What is Chemical Reactions and Equations Class 10: Complete Guide

Understanding Chemical Reactions: Definition and Examples

A chemical reaction is a process where one or more substances (reactants) change into new substances (products) with different properties. This change involves breaking and forming of chemical bonds.

Example:

When zinc reacts with hydrochloric acid, zinc chloride and hydrogen gas are formed:

$$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$$

This reaction shows zinc (metal) reacting with acid to produce a salt and gas.

Chemical reactions are everywhere — from cooking food to rusting iron. Recognising these changes helps understand the world scientifically.

What are Chemical Equations and How to Write Them

A chemical equation is a symbolic representation of a chemical reaction using chemical formulas and symbols.

It shows:

• The reactants on the left side
• The products on the right side
• An arrow ($\rightarrow$) indicating the direction of the reaction

Steps to write a chemical equation: 1. Write the correct formulas of reactants and products. 2. Use plus signs (+) to separate multiple reactants or products. 3. Place an arrow ($\rightarrow$) between reactants and products.

Example:

For the reaction of hydrogen and oxygen to form water:

$$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$$

This equation shows the number of molecules involved.

Balancing Chemical Equations: Law of Conservation of Mass

Balancing chemical equations ensures the same number of atoms of each element are present on both sides of the equation. This follows the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.

How to balance equations:

• Count atoms of each element on both sides.
• Use coefficients (numbers before formulas) to balance atoms.
• Never change the chemical formulas.

Example:

Unbalanced: $$\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$$

Balanced: $$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$$

Here, 4 hydrogen atoms and 2 oxygen atoms are on both sides.

Types of Chemical Reactions in Class 10 Science

Class 10 NCERT classifies chemical reactions into several types:

• Combination Reaction: Two or more substances combine to form one product.
• Example: $$\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$$

• Decomposition Reaction: One substance breaks down into two or more products.
• Example: $$2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2$$

• Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
• Example: $$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$$

• Double Displacement Reaction: Exchange of ions between two compounds.
• Example: $$\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$$

• Redox Reactions: Involve simultaneous oxidation and reduction.

Understanding these types helps in identifying reactions in experiments and daily life.

Indicators of Chemical Reactions: How to Identify Them

Chemical reactions often show visible signs called indicators. These help identify that a chemical change has occurred:

• Change in Colour: For example, burning magnesium turns white.
• Formation of Gas: Bubbles or fizzing, like when acid reacts with metal.
• Formation of Precipitate: A solid forms in a solution, e.g., silver chloride in a reaction.
• Change in Temperature: Reaction may release or absorb heat.
• Change in Odour: New smell indicates new substances.

Recognising these indicators is important for experiments and understanding chemical processes.

Worked Example: Writing and Balancing a Chemical Equation

Example: Write and balance the chemical equation for the reaction between iron and sulphur to form iron sulphide.

Step 1: Write the reactants and product formulas:

• Iron: Fe
• Sulphur: S
• Iron sulphide: FeS

Step 2: Write the unbalanced equation:

$$\text{Fe} + \text{S} \rightarrow \text{FeS}$$

Step 3: Count atoms on both sides:

• Left: 1 Fe, 1 S
• Right: 1 Fe, 1 S

Step 4: Equation is already balanced.

Final balanced equation:

$$\text{Fe} + \text{S} \rightarrow \text{FeS}$$

This simple reaction is a combination reaction.

Comparison Table: Types of Chemical Reactions