Chemical Reactions and Equations

Chips contain oils and fats that can react with oxygen in air. This oxidation causes rancidity, producing an unpleasant smell and taste. Nitrogen is relatively unreactive and reduces the amount of oxygen in the packet. Therefore, option A is correct; nitrogen does not make chips sweet or increase their moisture.

Antioxidants slow down oxidation reactions. Therefore, they help prevent or reduce rancidity in foods containing oils and fats. Freezing and melting are physical changes of state, while filtration is a method of separating mixtures; antioxidants do not control these processes.

In the usual school notation for a chemical equation, a downward arrow indicates that an insoluble solid has separated from the solution. Such a solid is called a precipitate, so the symbol denotes formation of a precipitate. A gas is commonly shown by an upward arrow, while heat and light are written as energy conditions or products rather than represented by this downward arrow. Hence option B is correct.

In a chemical equation, an upward arrow
(


\uparrow


) written with a product indicates that a gas is formed and evolves. For example, hydrogen gas is produced when zinc reacts with dilute hydrochloric acid. In contrast, a downward arrow
(


\downarrow


) indicates the formation of a precipitate.

The state symbol (s) denotes that a substance is in the solid state. In the same notation, (l) means liquid, (g) means gas, and (aq) means the substance is dissolved in water as an aqueous solution. For example, CaCO3(s) shows solid calcium carbonate. These symbols describe physical state and do not by themselves indicate whether a chemical reaction has occurred. Therefore option A is correct.

The lowercase symbol (l) is used in a chemical equation to show that a substance is in the liquid state. The other symbols have different meanings: (s) denotes a solid, (g) denotes a gas, and (aq) denotes a substance dissolved in water. Thus the letter l is not a numerical one or an abbreviation for gas; it specifically identifies liquid state. Option B is correct.

In a chemical equation, the symbol (g) placed after a formula indicates that the substance is in the gaseous state. The symbols (s) and (l) represent solid and liquid states, respectively, while (aq) means the substance is dissolved in water. Thus (aq) does not mean gas; it identifies an aqueous solution. Therefore option C correctly represents the gaseous state.

The symbol (aq) means aqueous, so it is used when a substance is dissolved in water. For example, NaCl(aq) represents sodium chloride present as ions in an aqueous solution. The symbols (s), (l), and (g) instead indicate solid, liquid, and gaseous states. Therefore (aq) is the only option that describes a substance dissolved in water, making option D correct.

A chemical formula represents the elements in a substance and their ratio of atoms. For example, changing H₂O to H₂O₂ changes water into hydrogen peroxide. Therefore, an equation is balanced only by changing coefficients, as in 2H₂ + O₂ → 2H₂O, not by changing the subscripts in formulae.

A balanced chemical equation follows the law of conservation of mass. Therefore, the number of atoms of each element is the same on both sides of the equation. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), there are 4 hydrogen atoms and 2 oxygen atoms on each side. The physical states of reactants and products do not need to be the same; they may be solids, liquids, or gases.

The law of conservation of mass states that the total mass of the reactants equals the total mass of the products in a chemical reaction. Substances may change form, but mass is neither created nor destroyed. Therefore, a chemical equation is balanced so that the number of atoms on both sides is equal. A colour change or gas formation occurs only in some reactions, not in every reaction.

A word equation gives the names of reactants and products. To convert it into a chemical equation, the correct chemical formula of each substance is written; for example, H₂ for hydrogen and O₂ for oxygen. Names of apparatus or prices of substances are not part of a chemical equation.

A skeletal equation shows the chemical formulae of reactants and products, but the number of atoms of each element may not be equal on both sides. It must be balanced by placing suitable coefficients to obey the law of conservation of mass. A catalyst may change the rate of a reaction, but it does not balance an equation.

Combustion is an exothermic oxidation reaction. When a fuel reacts with oxygen, chemical energy stored in the fuel is converted mainly into heat and often into visible light, such as the flame of a candle. Oxygen is a reactant consumed during combustion, not a form in which energy is released. Thus options B, C, and D confuse a reactant with energy output. Option A is correct.

If the surroundings become cooler during a reaction, the reaction may be absorbing heat from them. A reaction that takes in heat is called endothermic, so option A is the best answer. An exothermic reaction releases heat and generally warms its surroundings. Combustion is a particular type of usually exothermic reaction, while rusting is a specific slow oxidation process; neither term directly describes heat absorption.

The governing concept is heat exchange during a chemical reaction. If the mixture becomes warmer without receiving heat from an external source, energy has been released by the reacting substances to their surroundings. Such a reaction is called exothermic, so option B is correct. An endothermic reaction absorbs heat, while photochemical and electrolytic reactions are classified by light or electricity, not simply by a temperature rise.

When iron rusts, it reacts with oxygen and moisture to form a new substance, hydrated iron oxide (rust). Therefore, rusting is a chemical change. In contrast, boiling water and melting wax are only changes of state, while dissolved salt can be recovered from water by evaporation.

The governing concept is the reactivity series. Iron is more reactive than copper, so it displaces copper from copper sulphate: Fe + CuSO₄ → FeSO₄ + Cu. Ferrous sulphate makes the solution green, and copper appears as a reddish-brown coating on the nail; therefore option A is correct. The other observations do not match the products or colours of this reaction.

The governing idea is that chemical reactions may be identified through observable evidence, but no single sign is universal. A reaction may produce a colour change, gas, precipitate, temperature change, or smell, and another reaction may show a different combination. Therefore option A is correct: several relevant observations should be considered before concluding that a reaction has occurred. The other choices incorrectly reject all evidence or overvalue colour alone.

A balanced chemical equation follows the law of conservation of mass. Therefore, the number of atoms of each element is equal on the reactant and product sides. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), both sides have 4 hydrogen atoms and 2 oxygen atoms. It is not enough for only the total number of atoms to match; each element must be balanced separately.