Chemical Reactions and Equations

The governing concept is the classical definition of oxidation in terms of oxygen and hydrogen. Oxidation means addition of oxygen to a substance or removal of hydrogen from it. For example, conversion of copper to copper oxide involves addition of oxygen. Therefore, option A is correct. Option B defines reduction, while dissolving and melting are physical changes, not necessarily oxidation.

The governing concept is the classical meaning of reduction in a redox reaction. Reduction occurs when oxygen is removed from a substance or hydrogen is added to it. For example, removal of oxygen from a metal oxide can produce the metal. Hence option A is correct. Option B describes oxidation, whereas melting and gas bubbles may be physical or other reaction observations and do not define reduction.

A reaction in which one substance loses electrons and is oxidised while another gains electrons and is reduced is called a redox reaction. Oxidation and reduction occur together. For example, in
\(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\), zinc is oxidised and copper ions are reduced. A neutralisation reaction occurs between an acid and a base; oxidation and reduction are not necessary in it.

Iron reacts with oxygen and moisture to form rust, a hydrated iron oxide. Since rust is a new substance with properties different from iron, rusting is a chemical change. In contrast, a physical change alters only the form or state of a substance without producing a new substance.

Rusting of iron requires both oxygen and water (moisture). Air supplies oxygen, while moisture helps the reaction occur; therefore, iron rusts when exposed to air and moisture. Heat alone is not sufficient for rusting. Exam tip: Remember the two essential conditions for rusting—oxygen and water.

Oils and fats slowly undergo oxidation when exposed to oxygen in air, producing an unpleasant smell and taste. This process is called rancidity. Corrosion is the deterioration of metals due to environmental action, so it does not apply to oils and fats.

Rancidity occurs when fats and oils undergo oxidation, producing an unpleasant smell and taste. Antioxidants slow down oxidation and are therefore used to prevent rancidity. Preservatives mainly inhibit the growth of microorganisms; they do not directly prevent oxidation of fats and oils.

Chips contain oils that can react with oxygen and undergo oxidation, causing rancidity and an unpleasant smell and taste. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and prevents oxidation. Option B is incorrect because added moisture makes chips lose their crispness.

Bacteria convert lactose in milk into lactic acid. This coagulates milk proteins and forms curd, which has properties different from those of milk. Curd cannot be converted back into milk by simple physical methods. Therefore, this is a chemical change; in a physical change, no new substance is formed.

A colour change in a chemical reaction can indicate the formation of a new substance because the products may have properties different from those of the reactants. A change only in shape, state, or position does not usually form a new substance; these can be physical changes. Exam tip: Colour change, gas evolution, temperature change, and precipitate formation are common signs of a chemical reaction.

In respiration, glucose is oxidised to form carbon dioxide, water, and energy: \(\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy}\). Because energy is released, respiration is an exothermic reaction. In contrast, an endothermic reaction absorbs energy from the surroundings.

Natural gas consists mainly of methane. On burning in oxygen, methane forms carbon dioxide and water and releases heat and light: \(\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}\). A reaction in which a substance undergoes rapid oxidation in oxygen with release of energy is called combustion. Hence, this is a combustion reaction. In contrast, a decomposition reaction involves one compound breaking into simpler substances; here methane reacts with oxygen rather than breaking down.

Direct answer: A, (s), is correct. Chemical equations often use state symbols in parentheses after a chemical formula to show the physical state of a substance. The letter s stands for solid, so CaCO₃(s) means solid calcium carbonate. This information helps the reader understand whether a substance is a solid, liquid, gas, or dissolved in water during the reaction. Option B, (l), means liquid, as in H₂O(l). Option C, (g), means gas, as in O₂(g) or CO₂(g). Option D, (aq), means aqueous: the substance is dissolved in water, as in NaCl(aq). The state symbol is not a coefficient and does not show the number of particles; it gives the physical condition. A useful memory cue is “s = solid, l = liquid, g = gas.” Be careful not to confuse (aq) with a general liquid: aqueous specifically means dissolved in water, whereas (l) means the substance itself is in the liquid state.

