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Chemical Equations: Writing & Balancing | Science Class 10

Balancing a chemical equation involves adjusting the number of molecules or atoms of each substance so that the number of atoms of each element is equal on both sides of the equation.
This isn’t just a rule—it's a reflection of two fundamental scientific laws: the Law of Conservation of Mass (matter can’t be created or destroyed) and the Law of Constant Proportions (elements combine in fixed ratios). By adding the correct numbers, or coefficients, we ensure that the equation tells the true story of what’s happening in the reaction.

Chemical Equation and Rule for Writing Chemical Equations 

A chemical equation is a symbolic representation of a chemical reaction in which the reactants and products are denoted by their respective chemical formulae.

Example of a Chemical Equation

2H2 + O2 → 2H2O 

This equation describes the reaction between hydrogen and oxygen to form water. 

Key Components of Chemical Equation

  • Reactants: Substances that participate in a chemical reaction, written on the left side of the arrow (→).
  • Products: New substances formed as a result of the reaction, written on the right side of the arrow.
  • Skeletal Chemical Equation: A basic chemical equation showing only reactants and products without additional details like physical states.

Chemical Equations: Writing & Balancing | Science Class 10

Enhancing Chemical Equations

To make a chemical equation more informative, include:

1. Physical States: Indicate the state of each substance using standard symbols:

  • (g): Gaseous state

  • (l): Liquid state

  • (s): Solid state

  • (aq): Aqueous solution

2. Reaction Conditions: Specify conditions like temperature, pressure, or catalysts, typically written above or below the arrow.
Example: N2 + 3H2Chemical Equations: Writing & Balancing | Science Class 10 2NH3 + 22400 calorie heat

3. Special Symbols:

  • (↑): Indicates a gas evolved (e.g., H₂ (↑)).
    Example: 2Na(s) + 2H2O(l) → 2 NaOH(aq) + H2(g) or H2 (↑). This shows sodium reacting with water to produce sodium hydroxide and hydrogen gas.

  • (↓) : Indicates a precipitate formed (e.g., AgCl (↓)).
    Example: NaCl + AgNO3 → NaNO3 + AgCl (↓)

  • (→): Represents an irreversible reaction.
    Example: 2H2 + O2 → 2H2

  • (⇌): Represents a reversible reaction.

Chemical Equations: Writing & Balancing | Science Class 10In the above reaction, P4O10 & water are reactants and Phosphoric acid is the product.

Rules for Writing Informative Chemical Equations

  1. Write the correct chemical formulas for reactants and products.

  2. Place reactants on the left and products on the right, separated by an arrow (→ or ⇌).

  3. Include physical states using (s), (l), (g), or (aq).

  4. Indicate special outcomes like gas evolution (↑) or precipitate (↓).

  5. Specify reaction conditions (e.g., temperature, pressure, catalyst) above or below the arrow.

  6. Balance the chemical equation correctly.

Rules for Writing a Word Equation

A word equation is a way to describe a chemical reaction using the names of the substances involved instead of their chemical formulas. 

Here are the key rules for writing a word equation:

  1. The substances involved in a chemical reaction (reactants) are always placed on the left side of the arrow.
  2. The substances produced after the reaction (products) are always placed on the right side of the arrow.
  3. A plus sign (+) is used between reactants or products if there are two or more.
  4. An arrow (→) is positioned between the reactants and products; it indicates the direction of the reaction. This arrow is read as "to yield" or "to form".
  5. Optionally include physical states (e.g., gas, liquid) in parentheses.
  6. Keep it clear and accurate.

Example: Methane + Oxygen → Carbon Dioxide + Water

Steps to Balance Chemical Equations

A balanced chemical equation has the same number of atoms of each element on both sides. The process to achieve this is known as the "hit-and-trial method," where we try various coefficients to balance the equation using the smallest whole numbers. 

Here are the steps for balancing simple equations:

Step 1. Count the no. of atoms of various elements on both sides of the equation

Example: Fe + H2O → Fe3O4 + H2

Chemical Equations: Writing & Balancing | Science Class 10

Step 2. Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.
According to this rule Fe3O4 has maximum number of atoms & oxygen has 4 atoms so it is selected.

Chemical Equations: Writing & Balancing | Science Class 10

So the partly balanced equation is
Fe + 4H2O → Fe3O4 + H2

Step 3. Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.

Chemical Equations: Writing & Balancing | Science Class 10

Now the equation becomes as
Fe + 4H2O → Fe3O4 + 4H2

Step 4. Balance the Iron atoms similarly.

Chemical Equations: Writing & Balancing | Science Class 10

Now the equation is
3Fe + 4H2O → Fe3O4 + 4H2

Step 5. Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.

Chemical Equations: Writing & Balancing | Science Class 10

Step 6. To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq) is written.
Now the equation becomes as
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

Solved Examples


Q.1. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans.
(a) N2(g) + 3H2(g) Chemical Equations: Writing & Balancing | Science Class 10 2NH3(g)
(b) 2H2S + 3O2 → 2H2O + 2SO2
Chemical Equations: Writing & Balancing | Science Class 10
(d) 2K (s) + 2H2O (1) → 2KOH + H2 (g)

Q.2. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
Ans.
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

Q.3. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride.
Ans.

Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10

Q.4. Write the balanced chemical equation for the following and identify the type of reaction in each case -
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Ans.
(a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(aq)
This reaction is a double-displacement reaction.
(b) ZnCO3(s) → ZnO(s) + CO2(g)
This reaction is a decomposition reaction

Q.5. Why should a magnesium ribbon be cleaned before burning in air?
Ans.
Magnesium reacts with the constituent gases of the atmosphere to form various compounds which get deposited over its surface. The ribbon is cleaned before burning to remove the layer of these compounds so that pure magnesium can burn in air.

The document Chemical Equations: Writing & Balancing | Science Class 10 is a part of the Class 10 Course Science Class 10.
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FAQs on Chemical Equations: Writing & Balancing - Science Class 10

1. What is a chemical equation?
Ans. A chemical equation is a symbolic representation of a chemical reaction, showing the reactants on the left side and the products on the right side, with the chemical formulas and coefficients of each substance involved.
2. What are the rules for writing a word equation?
Ans. The rules for writing a word equation include using appropriate chemical names for all substances, writing the reactants on the left and products on the right, and ensuring the equation is balanced in terms of the number of atoms of each element.
3. What are the steps to balance chemical equations?
Ans. The steps to balance chemical equations include writing the unbalanced equation, adjusting the coefficients of the substances to ensure the same number of each type of atom on both sides, and checking the equation to confirm it is balanced.
4. Why is it important to include the symbols of physical states of substances in a chemical equation?
Ans. Including the symbols of physical states (such as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous) in a chemical equation provides additional information about the physical state of each substance involved in the reaction.
5. Can you provide a solved example of writing and balancing a chemical equation?
Ans. Sure. For example, the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O) can be written and balanced as: 2H2(g) + O2(g) → 2H2O(l)
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