Chemical Reactions and Equations

In a chemical reaction, the chemical properties of one or more substances change and new substances are formed. Therefore, option B is correct. In a physical change, the size, state, or appearance of a substance may change, but no new substance is formed. In a mixture or solution, substances are generally combined without a chemical reaction.

Evolution of a gas can be a common sign of a chemical reaction because new substances may form during the reaction. For example, an acid reacting with a carbonate produces carbon dioxide gas. In contrast, folding paper, breaking ice, and filtering water are physical changes because no new substance is formed.

The governing concept is the standard structure of a chemical equation. The substances written on the left side of the reaction arrow are the starting materials that undergo chemical change, so they are called reactants. The substances formed on the right side are products. A catalyst may speed up a reaction without being consumed, and a solvent is the medium in which a reaction may occur; neither term names all substances on the left. Therefore, option B is correct.

In a chemical equation, reactants are written on the left of the arrow, while the new substances formed are written on the right. These new substances are called products. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), \(H_2O\) is the product. In contrast, reactants are the substances present before the reaction begins.

According to the law of conservation of mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, a balanced equation has the same number of atoms of each element on the reactant and product sides, so total mass is conserved. The law of conservation of energy also applies to reactions, but balancing an equation specifically requires equal numbers of atoms.

In a balanced chemical equation, the number of atoms of each element is the same on both sides because of the law of conservation of mass. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), both sides contain 4 hydrogen atoms and 2 oxygen atoms. The total number of molecules need not be the same: here there are 3 molecules on the left and 2 on the right.

The governing concept is the classification of chemical reactions by the number and arrangement of reactants and products. A combination reaction occurs when two or more reactants join to produce one single product, represented generally as A + B → AB. Therefore, option A is correct. In decomposition, one compound breaks into multiple products; displacement involves replacement by a more reactive element; double displacement exchanges ions between two compounds.

In the reaction \\(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\\), two reactants, calcium oxide and water, combine to form one product, calcium hydroxide. Therefore, it is a combination reaction. In a decomposition reaction, one compound breaks down into two or more simpler substances, whereas substances are combining here.

The governing concept is the classification of chemical reactions by the number and nature of reactants and products. When one compound splits into two or more simpler substances, the process is a decomposition reaction, represented generally as AB → A + B. Therefore, option B is correct. A combination reaction forms one product, displacement involves replacement, and neutralisation produces salt and water.

In \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\), a single compound, calcium carbonate, breaks down on heating into two simpler substances. Therefore, it is a decomposition reaction; since heat causes it, it is also called thermal decomposition. In contrast, a combination reaction forms one product from two or more reactants.

The governing idea is that decomposition reactions may be named according to the energy source that causes them. If heat makes one compound split into simpler substances, the process is thermal decomposition. For example, calcium carbonate decomposes on heating: CaCO₃ → CaO + CO₂. Thus option A is correct. Electrolytic decomposition requires electricity, photolytic decomposition requires light, and double displacement is a different reaction pattern.

When electric current is passed through water, it decomposes into hydrogen and oxygen: \(2H_2O \rightarrow 2H_2 + O_2\). The breakdown of a compound using electrical energy is called an electrolytic decomposition reaction, so option A is correct. In thermal decomposition, the compound breaks down due to heating; here, electrical energy causes the decomposition.

Silver chloride is photosensitive. In sunlight, it undergoes photolytic decomposition and forms grey metallic silver, causing the colour change: \(2AgCl \xrightarrow{sunlight} 2Ag + Cl_2\). In a combination reaction, substances join to form a product, whereas here a compound is breaking down.

Silver chloride is white. When kept in sunlight, it undergoes photodecomposition, forming metallic silver. This makes its colour change from white to grey.

\(2\text{AgCl}(s) \xrightarrow{\text{sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)\)

Green is incorrect because no green substance is produced in this reaction.

When a more reactive metal removes a less reactive metal from its compound or salt solution, the reaction is called a displacement reaction. For example, zinc displaces copper from copper sulphate solution. A combination reaction forms a product by joining substances, so it does not describe this process. Exam tip: A metal higher in the reactivity series can displace a metal below it from its salt solution.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The released copper is deposited on the iron nail. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Zinc and magnesium are more reactive metals, but they are not present in the solution, so they cannot be deposited.

Calcium oxide reacts with water to form calcium hydroxide and releases heat: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + heat}\). Therefore, it is an exothermic reaction. In option B, heat must be supplied to decompose calcium carbonate, so that reaction is endothermic.

In a double displacement reaction, ions of two ionic compounds exchange partners to form two new compounds. Its general form is \,\(AB + CD \rightarrow AD + CB\)\,. In contrast, in a single displacement reaction, one element replaces another element in a compound.

The governing concept is precipitation in solution reactions. When dissolved reactants react and produce a solid that remains insoluble in the solvent, that solid is called a precipitate; the reaction may be called a precipitation reaction. Therefore, option A is correct. A solute dissolves in a solvent, the solvent does the dissolving, and a catalyst changes reaction rate without being consumed as the product.

This is a double-displacement reaction: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)}\). Barium sulphate is insoluble in water, so it forms a white precipitate. Sodium chloride remains dissolved, whereas barium carbonate would require carbonate ions.

The governing concept is energy transfer during a chemical reaction. When the reacting system gives heat to the surroundings, the reaction is called exothermic and its enthalpy change is negative under the usual convention. Combustion is a familiar example because it releases noticeable heat. Therefore, option A is correct. An endothermic reaction absorbs heat, while displacement and decomposition describe reaction patterns rather than this heat-flow property.

The governing concept is the direction of heat transfer between a reacting system and its surroundings. If the system absorbs heat from the surroundings, the reaction is endothermic; its enthalpy change is positive under the usual convention. Hence option B is correct. An exothermic reaction releases heat, while displacement and neutralisation classify reactions by substance exchange or products, not by absorption of heat.

A fuel reacts with oxygen and releases heat and usually light; this process is called a combustion reaction. For example, methane burns as \(\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}\). Although reactants combine in some combustion reactions, the specific classification for burning a fuel in oxygen is combustion.

The governing concept is a combination and combustion reaction involving oxygen. Magnesium burns with a dazzling white flame and combines with oxygen in air to form white magnesium oxide. The balanced equation is 2Mg + O₂ → 2MgO, so option A is correct. Magnesium chloride would require chlorine, magnesium sulphate would require sulphate ions, and magnesium carbonate is not the direct product of burning magnesium in air.

On exposure to air, a thin layer of magnesium oxide forms on the surface of magnesium ribbon. This coating prevents direct contact between magnesium and oxygen. Cleaning the ribbon with sandpaper removes the oxide layer, so magnesium burns readily and forms magnesium oxide. Cleaning does not increase its melting point or produce oxygen.