Matter, Atoms and Molecules RRB PYQ

Statement A is true; a balanced chemical equation ensures that the number of atoms of each element is the same on both sides of the equation, conserving mass. Statement B is also true; an unbalanced chemical equation indicates that there is an unequal number of atoms of one or more elements in the reactants and products, which does not comply with the law of conservation of mass.

The formation of water from hydrogen and oxygen is a combination reaction. In this process, two substances, hydrogen gas (H2) and oxygen gas (O2), combine to form a single product, water (H2O). This reaction is exothermic, releasing energy in the form of heat and light, and follows the equation: 2H2 + O2 → 2H2O.

A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides, not protons. Hence, statement A is false. Similarly, an unbalanced chemical equation has an unequal number of atoms, not protons, for one or more elements, so statement B is also false.

A combination reaction occurs when two or more elements or compounds combine to form a single product. The symbolic form of this reaction is represented as A + B → C. This type of reaction is often exothermic, releasing new bonds' energy. For example, calcium oxide (CaO) and water (H2O) form calcium hydroxide Ca(OH)2 in a combination reaction.

In an exothermic reaction, energy is released in the form of heat to the surroundings. This means that the products of the reaction have lower energy than the reactants, resulting in a temperature increase in the surrounding environment. Combustion and many oxidation reactions are common examples of exothermic reactions.

A precipitate is an insoluble substance that forms during a chemical reaction in a solution. It occurs when the solubility of a substance decreases, either due to changes in temperature or the addition of a precipitating agent. As the solution becomes supersaturated, the solid separates from the solution, forming a precipitate.

Electrolytic decomposition is a chemical reaction where a single substance breaks down into two or more simpler substances upon application of electrical energy. In this process, an electric current is passed through a compound—typically in its molten state or as an aqueous solution—to provide the necessary activation energy to break its chemical bonds.

In the given double displacement reaction, Ag and Na ions exchange partners. Initially, Cl is with Na, so after the reaction, it pairs with Ag. Similarly, NO3 is initially with Ag, so after the reaction, it pairs with Na. The correct balanced equation is: AgNO3 + NaCl → AgCl + NaNO3. Thus, p is NO3 and q is Cl.

The reaction 2Mg + O2 → 2MgO is a classic example of a combination reaction (also known as a synthesis reaction). In this reaction, two separate reactants—magnesium (Mg) and oxygen (O2)—combine to form a single, more complex product, magnesium oxide (MgO).

All three reactions are examples of exothermic reactions. Making of slaked lime involves calcium oxide reacting with water, releasing heat. Burning natural gas involves methane reacting with oxygen, producing carbon dioxide, water, and heat. Respiration is also exothermic, as glucose reacts with oxygen in cells to produce carbon dioxide, water, and release energy.