Chemical Changes and Equations: God almighty has ​created a beautiful world where all living things are born, grow, and eventually wither or die. Have you observed and analysed your everyday life from the perspective of a chemist? In the morning, we turn on the cooking gas, and it burns immediately. Simply put, we encounter a multitude of changes, which can be both physical and chemical in nature.

As everyone knows, a physical change can be easily reversed, but a chemical change is not easily reversible. Have you ever wondered why? In reality, chemical changes produce new substances, and it is difficult to regenerate the original substances known as reactants. All chemical changes are accompanied by chemical reactions, and these are represented with the help of chemical equations.

Chemical Reactions

Chemical changes are the processes by which original substances lose their nature and identity and are replaced by new chemical substances with different properties. A chemical reaction is a process that involves a chemical change. 

During the chemical reaction, there is a rearrangement of atoms between the reacting substances to form new substances with completely different properties. In chemical reactions, chemical bonds are broken in the reactant molecules, and new chemical bonds are formed to the products. It simply involves rearranging the atoms.

Learn All the Concepts on Chemical Reactions

Reaction Between Magnesium and Oxygen

The burning of magnesium in the air is an example of a chemical reaction.

Activity: Burning of Magnesium ribbon in the air.

Procedure:

1. Scrub a \({\rm{2}}\,{\rm{cm}}\) long magnesium ribbon with sandpaper to clean it. 
2. With a pair of tongs, hold it. Burn it with a spirit lamp or burner and collect the ash in a watch-glass, as shown in Figure.
3. Burn the magnesium ribbon while keeping it as far away from your eyes as possible. 
4. What observations have you made?

Observation: It is observed that magnesium ribbon burns with dazzling white light to form a white powder. This powder is magnesium oxide.

Conclusion: On heating, magnesium combines with oxygen present in the air to form magnesium oxide. Burning magnesium in the air to form magnesium oxide is an example of a chemical reaction. In this chemical reaction, the reactant magnesium and oxygen combine to form magnesium oxide, the properties of which are completely different from those of the reactants magnesium and oxygen.

Characteristics of Chemical Reaction

The easily observable features which take place as a result of chemical reactions are known as characteristics of chemical reactions. The important characteristics of chemical reactions are:

1. Change in the state: Some chemical reactions are accompanied by changes in state. For example, burning magnesium ribbon forms magnesium oxide, which is a white powder, while magnesium is a metal.

2. Change in colour: Some chemical reactions are indicated by a change in colour. For example, rust is brown in colour while the iron is grey-black in colour.

3. Evolution of gas: The evolution of a gas characterises some chemical reactions. When dilute hydrochloric acid is added to sodium carbonate, brisk effervescence occurs with the evolution of carbon dioxide gas. When carbon dioxide gas is passed through lime water, it turns milky.

4. Change in temperature: Temperature change is one of the characteristics of the chemical reaction; for example, when quicklime is mixed with water, slaked lime is formed, and a lot of energy is generated, and the reaction mixture becomes hot.

5. Formation of precipitate: Some reactions are characterised by the formation of a precipitate. For example, when a silver nitrate solution is added to a sodium chloride solution, a white precipitate of silver chloride forms.

Chemical Equations

A chemical equation represents a chemical reaction using symbols and formulae of the substances involved.

A chemical equation can be represented in two ways:

1. In terms of words (called word equation): When zinc metal reacts with dilute sulphuric acid, zinc sulphate and hydrogen gas are formed. This reaction can be written in words as 

Zinc\(+\)Dil. sulphuric acid\(→\)Zinc sulphate\(+\)Hydrogen

2. In terms of symbols and formulae (called chemical equation): In the word equation, by putting the symbols and formulae of all the substances, we get the following chemical equation.

\(\underbrace {{\rm{Zn}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}}_{{\rm{Reactants }}} \to \underbrace {{\rm{ZnS}}{{\rm{O}}_4} + {{\rm{H}}_2}}_{{\rm{Products }}}\)

This equation is called a chemical equation.

Writing a Chemical Equation

To represent a chemical equation, we write: 

1. The reactant symbols and formulae are on the left-hand side (LHS), with a plus sign between them.
2. The product symbols and formulae are on the right-hand side (RHS), with a plus sign between them.
3. An arrow \((→)\) sign is put between the reactants and the products, pointing from reactants towards products.

Balanced and Unbalanced Chemical Equation

Balanced Chemical Equation: A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

For example,

\({\rm{C}}{{\rm{H}}_4} + 2{{\rm{O}}_2} \to {\rm{C}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}}\)

Let us examine the number of atoms of different elements on both sides of the arrow.

