Chemical Reactions and Equations

Correct answer: A. When the substance is dissolved in water

The state symbol (aq) means aqueous: the substance is dissolved in water and exists as part of an aqueous solution. For example, NaCl(aq) represents sodium chloride dissolved in water. Therefore option A is correct. The symbols (s), (l), and (g) separately indicate solid, pure liquid, and gaseous states; (aq) is not simply a general symbol for every liquid.

Correct answer: A. Only a change in the position of a substance

A chemical reaction produces one or more new substances, and observations such as gas evolution, a colour change, or precipitate formation may provide evidence of that change. Merely moving a substance from one place to another changes its position, not its composition or identity. It is therefore a physical change and is not evidence by itself that a chemical reaction occurred. Option A is correct.

Correct answer: A. Oxidation

According to the classical definitions of redox reactions, removal of hydrogen from a substance is oxidation, while addition of hydrogen is reduction. For example, removal of hydrogen from hydrogen sulfide during oxidation changes the substance into sulfur or another oxidised product. Neutralisation concerns acid–base reaction, and displacement describes replacement of one element by another, so neither explains hydrogen removal here.

Correct answer: A. A new reddish-brown substance, hydrated iron(III) oxide, is formed.

Rusting is a chemical change because iron reacts slowly with oxygen in the presence of moisture and produces a new reddish-brown material, commonly represented as hydrated iron(III) oxide. The new substance has properties different from those of iron and cannot be removed simply by reversing a shape or position change. Therefore, options B, C and D describe possible physical changes, not the chemical transformation involved in rusting.

Correct answer: A. A gas is evolving

In the conventional notation used in school chemical equations, an upward arrow (↑) beside a product indicates that a gas is evolved during the reaction. For example, acid reacting with a carbonate may produce carbon dioxide gas, shown with this symbol. A downward arrow (↓), not an upward arrow, is commonly used to indicate an insoluble precipitate. The symbols do not directly represent heat absorption or dissolution.

Correct answer: A. Formation of a precipitate

A downward arrow (↓) in a chemical equation conventionally shows that an insoluble solid has formed from a solution and separated as a precipitate. For example, mixing suitable soluble salts can produce solid barium sulfate, written with a downward arrow. An upward arrow (↑) is commonly used for gas evolution, while heat and light require separate conditions or descriptions rather than this precipitation symbol.

Correct answer: A. Water freezing into ice

The governing concept is the difference between physical and chemical change. When water freezes, its particles become arranged in a solid form, but its chemical composition remains H₂O. Thus only the state changes from liquid to solid and no new substance is produced, making option A correct. Rusting, curdling, and burning involve chemical changes that produce substances with new properties.

Correct answer: A. Evolution of a gas

A common visible indicator of a chemical reaction is the evolution of a gas. If a gaseous product is formed in the liquid mixture, it escapes as bubbles, so option A is the best answer. Melting changes the physical state of a solid, a precipitate is an insoluble solid that settles or remains suspended, and a colour change alone does not specifically explain bubbles. Therefore B, C, and D do not fit.

Correct answer: A. Exothermic reaction

The governing concept is the direction of energy transfer. In an exothermic reaction, the chemical system releases energy to its surroundings; that energy may appear as heat and, in some reactions, as light. Because both heat and light are released here, option A is correct. An endothermic reaction absorbs energy, while displacement and double-displacement describe reactant replacement patterns, not necessarily whether heat or light is released.

Correct answer: A. Physical change

The governing criterion is whether the chemical composition changes. In a physical change, a substance may be cut, bent, crushed, reshaped, or changed in state while its composition remains unchanged. Since only the shape changes and no new substance forms, option A is correct. A chemical change forms substances with new compositions; redox and precipitation are particular chemical processes and therefore do not describe this situation.

Correct answer: B. The substance is dissolved in water

The symbol (aq) means aqueous: the substance is present in a solution made with water, usually dissolved in water. For example, NaCl(aq) represents sodium chloride dispersed as ions in water. Thus option B is correct. The symbols (s), (l), and (g) separately indicate solid, liquid, and gaseous states, so options A, C, and D confuse aqueous solution with ordinary physical states.

Correct answer: A. When gas forms and evolves

In a chemical equation, an upward arrow is commonly used to show that a gaseous product is evolved and escapes from the reaction mixture. Therefore option A is correct. A precipitate is an insoluble solid and is conventionally indicated by a downward arrow, not an upward arrow. Freezing and filtration describe physical processes and do not determine this gas-evolution symbol.

