A chemical reaction is a process in which the bonds within the reactant atoms are broken, and the substituent bonds within the product atoms are crystallized to crystallize a substituent. The method of representing a chemical reaction with the help of symbols and formulas of the substances involved in it is called a chemical reaction. Chemical reactions are related to chemical changes. While a chemical change occurs, a chemical reaction takes place. Chemical reactions are usually irreversible and a new product is formed. Atoms are rearranged during a chemical reaction.

• The substance which combines or reacts is known as a reactant.
• The new substance produced in a reaction is called a product.

A chemical equation is a concise method of representing a chemical reaction.

Characteristics of Chemical reactions are,

• Evolution of gas
• Formation of a precipitate
• Change in color
• Change in temperature
• Change in state

Balanced and Unbalanced Chemical Equations

1. In a balanced chemical equation, the reactants and products have the same number of atoms of different elements.
2. In an unbalanced chemical equation, there are unequal numbers of atoms of one or more elements in the reactants and products.
3. In a balanced chemical equation, the reactants and products have equal masses of different elements.
4. In an unbalanced equation, the reactants and products have unequal masses of different elements.
5. The number of atoms of different types in the reactants should be equal to the number of atoms of the same type in the products.
6. The chemical equations are balanced to satisfy the law of conservation of mass in a chemical reaction.

Never change the formula of an element or compound to balance an equation.

The process of making the number of different types of atoms equal on both sides of an equation is called equation balancing.

The equation which gives more information about the chemical reaction is called a more informative or informative equation. A chemical reaction can be made more informative in three ways:

• By indicating the “physical states” of the reactants and products.
• By indicating the “heat change” taking place in the reaction.
• By indicating the “conditions” under which the reaction takes place.

Important Example on Writing of Balanced Chemical Equations

The following steps must be followed to write equations for chemical equations:

Keeping these things in mind, let’s solve an example:

Example: Write a balanced equation for the following reaction : Methane burns in oxygen to form carbon dioxide and water.

Solution:

Step 1: Write the chemical reaction in the form of a word equation with the reactants on the left and the products on the right.

This reaction can be written as a one-term equation:

Carbon dioxide + Water → Methane + Oxygen

• Formula of methane is CH₃
• Formula of oxygen is O₂
• Formula of carbon dioxide is CO₂
• Formula of water is H₂O

Step 2: Substitute the symbols and formulas for all the reactants and products into the word equation.

On writing the formulas of all the substances in the above word equation, we get:

CH₄ + O₂ → CO₂ + H₂O

Let us calculate the structure of different atoms in the reactants and products:

	In reactants	In products
Number of C atoms	1	1
Number of H atoms	4	2
Number of O atoms	2	3

Step 3: Balance the equation by multiplying the symbols and formulas by the smallest possible number of digits.

The composition of carbon atoms is the indistinguishable on both sides (1 each) but the composition of hydrogen titles isn’t equal to that of the oxygen atoms of copper. There are 4 hydrogen atoms on the reactant side but only 2 hydrogen atoms on the product side. on the right side. To deposit 4 hydrogen atoms on the right, we multiply H₂O by 2 and write 2H₂O. Therefore,  

CH₄ + 2O₂ → CO₂ + H₂O

Counting the number of various atoms on both the sides again, we get :

	In reactants	In products
Number of C atoms	1	1
Number of H atoms	4	4
Number of O atoms	2	4

Now, only the number of oxygen atoms is unequal. There are 2 oxygen atoms on the left but 4 on the right. To have 4 oxygen atoms on the left, we multiply O₂ by 2 and write 2O₂.

CH₄ + 2O₂ →CO₂ + 2H₂O

Let’s count the number of different atoms on both sides once again

	In reactants	In products
Number of C atoms	1	1
Number of H atoms	4	4
Number of O atoms	4	4

In this chemical equation, there are equal number of different types of atoms in the reactants and products, so it is a balanced equation.

Detailed description for better understanding of above example-

The above equation can be made more informative by indicating the physical states of the reactants and products as well as the heat changes that take place in the reaction as discussed below:

• Methane is a gas- CH₄(g) 
• Oxygen is a gas- O₂(g)
• Carbon dioxide is a gas- CO₂ (g) 

What about the physical state of H₂O?

If the reaction takes place in an aqueous medium, then H₂O is in the liquid state and we write H-O(1) for it. If the reaction takes place in the vapor phase, then H₂O is in the gaseous state and is denoted as H₂O (g). In this case, methane gas burns into oxygen gas to form carbon dioxide gas and water vapor or stream. So the water here is in the gaseous state and is H₂O (g). If we put the physical states of all the reactants and products, then the equation can be written as-

CH₄(g) + 2O₂(g) → CO₂(g) +2H₂O(g)

Now, the heat that takes place in this reaction changes. When methane burns in oxygen to form carbon dioxide and water, a lot of heat is also produced, so it is an exothermic reaction. An exothermic reaction is indicated by writing a “+heat” sign on the product side. So, the above equation can finally be written as:

CH₄(g) + 2O₂(g) → CO₂(g) +2H₂O(g) + Heat

This equation now gives the physical states of those reactants and products as well as the heat changes that take place in the reaction, so it is a more informative equation.

Sample Questions

Question 1: List any four changes around you through which we can determine whether a chemical reaction has taken place or not.

Answer:

1. Respiration of animals, 
2. Digestion of food, 
3. Burning of coal, and
4. Rusting of iron. 

Question 2: Mention any three observations which help in determining whether a chemical reaction has taken place or not. Write an activity that can justify at least two of the comments listed.

Answer:

Observations: 

1. Change in state, 
2. Change in color, 
3. Evolution of a gas, 
4. Change in temperature.

Activity: 

• Some zinc granules are taken in a conical flask.
• A small amount of dilute hydrochloric acid is poured over it.
• It is observed that the zinc granules start decreasing in size and hydrogen gas is released.
• The temperature of the flask also rises slightly.

Question 3: Mention three ways by which a general chemical equation can be made more informative. 

Answer:

There are three ways to make a general chemical equation more informative:

1. By balancing the chemical reaction, the number of atoms of each reactant and product can be made equal.
2. More information about the physical states of reactants and products can be obtained by referring to their physical states (s, l, g, aq).
3. The heat must be specified in favor of the reactants or products in order to identify an endothermic or exothermic reaction.

Question 4: Seema bought the packet of chips and opened it. Suddenly his friend Shanu came. She started playing and forgot to eat chips. The next day when he ate the chips, he felt that the taste was not good and he was not feeling well. He asked his mother to take him to the doctor. The doctor told him that this happened due to eating stale chips. He gave medicines due to which Seema got well within a few days.

(a) Why do chips stay fresh longer in sealed packets?

(b) Why did the open packet chips go bad?

(c) What value do you learn from this passage?

Answer:

(a) This is because sealed packets of chips are usually filled with a gas such as nitrogen, which prevents the chips from oxidizing.

(b) The fats and oils present in the open packet of chips get oxidized. They become rancid and their smell and taste change.

(c) Food items should be stored in airtight containers or in the refrigerator to prevent oxidation of the fats and oils present in them. Once the chips packets/food item gets spoiled, their smell and taste change. They should not be eaten because they are not good for our health.

Question 5: What is the physical state of H₂O in a chemical reaction?

Answer:

If the reaction takes place in an aqueous medium, then H₂O is in the liquid state and we write H-O(1) for it. If the reaction takes place in the vapor phase, then H₂O is in the gaseous state and is denoted as H₂O (g).