Element	Number of Atoms in Reactants(LHS)	Number of atoms in Products(RHS)
\({\rm{C}}\)	\(1\)	\(1\)
\({\rm{H}}\)	\(4\)	\(4\)
\({\rm{O}}\)	\(4\)	\(4\)

As the number of atoms of each element is the same on both sides of the arrow, the above equation is a balanced chemical equation.

Unbalanced Chemical Equation: An equation is one in which the number of atoms of different elements is not equal on the two sides of the equation is called unbalanced or skeletal. An unbalanced chemical equation is also called a skeletal equation.

For example,

\({\rm{KCl}}{{\rm{O}}_3} \to {\rm{KCl}} + {{\rm{O}}_2}\)

In the below table, the number of atoms of different elements on both sides is given.

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)
\({\rm{K}}\)	\(1\)	\(1\)
\({\rm{Cl}}\)	\(1\)	\(1\)
\({\rm{O}}\)	\(3\)	\(2\)

As the number of atoms of \({\rm{K}}\) and \({\rm{Cl}}\) is equal on both sides, but \({\rm{O}}\) – atoms are not equal on both sides; hence the above equation is unbalanced.

Balancing of a Chemical Equation

Balancing a chemical equation means making the number of atoms of each element equal on both sides of the equation.

Importance of Balanced Chemical Equation

The importance of a balanced chemical equation stems from the fact that it obeys the law of conservation of mass, which states that in a chemical reaction, the total mass of all the products equals the total mass of all the reactants.

Steps Involved in the Balancing of a Chemical Equation

The Hit and Trial Method is used to balance a chemical equation. This method, which is useful for balancing simple chemical equations, involves counting the number of atoms of each element on both sides and attempting to equalise them.

The following steps are involved in the balancing of a chemical equation:

Step –I: To write the word equation: Write the chemical equation as a word equation. Keep the reactants on the left side (LHS) and the products on the right side (RHS). Separate them with an arrow \((→)\) head pointing from the reactants towards the products. 

Step –II: Create the skeletal chemical equation: record the symbols and formulas for the various reactants and products. This gives us the skeletal chemical equation.

Step –III: Put the formulas for each reactant and product in a box. While balancing the equation, make no changes to the contents of the boxes.

Step –IV: In the unbalanced equation, count the number of atoms of each element on the LHS (Reactants) and RHS (Products).

Step –V: Choose the largest formula to start balancing: It is often convenient to start with the compound that contains the maximum number of atoms; this can be a reactant or a product.

Step –VI: To Balance Different Elements: After you have selected the compound with the largest formula, balance the element of that compound first, and then balance the other elements one at a time. To balance the atoms of an element, put an integer coefficient in front of the compound’s formula.

Step –VII: To check the correctness of the balanced equation: Finally, to check the correctness of the balanced equation, count the atoms of each element on either side of the equation. If the number of atoms of the elements is the same on both sides, then the equation is balanced.

Example to illustrate Balancing of Equation by Hit and Trial Method.

Hydrogen gas combines with chlorine to give hydrogen chloride.

Step –I: Write the chemical equation in word form:

Hydrogen\(+\)Chlorine\(→\)Hydrogen chloride

Step-II: Writing the skeletal chemical equation: 

\({{\rm{H}}_2} + {\rm{C}}{{\rm{l}}_2} \to 2{\rm{HCl}}\)

Step-III: Balancing of atoms on both sides: There are \({\rm{2H}}\) atoms and \({\rm{2Cl}}\) atoms on LHS but only \({\rm{1H}}\)-atom and \({\rm{1Cl}}\)-atom on the RHS. To balance \({\rm{H}}\)-atoms and \({\rm{Cl}}\)-atom, multiply \({\rm{HCl}}\) by \(2,\) we get

\({{\rm{H}}_2} + {\rm{C}}{{\rm{l}}_2} \to 2{\rm{HCl}}\)

Step-IV: To check the correctness of the balanced equation:

Element	Number of atoms on LHS	Number of atoms on RHS
\({\rm{H}}\)	\(2\)	\(2\)
\({\rm{Cl}}\)	\(2\)	\(2\)

Since the number of atoms of each element is equal on both sides of the equation, it is balanced. Hence the balanced chemical equation is:

\({{\rm{H}}_2} + {\rm{C}}{{\rm{l}}_2} \to 2{\rm{HCl}}\)

Making a Chemical Equation More Informative

A chemical equation can be made more informative by adding the following additional information to get a clear picture of a chemical reaction.

1. The physical state of reactants and products: The physical state of reactants and product species should be represented by symbols, e.g. 

