Chapter 1 Chemical Reactions

A chemical reaction is a process in which one or more substances, called reactants, are converted into one or more different substances called products. You can identify a chemical reaction through several observable signs such as change in colour, evolution of gas, formation of precipitate, change in temperature, or emission of light. For example, when iron rusts, its colour changes from grey to reddish-brown, indicating a chemical reaction. The burning of magnesium ribbon in air produces a bright white light and leaves behind a white powdery ash, which is magnesium oxide. These physical changes signal that new substances with different properties have formed.

Chapter 1 covers five major types of chemical reactions. Combination reactions involve two or more substances combining to form a single product. Decomposition reactions break one substance into two or more simpler substances and may require heat, light, or electricity. Displacement reactions occur when a more reactive element replaces a less reactive one from a compound. Double displacement reactions involve exchange of ions between two compounds, often forming a precipitate or gas. Redox reactions involve simultaneous oxidation and reduction. Understanding each type with examples like burning of coal, electrolysis of water, and reaction of iron with copper sulphate helps students answer exam questions confidently.

Balancing a chemical equation means ensuring the number of atoms of each element is equal on both sides of the equation. This is based on the Law of Conservation of Mass, which states that matter can neither be created nor destroyed. To balance an equation, first write the skeletal equation, then count atoms on each side, and adjust coefficients to make them equal. For example, the reaction of hydrogen with oxygen: 2H₂ + O₂ → 2H₂O. Here, both sides have 4 hydrogen atoms and 2 oxygen atoms. Balanced equations are necessary for correctly calculating the amounts of reactants and products in a chemical reaction.

Oxidation is the process in which a substance gains oxygen or loses hydrogen or loses electrons. Reduction is the opposite — a substance loses oxygen or gains hydrogen or gains electrons. These two processes always occur simultaneously in what we call a redox reaction. A classic example is the reaction of copper oxide with hydrogen: CuO + H₂ → Cu + H₂O. Here, copper oxide is reduced to copper (loses oxygen), and hydrogen is oxidised to water (gains oxygen). Another example is the rusting of iron, where iron is oxidised. The substance that causes oxidation is called an oxidising agent, and the one that causes reduction is the reducing agent.

Exothermic reactions release energy in the form of heat or light during the chemical process. Endothermic reactions absorb energy from the surroundings, causing the temperature to drop. In Chapter 1, burning of natural gas (methane) is a classic exothermic reaction where heat is released — this is why we use gas stoves for cooking. The dissolution of quick lime in water also releases enormous heat. On the other hand, the decomposition of calcium carbonate on heating into calcium oxide and carbon dioxide is an endothermic reaction since it requires continuous heat supply. Photosynthesis in plants is another example of an endothermic reaction where sunlight energy is absorbed.