CBSE Class 10 Science Chemical reactions and equations Notes

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CHEMICAL REACTIONS AND EQUATIONS

CHEMICAL REACTIONS

Any change can be classified as physical change and chemical change. Physical changes can be easily reversed but, it is not easy to reverse a chemical change. In chemical changes, new substances are formed and it is difficult to regenerate the original substances. Chemical changes are more permanent than physical changes. Chemical reaction involves chemical changes. Chemical reactions are the processes in which new substances with new properties are formed. During a chemical reaction, atoms of one element do not change in0to those of another element. Only a rearrangement of atoms takes place in a chemical reaction. Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

The burning of magnesium in air to form magnesium oxide is an example of chemical reaction.

REACTANTS AND PRODUCTS

The substances which take part in a chemical reaction are called reactants. The new substances produced as a result of chemical reaction are called products. In the above chemical reaction, there are two reactants : Magnesium and Oxygen but only one product : Magnesium oxide.

CHARACTERISTICS OF CHEMICAL REACTIONS

In a chemical reaction, reactants are transformed into products.

The important characteristics of chemical reaction are:

• Evolution of a gas
• Formation of a precipitate
• Change in colour
• Change in temperature and
• Change in state.

Any one of these characteristics can tell us whether a chemical reaction has taken place or not.

CHEMICAL EQUATIONS

The method of representing a chemical reaction with the help of symbols and formulas of the substances involved in it is known as chemical equation. A word-equation shows change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, and shows the direction of the reaction.

Example: A + B → C + D

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation:
Hydrogen + Oxygen ⇨ Water
H₂ + O₂ → H₂O
In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations. Chemical Equation is a way to represent the chemical reaction in concise and informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number of atoms of each element equal on both sides.
Example: Zn + H2SO₄ → ZnSO₄ + H₂
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H₂O → Fe3O₄ + H₂
In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

BALANCING A CHEMICAL EQUATION:

To balance the given or any chemical equation, follow these steps:
Fe + H₂O → Fe₃O4 + H₂
Write the number of atoms of elements present in reactants and in products in a table; as shown Balance the atom which is the maximum in number; on either side of chemical equation.

In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H₂O → Fe3O₄ + H₂
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

Fe + 4 x H₂O → Fe3O₄ + 4 x H₂
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
• Gaseous state is represented by symbol ‘g’
• Liquid state is represented by symbol ‘l’
• Solid state is written by symbol ‘s’
• Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

INTEXT QUESTIONS PAGE NO. 6

Q1: Why should a magnesium ribbon be cleaned before it is burnt in air?
Answer : Magnesium is an extremely reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air.

Question 2: Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer :

Question 3: Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

TYPES OF CHEMICAL REACTION
Chemical reactions can be classified in following types:

• Combination Reaction
• Decomposition Reaction
• Displacement Reaction
• Double Displacement Reaction
• Oxidation and Reduction Reaction

COMBINATION REACTION
Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.

Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.
Mg + O₂ → 2MgO
Magnesium + Oxygen ⇨ Magnesium oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C + O₂ → CO₂
Carbon + Oxygen ⇨ Carbon dioxide
When hydrogen reacts with chlorine, hydrogen chloride is formed.
H₂ + Cl₂ → 2HCl
Hydrogen + Chlorine ⇨ Hydrogen chloride
When calcium oxide reacts with water, calcium hydroxide is formed
CaO + H₂O → Ca(OH)₂
Calcium oxide + Water → Calcium hydroxide
When carbon monoxide reacts with oxygen, carbon dioxide is formed.
2CO + O₂ → 2CO₂
Carbon monoxide + Oxygen → Carbon dioxide

DECOMPOSITION REACTION

Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:

Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide
CaCO₃ → CaO + CO₂
Calcium carbonate → Calcium oxide + Carbon dioxide

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