In a chemical change, a new substance with different properties is formed. Therefore, formation of a new substance indicates a chemical change. In contrast, melting of ice is only a change of state: it remains water and no new substance is produced. Evaporation of water and breaking of glass are also physical changes.

Direct answer: D, (aq), is correct. In a chemical equation, (aq) is the state symbol for aqueous. It tells us that the substance is dissolved in water, not merely that it is a liquid. For example, NaCl(aq) represents sodium chloride dissolved in water. Option A, (s), means solid; a substance such as calcium carbonate may be written CaCO₃(s). Option B, (l), means that the substance itself is in the liquid state, such as H₂O(l), and does not specifically mean a solute dissolved in water. Option C, (g), means gas, such as CO₂(g). State symbols give physical-state information and are placed after the formula in parentheses. A common confusion is between (l) and (aq): both may involve a fluid system, but (l) identifies a liquid substance, while (aq) identifies a substance present in water as a solution. Memory cue: “aq = aqua = water.” This helps connect the abbreviation with an aqueous solution.

In chemical equations, an upward arrow (↑) generally indicates that a gas is evolved during the reaction and escapes from the reaction mixture. For example, an acid reacting with a carbonate releases carbon dioxide gas. The arrow does not mean that a solid forms, water is produced, or heat is removed; those changes require different descriptions or symbols.

A downward arrow (↓) in a chemical equation generally indicates the formation of a precipitate, which is an insoluble solid produced in a solution. For example, mixing silver nitrate and sodium chloride forms solid silver chloride. The downward arrow therefore represents a solid settling out, whereas gas evolution is conventionally shown by an upward arrow.

A chemical equation in which the number of atoms of one or more elements differs on the reactant and product sides is called a skeletal equation. In a balanced chemical equation, the number of atoms of every element is equal on both sides; therefore, option B is not a skeletal equation.

A chemical formula shows the elements in a substance and their fixed ratio. Changing a subscript changes the identity of the substance; for example, H₂O is water, whereas H₂O₂ is hydrogen peroxide. Therefore, an equation is balanced by changing coefficients only, because coefficients change the amount of a substance, not its identity.

The addition of oxygen to a substance is called oxidation. For instance, magnesium combines with oxygen according to 2Mg + O₂ → 2MgO, so magnesium is oxidised. Reduction is generally associated with removal of oxygen or addition of hydrogen. Neutralisation involves acid and base, while displacement involves replacement of one element by another.

Removal of oxygen from a substance is called reduction. For example, in the reaction CuO + H₂ → Cu + H₂O, copper(II) oxide loses oxygen and is reduced to copper. Oxidation is the opposite idea when oxygen is added. Neutralisation concerns acid-base reactions, and decomposition describes one compound splitting into simpler substances, so neither is the best answer.

The balanced equation is 2H₂ + O₂ → 2H₂O. Two reactants, hydrogen and oxygen, combine to produce only one compound, water; this satisfies the definition of a combination reaction. A decomposition reaction starts with one compound and gives multiple products. Displacement reactions involve replacement, while double displacement exchanges ions between two compounds.

In photosynthesis, carbon dioxide reacts with water in the presence of sunlight and chlorophyll to form glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Thus, carbon dioxide is a gaseous reactant. Oxygen is mainly released as a product, while nitrogen and hydrogen are not the gaseous reactants in this standard equation.

A rise or fall in temperature indicates an energy change in the reaction. In an exothermic reaction, heat is released, so the temperature may rise; in an endothermic reaction, heat is absorbed, so the temperature may fall. A colour change can also be evidence of a chemical reaction, but a temperature change specifically indicates the release or absorption of heat, that is, an energy change.

An equation that states reactants and products by their names rather than by chemical formulae is called a word equation. For example: magnesium + oxygen → magnesium oxide. In contrast, a balanced chemical equation has the same number of atoms of each element on both sides.