble substance; therefore, a chemical reaction has occurred. A change of state does not create a new substance, a shape change only alters form, and dissolution produces a solution rather than a precipitate. Hence option A is correct.

The correct answer is two. In \,\(2H_2 + O_2 \rightarrow 2H_2O\), the left side has 4 hydrogen atoms and 2 oxygen atoms. Placing 2 before water gives the right side 4 hydrogen atoms and 2 oxygen atoms as well. A coefficient of 1 would not balance the numbers of hydrogen and oxygen atoms.

Magnesium burns with a bright white flame because it reacts with oxygen in the air. The balanced equation is 2Mg + O₂ → 2MgO, showing that oxygen combines with magnesium to form magnesium oxide. Nitrogen is abundant in air but is not the reacting gas in this standard combustion example. Chlorine and carbon dioxide are not the intended reactants. Therefore option B is correct.

The reaction is \(2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO}\). Here, two reactants, magnesium and oxygen, combine to form a single product, magnesium oxide. Therefore, it is a combination reaction. In a displacement reaction, one element replaces another element from its compound, which does not occur here.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released during this reaction, so the container becomes hot. Therefore, it is an exothermic reaction. In contrast, if heat were absorbed as in option A, the container would tend to become cooler.

In option A, zinc is more reactive than copper. Therefore, zinc displaces copper from \(CuSO_4\), forming \(ZnSO_4\) and \(Cu\). This is a displacement reaction. In contrast, option B is a combination reaction because two substances combine to form one product.

Silver chloride undergoes photodecomposition in sunlight: \(2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)\). The metallic silver formed is grey, so white silver chloride appears grey. Chlorine gas is also produced, but it does not cause the grey colour.

Silver chloride decomposes in sunlight to form silver and chlorine: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). A decomposition caused by light energy is called photolytic decomposition. In contrast, thermal decomposition occurs when a substance decomposes on heating.

On heating, lead nitrate undergoes thermal decomposition to form lead oxide, nitrogen dioxide, and oxygen:
\[2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO}+4\mathrm{NO_2}+\mathrm{O_2}\]
\(\mathrm{NO_2}\) is a brown gas, so nitrogen dioxide is correct. Oxygen is also formed, but it is colourless.

On heating, lead nitrate decomposes: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Since heat is supplied to initiate the reaction, it is called a thermal decomposition reaction. Decomposition caused by electrical energy is electrolysis, which is not involved here.