Class 10 · Chapter 1 · Chemistry
Chemical Reactions and Equations Cheat Sheet
Laws
Law of conservation of mass
Mass can neither be created nor destroyed in a chemical reaction.
The total mass of the reactants equals the total mass of the products, so every chemical equation must be balanced.
Reactions
Combination of quick lime and water
Quick lime (CaO) + water → slaked lime (Ca(OH)₂). Heat is released, so it is exothermic.
Combustion of methane
Methane burns in oxygen to give carbon dioxide and water, with the evolution of heat and light.
Thermal decomposition of calcium carbonate
Limestone breaks down on heating into quick lime and carbon dioxide. Δ above the arrow means heating.
Decomposition of ferrous sulphate
Green ferrous sulphate crystals lose water and turn brown; SO₂ and SO₃ are gases with a pungent smell.
Decomposition of lead nitrate
Heating gives brown fumes of NO₂ gas along with PbO and O₂. Δ means heat.
Electrolytic decomposition of water
An electric current splits water into hydrogen and oxygen; the label above the arrow shows the condition.
Photolytic decomposition of silver chloride
White AgCl turns grey in sunlight. Used in black-and-white photography.
Displacement of copper by iron
More reactive iron displaces copper from copper sulphate; the blue colour fades.
Double displacement reaction
A white precipitate of barium sulphate (↓) forms in a double displacement reaction.
Neutralisation reaction
An acid and a base react to give a salt and water.
Redox reaction
Carbon is oxidised and zinc oxide is reduced, so oxidation and reduction occur together.
Respiration (exothermic)
Glucose combines with oxygen to release energy — an exothermic reaction.
Definitions
- Chemical reaction
- A process in which one or more substances change into new substances with different properties.
- Reactants
- The substances that take part in a chemical reaction, written on the left of the arrow.
- Products
- The new substances formed in a chemical reaction, written on the right of the arrow.
- Chemical equation
- A shorthand way of representing a chemical reaction using symbols and formulae.
- Balanced chemical equation
- An equation in which the number of atoms of each element is equal on both sides.
- Skeletal equation
- An unbalanced chemical equation.
- Combination reaction
- Two or more reactants combine to form a single product.
- Decomposition reaction
- A single compound breaks down into two or more simpler substances.
- Displacement reaction
- A more reactive element displaces a less reactive element from its compound.
- Double displacement reaction
- Two compounds exchange ions to form two new compounds.
- Precipitation reaction
- A reaction in which an insoluble solid (precipitate) is formed.
- Oxidation
- Gain of oxygen or loss of hydrogen by a substance.
- Reduction
- Loss of oxygen or gain of hydrogen by a substance.
- Redox reaction
- A reaction in which oxidation and reduction take place together.
- Exothermic reaction
- A reaction in which heat is released.
- Endothermic reaction
- A reaction in which heat is absorbed.
- Corrosion
- The gradual eating away of a metal surface by the action of air, moisture or chemicals; e.g. rusting of iron.
- Rancidity
- The oxidation of fats and oils in food, giving it an unpleasant smell and taste.
Key Points
- State symbols are written after formulae: (s) solid, (l) liquid, (g) gas and (aq) aqueous solution.
- An arrow pointing upwards (↑) shows a gas evolved; an arrow downwards (↓) shows a precipitate formed.
- Conditions such as heat (Δ), light, electricity or a catalyst are written above the arrow.
- Equations are balanced to obey the law of conservation of mass.
- Types of reactions: combination, decomposition (thermal, electrolytic, photolytic), displacement, double displacement and oxidation–reduction.
- Rusting of iron needs both oxygen and moisture; it is prevented by painting, oiling, greasing or galvanisation.
- Rancidity is prevented by adding antioxidants, storing food in airtight containers, flushing with nitrogen and refrigeration.