Chemical Reactions and Equations

Rancidity is the unpleasant smell and taste produced when fats and oils undergo oxidation. Filling a packet with nitrogen removes or greatly reduces the oxygen available around the food, so oxidation occurs more slowly and the food remains acceptable for longer. Nitrogen is not increasing oxygen, forming water, or producing a precipitate. Therefore, option A correctly explains the preservation method.

In a thermochemical equation, heat written on the product side means that the reaction releases heat to the surroundings. The surroundings, including the container, therefore become warm. Such a reaction is called exothermic, so option A is correct. An endothermic reaction would absorb heat and generally cause cooling. Electrolytic decomposition and precipitation describe different reaction features and do not follow from the temperature observation.

In combustion, a combustible substance generally reacts with oxygen and releases heat, often with light. Therefore, oxygen is the correct answer. Carbon dioxide does not support combustion, which is why it is used in fire extinguishers. Exam tip: Remember that combustion needs both a fuel and oxygen from air.

In a precipitation reaction, ions from two aqueous solutions combine to form an insoluble solid called a precipitate. Therefore, option A is correct. Formation of a gas is characteristic of a gas-evolution reaction, not a precipitation reaction. Exam tip: If mixing solutions produces a visible solid, identify it as a precipitation reaction.

Oxidation involves the addition of oxygen to a substance or the removal of hydrogen from it, so option A is correct. Removal of oxygen or addition of hydrogen is reduction, making option B incorrect. For example, copper reacts with oxygen to form copper oxide. Exam tip: remember that gain of oxygen indicates oxidation.

In reduction, oxygen is removed from a substance or hydrogen is added to it, so option A is correct. Option B describes oxidation in the usual Class 10 sense. For example, removal of oxygen from copper(II) oxide forms copper. Exam tip: remember that removal of oxygen indicates reduction.

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners to form two new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). Therefore, option A is correct. Option B describes a displacement reaction, where one element replaces another. Exam tip: Identify a double displacement reaction when ions of two compounds interchange.

A chemical equation is balanced to obey the law of conservation of mass. In a balanced equation, the number of atoms of every element is the same on the reactant and product sides, so total mass remains unchanged. Balancing does not aim to increase products or reduce reactants. Exam tip: change only coefficients while balancing, never the subscripts in chemical formulae.

In a displacement reaction, a more reactive element removes a less reactive element from its compound. For example, zinc displaces copper from copper sulfate: \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\). Option B describes a double displacement reaction, while option C is a decomposition reaction. Exam tip: an element higher in the reactivity series can displace an element below it from its salt solution.