Science Part 1 • Class 10th
Learn how to identify, write, balance and classify chemical reactions.
During a physical change, generally only the state of matter changes and the composition remains the same.
A physical change is often reversible.
During a chemical change, the composition of matter changes and the change is generally permanent.
| Physical changes | Chemical changes |
|---|---|
| Ice → water | Cooking of food |
| Water → ice | Milk → curd |
| Evaporation of water | Digestion of food |
| Breaking glass | Ripening of fruit |
A chemical reaction is a process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds.
The substances taking part in a chemical reaction are called as reactants.
The substances formed as a result of a chemical reaction by formation of new bonds are called as products.
A chemical reaction can first be represented using words. This is a word equation.
It can then be written in condensed form using chemical formulae.
The representation of a chemical reaction in a condensed form using chemical formulae is called as a chemical equation.
Some reactions require heat, a specific temperature, pressure or catalyst.
Here Δ indicates heating.
Here nickel acts as a catalyst.
A chemical equation in which the number of atoms of each element is the same on both sides of the equation is called as a balanced equation.
An equation in which the number of atoms of each element is not the same on both sides is called as an unbalanced equation.
Total mass of each element remains the same on both sides. This follows the law of conservation of mass.
Example:
Na is not balanced. Put coefficient 2 before NaOH.
Hydrogen is not balanced. Put coefficient 2 before water.
While balancing an equation, do not change the formula of a compound. Change only appropriate coefficients.
Chemical reactions are classified into four types according to the nature and number of reactants and products.
1. Combination
2. Decomposition
3. Displacement
4. Double displacement
The reaction in which two or more reactants combine to form a single product is called as a combination reaction.
Many reactants → One product
The chemical reaction in which two or more products are formed from a single reactant is called as a decomposition reaction.
Calcium carbonate decomposes on heating. Carbon dioxide turns freshly prepared lime water milky.
One reactant → Two or more products
Water can be decomposed by passing electric current through acidulated water.
This decomposition by electric current is called as electrolysis.
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 as a displacement reaction.
The reaction in which the ions in the reactants are exchanged to form a precipitate is called as a double displacement reaction.
Silver chloride is formed as a white precipitate. Ions in the reactants exchange with each other.
Ion exchange → precipitate → double displacement reaction
The process in which heat is absorbed from the outside is called as an endothermic process.
The process in which heat is given out is called as an exothermic process.
| Endothermic | Exothermic |
|---|---|
| Heat absorbed | Heat released |
| Example: CaCO₃ decomposition | Example: CaO + H₂O |
A chemical reaction in which heat is absorbed from the surroundings or supplied continuously from outside is called as an endothermic reaction.
Examples of endothermic processes include melting of ice and dissolution of potassium nitrate in water.
A chemical reaction in which heat is given away when reactants are transformed into products is called as an exothermic reaction.
Examples of exothermic processes include formation of ice from water and dissolution of sodium hydroxide in water.
Some chemical reactions occur rapidly while some occur slowly.
Burning of cooking gas; effervescence on adding baking soda to dilute acid; formation of a white precipitate.
Rusting of iron; erosion of rocks; formation of alcohol under proper conditions.
Therefore, the rate of different chemical reactions is different.
Powder reacts faster than larger pieces because smaller particles expose a larger surface.
Particle size ↓ → Rate of reaction ↑
Concentrated HCl reacts faster with calcium carbonate than dilute HCl.
Concentration ↑ → Rate of reaction ↑
Increasing temperature increases the rate of a chemical reaction.
Temperature ↑ → Rate of reaction ↑
The substance in whose presence the rate of a chemical reaction increases, without causing any chemical change to it, is called as a catalyst.
Manganese dioxide increases the rate of decomposition of potassium chlorate and itself does not undergo chemical change.
The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called as an oxidation reaction.
Oxidation means losing one or more electrons.
The substances which bring about an oxidation reaction by making oxygen available are called as oxidants or oxidizing agents.
Common oxidants include:
Nascent oxygen is represented by [O] and is the reactive form of oxygen before formation of O₂.
The chemical reaction in which reactants gain hydrogen is called as a reduction reaction.
The reaction in which a reactant loses oxygen to form the product is also called as a reduction reaction.
Reduction means gaining one or more electrons.
A reaction in which oxidation and reduction take place simultaneously is called as a redox reaction.
| Substance | Change |
|---|---|
| CuO | Reduction |
| H₂ | Oxidation |
Oxidation + Reduction = Redox reaction
| Oxidation | Reduction |
|---|---|
| Loss of electrons | Gain of electrons |
| Positive charge increases | Positive charge decreases |
| Negative charge decreases | Negative charge increases |
The damage of metals due to oxidation caused by various components of the atmosphere is called as corrosion.
Corrosion is an oxidation process. Rusting of iron is an important example.
Both water and air are necessary for rusting. Rusting occurs rapidly in the presence of salt.
Rust is a reddish coloured solid layer formed on iron.
Rust is formed through an electrochemical reaction. Different regions of the iron surface act as anode and cathode.
The foul odour and change in taste produced when oil or ghee undergoes air oxidation is called as rancidity.
Revise balancing, four reaction types, endothermic/exothermic reactions, rate factors, oxidation/reduction, corrosion and rancidity.
Combination → Many reactants, one product
Decomposition → One reactant, many products
Displacement → More reactive element displaces less reactive element
Double displacement → Exchange of ions
Nature • Particle size • Concentration • Temperature • Catalyst
Oxidation + Reduction = Redox
Metal damage due to atmospheric oxidation.
Foul odour and change in taste due to air oxidation of oil/ghee.