CLASS 10 SCIENCE
CHAPTER 1 – CHEMICAL REACTIONS AND EQUATIONS
Complete Notes • Definitions • Examples • Equations • Exam Points
1. CHEMICAL REACTION
A chemical reaction is a process in which one or more substances, called reactants, are
transformed into one or more new substances, called products. The products have
different properties from the reactants.
Example: Magnesium burns in oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
Signs of a chemical reaction: change in state, change in colour, evolution of a gas,
change in temperature, and formation of a precipitate.
2. CHEMICAL EQUATIONS
A chemical equation represents a chemical reaction by using symbols and chemical
formulae. The substances on the left are reactants and those on the right are products.
Word equation: Magnesium + Oxygen → Magnesium oxide
Symbol equation: Mg + O₂ → MgO
A chemical equation should be balanced because atoms are neither created nor
destroyed during a chemical reaction. This follows the law of conservation of mass.
3. BALANCING A CHEMICAL EQUATION
While balancing, coefficients are placed before formulae. The subscripts inside a
chemical formula must not be changed.
Example: H₂ + O₂ → H₂O
There are 2 oxygen atoms on the left but only 1 on the right. Put 2 before H₂O: H₂ + O₂
→ 2H₂O. Now hydrogen becomes 4 on the right, so put 2 before H₂.
Balanced equation: 2H₂ + O₂ → 2H₂O
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. Heat may be shown
above the arrow.
4. TYPES OF CHEMICAL REACTIONS
A. Combination Reaction
A reaction in which two or more substances combine to form a single product is called a
combination reaction.
CaO + H₂O → Ca(OH)₂ + heat
CHAPTER 1 – CHEMICAL REACTIONS AND EQUATIONS
Complete Notes • Definitions • Examples • Equations • Exam Points
1. CHEMICAL REACTION
A chemical reaction is a process in which one or more substances, called reactants, are
transformed into one or more new substances, called products. The products have
different properties from the reactants.
Example: Magnesium burns in oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
Signs of a chemical reaction: change in state, change in colour, evolution of a gas,
change in temperature, and formation of a precipitate.
2. CHEMICAL EQUATIONS
A chemical equation represents a chemical reaction by using symbols and chemical
formulae. The substances on the left are reactants and those on the right are products.
Word equation: Magnesium + Oxygen → Magnesium oxide
Symbol equation: Mg + O₂ → MgO
A chemical equation should be balanced because atoms are neither created nor
destroyed during a chemical reaction. This follows the law of conservation of mass.
3. BALANCING A CHEMICAL EQUATION
While balancing, coefficients are placed before formulae. The subscripts inside a
chemical formula must not be changed.
Example: H₂ + O₂ → H₂O
There are 2 oxygen atoms on the left but only 1 on the right. Put 2 before H₂O: H₂ + O₂
→ 2H₂O. Now hydrogen becomes 4 on the right, so put 2 before H₂.
Balanced equation: 2H₂ + O₂ → 2H₂O
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. Heat may be shown
above the arrow.
4. TYPES OF CHEMICAL REACTIONS
A. Combination Reaction
A reaction in which two or more substances combine to form a single product is called a
combination reaction.
CaO + H₂O → Ca(OH)₂ + heat