\(\left( {\rm{s}} \right)\) for solid-state

\(\left( {\rm{l}} \right)\) for liquid state 

\(\left( {\rm{g}} \right)\) for gaseous state

\(\left( {\rm{aq}} \right)\) for aqueous solution, i.e, a solution made in water. 

These symbols are written in brackets after the symbols/formulae of the reactants or products. For example, 

\(2{\rm{Na}}({\rm{s}}) + 2{{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to 2{\rm{NaOH}}({\rm{aq}}) + {{\rm{H}}_2}(\;{\rm{g}}) \uparrow \)

2. The concentration of the acid: If acid is present as one of the reactants, it may be dilute or concentrated. The symbol ‘dil’ is used for dilute, and ‘conc’ is used for concentrated acid.

For example, 

\({\rm{Zn}}({\rm{s}}){\rm{ + dil}}.{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to {\rm{ZnS}}{{\rm{O}}_{\rm{4}}}({\rm{aq}}){\rm{ + }}{{\rm{H}}_{\rm{2}}}({\rm{g}}) \uparrow \)

3. Heat changes accompanying the chemical reaction: There are two types of reactions on the basis of heat changes involved: exothermic and endothermic reactions.

a) Exothermic reactions: The reaction in which heat is evolved are known as exothermic reactions. In such cases, the sign ‘\(+\) Heat’ is written along with the products.

For example, 

1. The burning of coal is an exothermic reaction because heat is evolved during the burning of coal.

\(\mathop {{\rm{C(s)}}}\limits_{{\rm{Coal}}} {\rm{ + }}\mathop {{{\rm{O}}_{\rm{2}}}{\rm{(g)}}}\limits_{{\rm{Oxygen}}} \to \mathop {{\rm{C}}{{\rm{O}}_{\rm{2}}}{\rm{(g)}}}\limits_{{\rm{Carbon}}\,{\rm{dioxide}}} {\rm{ + Heat}}\)

b) Endothermic reactions: The reaction in which heat is absorbed are known as endothermic reactions. In such cases, the sign ‘\(+\) Heat’ is written along with the reactants.

For example,

Photosynthesis is an example of an endothermic reaction because sunlight is absorbed by the green plants during the process of photosynthesis.

4. Conditions under which the reaction will proceed: When a reaction requires heat, the heat symbol delta (Δ) is placed on the arrow in the equation. When the reaction takes place in the presence of a catalyst, the catalyst symbol or the formula of catalyst is also written above or below the arrow symbol in the equation. 

For example,

\({\rm{2KCl}}{{\rm{O}}_{\rm{3}}}{\rm{(s)}}\mathop \to \limits_{{\rm{Mn}}{{\rm{O}}_{\rm{2}}}}^\Delta 2{\rm{KCl(s)}} + 3{{\rm{O}}_2}{\rm{(g)}} \uparrow \)

Summary

Chemical changes are the processes by which original substances lose their nature and identity and are replaced by new chemical substances with different properties. A chemical reaction is a process that involves a chemical change. Burning of magnesium, rusting of iron, photosynthesis etc., are examples of chemical reactions. The method of representing a chemical reaction using symbols and formulae of the substances involved is known as a chemical equation.

FAQ’s on Chemical Changes and Equations

Q.1. What are chemical reactions and equations?
Ans: Chemical changes are the processes by which original substances lose their nature and identity and are replaced by new chemical substances with different properties. A chemical reaction is a process that involves a chemical change. A chemical equation is a method of representing a chemical reaction using symbols and formulae of the substances involved.

Q.2. What are 10 different examples of chemical changes?
Ans: The \(10\) different examples of chemical changes are as follows:
1. Burning of wood
2. Burning of magnesium
3. Cooking of food
4. Curdling of milk
5. Neutralisation reaction
6. Ripening of fruits
7. Rusting of iron
8. Digestion of food
9. Photosynthesis
10. Respiration

Q.3. Difference between Chemical Changes and Equations.
Ans: A chemical change is a process in which the original substances lose their nature and identity and form new chemical substances with different properties. In comparison, the symbolic representation of chemical reactions is known as chemical equations.

Q.4. What is a chemical equation? Give an example?
Ans: A chemical equation is a method of representing a chemical reaction using symbols and formulae of the substances involved.
For example, the chemical equation for the reaction between zinc and sulphuric acid can be written as
\({\rm{Zn}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{ZnS}}{{\rm{O}}_4} + {{\rm{H}}_2}\)

Q.5. What are the 2 parts of a chemical equation?
Ans: The two parts in a chemical equation are reactants (it is present on the left-hand side of the reaction) and products (it is present in the right-hand side reaction).