Chemical Reactions and Equations

Photosynthesis is a chemical reaction because reactants are converted into new products. Using light energy and chlorophyll, plants combine carbon dioxide and water to form glucose and release oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in simplified form. Option B identifies new-substance formation; leaf shape or evaporation does not establish this reaction.

A subscript tells how many atoms of an element are present in one formula unit or molecule. For example, H₂O and H₂O₂ are different substances because their compositions differ. Changing a subscript during balancing would therefore change the reactant or product itself and violate the original reaction. Only coefficients placed before formulae may be changed to equalise atom counts.

The law of conservation of mass states that atoms are not created or destroyed in a chemical reaction. When an equation is balanced, the coefficient-adjusted number of atoms of every element is identical on the reactant and product sides. Consequently, the total mass is conserved, although atoms may be rearranged into new substances. Therefore option D is correct; physical state or colour is not the basis of balancing.

Balancing a skeletal equation means making the number of atoms of every element equal on both sides while preserving the identities of the substances. This is done by adding suitable whole-number coefficients before formulae, such as changing H₂ + O₂ → H₂O to 2H₂ + O₂ → 2H₂O. Subscripts must not be altered because that would create different substances. Hence A is correct.

If the outside of a reaction container feels cold, the reaction mixture is taking energy from the surroundings faster than it releases energy to them. This indicates an endothermic reaction, in which heat is absorbed. Therefore option B is the best conclusion. A released-heat process is exothermic and would generally warm the container; coldness alone cannot prove combustion or rusting.

A container becoming hot indicates that energy is being transferred from the reacting system to its surroundings. This is evidence that heat has been released and the reaction is likely exothermic, so option C is correct. However, the observation alone does not prove combustion; other exothermic reactions can also warm the container. It also does not represent mere state change or absence of reaction.

The formation of bubbles may indicate that a gas is being produced or released, so the most careful conclusion is that gas formation is possible. Bubbles do not by themselves prove that a precipitate has formed; a precipitate is usually observed as a solid. Bubbles can also arise on heating or when dissolved air escapes, so bubbles alone do not conclusively prove a chemical reaction.

In a chemical equation, an upward arrow (↑) indicates that a gaseous product is evolved during the reaction. For example, an acid reacting with a carbonate may release carbon dioxide gas, which can be shown by this symbol. A downward arrow indicates a precipitate, while heat is written over the reaction arrow rather than represented by an upward arrow. Therefore, option A is correct.

A downward arrow (↓) in a chemical equation shows that an insoluble solid has formed from the reacting solutions. This solid settles out and is called a precipitate; for example, silver chloride may form as a precipitate in a reaction involving soluble silver and chloride compounds. An upward arrow represents gas evolution, whereas combustion and light emission require different descriptions. Thus, option B is correct.

The aqueous state symbol, written as (aq), is used when a substance is dissolved in water and forms an aqueous solution. It does not mean that the substance is merely liquid; water is specifically the solvent. A solid is marked (s), a gas is marked (g), and a pure liquid is marked (l). Therefore, the condition described in option C is the correct one.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The blue copper sulphate solution turns green due to iron sulphate, and copper deposits on the nail. Therefore, this is a displacement reaction. In contrast, a combination reaction forms one product from two or more reactants.

When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). This is thermal decomposition because heat causes the breakdown, not light. Silver chloride and silver bromide undergo photolytic decomposition in sunlight. In contrast, iron reacting with copper sulfate is a displacement reaction.

Red-hot iron reacts with steam to form magnetite, \(Fe_3O_4\), and hydrogen. The balanced equation is \(3Fe + 4H_2O \to Fe_3O_4 + 4H_2\); each side contains 3 Fe, 4 O, and 8 H atoms. Option D balances the atom count but still incorrectly identifies the iron oxide as \(FeO\).

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Reddish-brown copper deposits on the nail, and green iron sulphate solution forms. This is not a combination reaction because it produces two products rather than a single product.

In a chemical reaction, reactants are converted into new substances with properties different from those of the reactants. Therefore, the formation of new substances with new properties is the most reliable indication of a chemical reaction. In contrast, changing position, shape, or container usually forms no new substance; these are physical changes.

In a closed container, no matter enters or leaves. During a chemical reaction, atoms are only rearranged, so the total mass of the reactants equals the total mass of the products. This illustrates the law of conservation of mass. The law of definite proportions concerns the fixed mass ratio of elements in a compound, not the total mass before and after a reaction.

An insoluble solid formed when two aqueous solutions are mixed is called a precipitate. Its formation indicates a precipitation reaction. In contrast, combustion involves burning in oxygen, whereas photolytic and electrolytic decomposition involve breakdown of a compound by light and electricity, respectively.

A change in colour, formation of a gas, and release of heat are common indicators of a chemical reaction. When these observations occur together, there is a strong possibility that a chemical reaction has taken place. In contrast, a change only in position is a physical change and does not indicate formation of a new substance.

According to the law of conservation of mass, total mass is neither created nor destroyed in a chemical reaction. Since the container is closed, no matter leaves or enters it; therefore, the total mass of the products equals the total mass of the reactants, which is 50 g. A value such as 25 g or 0 g would imply that mass has disappeared, which is not possible in this situation.

The balanced equation is \(2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}\). The reactant side has 4 hydrogen atoms and 2 oxygen atoms. \(2\mathrm{H}_2\mathrm{O}\) also contains 4 hydrogen atoms and 2 oxygen atoms, so 2 water molecules are formed. Writing only 1 water molecule would not balance the atoms.