Correct answer: B. Oxidation

The addition of oxygen to a substance is the classical definition of oxidation. For example, magnesium combines with oxygen to form magnesium oxide: 2Mg + O2 → 2MgO. Thus option B is correct. Reduction generally involves removal of oxygen or addition of hydrogen, while melting and filtration are physical processes and do not describe the chemical change caused by oxygen addition.

Correct answer: D. Reduction

In the classical school-level definition, addition of hydrogen to a substance is reduction. For example, hydrogen can remove oxygen from a metal oxide, converting it to the metal. Therefore option D is correct. Oxidation is commonly associated with addition of oxygen or removal of hydrogen. Precipitation forms an insoluble solid from a solution, and evaporation changes liquid into vapour; neither describes hydrogen addition.

Correct answer: A. Oxidation

Oxidation can be described as addition of oxygen or removal of hydrogen from a substance. Therefore, when hydrogen is removed, the process is generally identified as oxidation, making option A correct. Reduction is the opposite in this classical framework: it commonly involves addition of hydrogen or removal of oxygen. Double displacement and precipitation describe particular reaction patterns, not the specific removal of hydrogen.

Correct answer: C. Only the state changes

Ice and liquid water are the same substance, H₂O. During melting, the particles gain energy and change from an ordered solid state to a liquid state, but their chemical composition and identity remain unchanged. The process is reversible by freezing, so it is a physical change. Option C is correct; no new gas, precipitate, or substance is produced.

Correct answer: A. Oxidation

The governing concept is oxidation–reduction. Oxidation is identified by the addition of oxygen or the removal of hydrogen from a substance; loss of electrons is another related definition. Therefore, the reaction described is oxidation, so option A is correct. Reduction is the opposite process, involving removal of oxygen or addition of hydrogen. Displacement and neutralisation describe different reaction patterns.

Correct answer: B. The reaction releases heat

In a chemical equation, substances written on the product side are formed during the reaction. Therefore, when heat appears on the product side, heat is produced and released to the surroundings. Such a process is called an exothermic reaction, so option B is correct. If heat were written with the reactants, it would indicate that heat is required or absorbed. A precipitate is an insoluble solid, not a meaning of heat placement.

Correct answer: A. Heat is being absorbed

A reactant is a substance consumed or required in a reaction. Thus, when heat is written on the reactant side, the reaction requires an input of heat and absorbs it from an external source. This indicates an endothermic process, making option A correct. Heat is not a precipitate or a gas, and placing it on the reactant side does not mean that heat is released; release is represented by writing heat among the products.

Correct answer: A. Upward arrow indicates gas and downward arrow indicates precipitate

In conventional chemical-equation notation, an upward arrow (↑) placed beside a product indicates that a gas is evolved, while a downward arrow (↓) indicates that an insoluble solid precipitate has formed. Thus option A gives both meanings correctly. These arrows do not represent heat, light, liquid, or metal. State symbols such as (g), (l), (s), and (aq) provide different information about physical state.

Correct answer: A. When the substance is dissolved in water

The state symbol (aq) means aqueous. It is written when a substance is dissolved in water and exists as part of an aqueous solution, for example NaCl(aq). Therefore option A is correct. A solid substance is shown by (s), a liquid by (l), and a gas by (g). Light emission is not represented by a physical-state symbol, so options B, C and D do not give the meaning of (aq).

Correct answer: A. Reactants on the left and products on the right

A chemical equation represents a reaction from starting substances to formed substances. The starting substances are called reactants, while the new substances produced are called products. By convention, reactants are written on the left side of the arrow and products on the right side; conditions may be written above the arrow. Therefore, option A is correct, whereas the other options reverse or misstate the standard arrangement.

Correct answer: B. Placing suitable coefficients

Balancing a chemical equation means making the number of atoms of each element equal on both sides while keeping the identities of the substances unchanged. This is done by placing suitable whole-number coefficients before chemical formulae. Changing a subscript changes the substance itself, and removing a reactant or reversing the arrow changes the reaction rather than balancing it. Therefore, option B is the correct method.

Correct answer: A. Oxygen is removed

Copper oxide contains oxygen chemically combined with copper. When it changes into copper, that oxygen is removed from the compound, for example in CuO + H₂ → Cu + H₂O. Removal of oxygen is the traditional school-level definition of reduction. Addition of oxygen describes oxidation, while hydrogen removal and precipitate formation are not the defining features here. Therefore, option A is correct.

Correct answer: C. Hydrogen is added to it

Reduction can be described in several equivalent ways. At this level, it includes removal of oxygen, addition of hydrogen, gain of electrons, or a decrease in oxidation number. Among the given choices, addition of hydrogen is the direct reduction criterion, so option C is correct. Oxygen addition indicates oxidation, hydrogen removal indicates oxidation, and dissolving in water alone does not identify a redox change.