Chemical reactions and equations

CHEMICAL REACTIONS AND EQUATIONS 

A chemical reaction is a process in which one or more substances (called reactants) undergo a chemical change to form new substances (called products) with different properties. During a chemical reaction, chemical bonds are broken and new bonds are formed, leading to the formation of new substances.

Indicators of a Chemical Reaction

A chemical reaction is usually identified by one or more of the following changes:

• Change in colour

• Change in temperature (heat is released or absorbed)

• Evolution of a gas

• Formation of a precipitate

• Change in state

Example:
When zinc reacts with dilute hydrochloric acid, hydrogen gas is evolved.
Zn + 2HCl → ZnCl₂ + H₂↑

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Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction using chemical formulas instead of words.

Types of Chemical Equations

1. Word Equation
   Reactants → Products
   Magnesium + Oxygen → Magnesium oxide

2. Skeleton Equation
   Mg + O₂ → MgO

3. Balanced Chemical Equation
   2Mg + O₂ → 2MgO
   (Number of atoms of each element is equal on both sides)

Law of Conservation of Mass

According to this law, mass can neither be created nor destroyed in a chemical reaction.
Hence, every chemical equation must be balanced.

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Types of Chemical Reactions (Very Important for Exams)

1️⃣ Combination Reaction

Two or more substances combine to form a single product.
CaO + H₂O → Ca(OH)₂

2️⃣ Decomposition Reaction

A single compound breaks down into two or more simpler substances (usually using heat, electricity, or light).
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

3️⃣ Displacement Reaction

A more reactive element displaces a less reactive element from its compound.
Zn + CuSO₄ → ZnSO₄ + Cu

4️⃣ Double Displacement Reaction

Exchange of ions between two compounds, often forming a precipitate.
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

5️⃣ Oxidation and Reduction (Redox Reactions)

• Oxidation: Gain of oxygen or loss of hydrogen

• Reduction: Loss of oxygen or gain of hydrogen
  Both occur simultaneously.

Example:
CuO + H₂ → Cu + H₂O

• CuO is reduced

• H₂ is oxidised

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Effects of Oxidation in Daily Life

• Corrosion: Rusting of iron

• Rancidity: Spoiling of food containing fats and oils

Prevention methods include:

• Painting

• Galvanisation

• Using antioxidants