Revision: Types of Chemical Reactions Science SSLC (English Medium) Class 10 Tamil Nadu Board of Secondary Education

A chemical change is a process in which the composition of matter changes, resulting in the formation of one or more new substances.

The cell which converts electrical energy into chemical energy is called an electrolytic cell and the process which converts electrical energy into chemical energy is called electrolysis.

A chemical reaction that occurs in the presence of light is called a photochemical reaction.

The substances formed as a result of a chemical reaction by formation of new bonds are called products.

The substances taking part in chemical reaction are called reactants.

A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.

Any chemical change in matter that involves its transformation into one or more new substances is called a chemical reaction.

An electrochemical reaction is a chemical reaction involving the transfer of electrons, where oxidation and reduction occur simultaneously.

Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates.

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the products.

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

A chemical equation is a balanced account of a chemical transaction. It is not merely a qualitative statement, but it also gives quantitative information of a chemical reaction.

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The representation of a chemical reaction in a condensed form using chemical formulae is called as the chemical equation.

The reactions in which heat is absorbed are called endothermic reactions. The reactants absorb heat for form products.

When an acid reacts with a base salt and water are formed and this is called Neutralisation.
Double-decomposition – Neutralisation reaction :
“Is the chemical reaction between two compounds (acid and base) to interchange radicals and produce salt and water.”

A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

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When two or more reactants combine in a reaction to form a single product, it is a combination reaction.

A single reactant breaks down to give simpler products. This is a decomposition reaction.

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The chemical reaction in which two or more products are formed from a single reactant is a decomposition reaction.

When a decomposition reaction is carried out by heating, it is called thermal decomposition.

The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.

Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

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The reaction in which the ions in the reactants are exchanged to form a precipitate is a double displacement reaction.

pH scale is a scale for measuring the hydrogen ion concentration in a solution.

The pOH of a solution can be defined as the negative logarithm to the base 10, of the molar concentration of OH⁻ ions in solution.

pOH = -log₁₀[OH^-]

The pH of a solution is defined as the negative logarithm to the base 10, of the concentration of H⁺ ions in solution in mol dm^–3.

pH is expressed mathematically as

pH = -log₁₀ [H⁺] or pH = -log₁₀ [H₃O⁺]

• C + O₂ → CO₂
• 2H₂ + O₂ → 2H₂O
• 2Mg + O₂ → 2MgO

• Word equations use names; chemical equations use formulas.
• Reactants → Products, with arrow showing reaction direction.
• Use + between two or more reactants or products.
• Show states: (s), (l), (g), (aq); use ↑ for gas, ↓ for precipitate.
• Heat (Δ) or other conditions go above/below the arrow.

• Law of Conservation of Mass: In a chemical reaction, mass is neither created nor destroyed, so the number of atoms of each element must be equal on both sides.
• A skeletal (unbalanced) equation has unequal atoms of one or more elements on the LHS and RHS.
• Balancing is done using the hit-and-trial method, starting with the compound having the most atoms and balancing hydrogen and oxygen last.
• Only coefficients are changed while balancing; chemical formulas must not be altered.
• A balanced equation may also indicate physical states (s, l, g, aq) and reaction conditions, such as temperature, pressure, or a catalyst.

• A more reactive metal can replace a less reactive metal from its compound in solution.
• Zinc, iron, and lead can displace copper from copper sulphate or copper chloride solutions.
• During displacement, the displaced metal appears in elemental form, while the replacing metal forms its salt.
• In the reaction Fe + CuSO₄ → FeSO₄ + Cu, the iron nail turns brownish due to deposited copper, and the blue solution fades.
• Heat may be released during displacement reactions, indicating that the reaction is exothermic.

1. The pH scale (0–14) measures the concentration of H⁺ ions in a solution; values < 7 indicate acids, > 7 indicate bases, and 7 is neutral.
2. A universal indicator shows different colours at different pH levels, helping to determine the strength of an acid or base.
3. Strong acids/bases produce more H⁺ or OH⁻ ions in solution, while weak acids/bases produce fewer ions at the same concentration.