CBSE Class 10 Science Chemical Reactions And Chemical Equations Notes Set 06

These CBSE Class 10 Science notes for Chapter 1, Chemical Reactions and Equations, cover physical and chemical changes, writing and balancing equations, the four types of chemical reactions, exothermic and endothermic reactions, oxidation and reduction, corrosion and rancidity.

Important Notes

Physical and Chemical Change

Physical change: a change in physical appearance in which the composition of the substance stays the same and no new substance is formed, for example melting of wax, boiling of water, dissolving salt in water.

Chemical change: a change in which one or more new substances with different properties and composition are formed, for example burning of paper or magnesium, curdling of milk, digestion of food. A chemical change is also called a chemical reaction and is usually permanent.

A chemical reaction may be recognised by: change in state, formation of a precipitate, change in colour, change in temperature, or evolution of a gas.

Chemical Equations

1. Word equation: shows reactants and products in words, joined by an arrow, for example magnesium + oxygen → magnesium oxide.
2. Skeletal equation: uses symbols and formulae, but is not balanced, for example Mg + O₂ → MgO.
3. Balanced equation: the number of atoms of each element is equal on both sides, for example 2Mg + O₂ → 2MgO. Balancing follows the law of conservation of mass.
4. Balancing (hit and trial): write the skeletal equation, count atoms of each element, put numbers (coefficients) before formulae, never change the formulae, and check again.
5. Making an equation more informative: write physical states (s), (l), (g), (aq); conditions such as heat or sunlight above the arrow; use ↓ for a precipitate and ↑ for a gas.

Example: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

Types of Chemical Reactions

Decomposition examples: thermal: CaCO₃ →^heat CaO + CO₂; electrolytic: 2H₂O →^electricity 2H₂ + O₂; photochemical: 2AgCl →^sunlight 2Ag + Cl₂.

Precipitation reaction: a double displacement reaction in which an insoluble solid is formed, for example AgNO₃ + NaCl → AgCl↓ + NaNO₃.

Reactivity series (decreasing): K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au, Pt.

Exothermic and Endothermic Reactions

1. Exothermic: heat is released along with the products, for example C + O₂ → CO₂ + heat, burning of natural gas, respiration.
2. Endothermic: heat is absorbed, for example decomposition of CaCO₃, electrolysis of water, photosynthesis.

Oxidation and Reduction

1. Oxidation: gain of oxygen or loss of hydrogen by a substance.
2. Reduction: loss of oxygen or gain of hydrogen by a substance.
3. Redox reaction: oxidation and reduction take place together, for example CuO + H₂ → Cu + H₂O (CuO is reduced, H₂ is oxidised).
4. Oxidising agent is the substance that gets reduced; reducing agent is the substance that gets oxidised.

Corrosion and Rancidity

1. Corrosion: slow eating away of metals by air, moisture or chemicals. Corrosion of iron is rusting; rust is hydrated iron(III) oxide, Fe₂O₃.xH₂O. Silver turns black (Ag₂S) and copper turns green.
2. Rancidity: when oils and fats in food are oxidised, the food develops an unpleasant smell and taste.
3. Prevention of rancidity: adding antioxidants, keeping food in airtight containers, refrigeration, and flushing packets with nitrogen.