Writing Chemical Equations for CBSE Class 10 Science

Describing chemical reactions in full sentences can be cumbersome. For example, stating that "When magnesium ribbon is burned in oxygen, it gets converted to magnesium oxide" is descriptive, but inconvenient for quantitative chemical analysis. A chemical equation provides a concise, symbolic representation of a chemical change using formulas, symbols, and standard notations.

In CBSE Class 10 Science, learning how to convert word descriptions into concise skeletal chemical equations and enriching them with physical state symbols is a foundational skill examined across all chemistry chapters.

What You Will Learn

• Word equations vs. symbolic chemical equations
• Reactants, products, and standard directional arrow conventions
• Definition and limitations of a skeletal chemical equation
• Making equations more informative using state symbols ($(s)$, $(l)$, $(g)$, $(aq)$)
• Notations for gas evolution ($\uparrow$) and precipitation ($\downarrow$)
• Representing reaction conditions: catalysts, temperature, pressure, and sunlight
• Step-by-step solved examples and board exam conventions

1. From Word Equations to Chemical Equations

A word equation represents a chemical reaction by writing the names of reactants and products.

Example: Word Equation

$\text{Magnesium} + \text{Oxygen} \longrightarrow \text{Magnesium oxide}$

• Reactants: The initial substances that undergo chemical change are written on the left-hand side (LHS) with a plus sign ($+$) between them.
• Products: The new substances formed during the reaction are written on the right-hand side (RHS) with a plus sign ($+$) between them.
• Reaction Arrow ($\longrightarrow$): Placed between reactants and products, pointing toward the products, indicating the direction of the chemical transformation.

Writing the Chemical Equation

By substituting chemical formulas for words, the description becomes far more compact and universally readable: $\text{Mg} + \text{O}_2 \longrightarrow \text{MgO}$

Important: <u>In any chemical formula, subscripts indicate the fixed atomic ratio inside a molecule (e.g., $\text{O}_2$ is diatomic oxygen). You must NEVER change subscripts when writing or balancing an equation.</u>

2. Skeletal Chemical Equations

A chemical equation that simply represents the formulas of reactants and products without balancing the number of atoms is called a skeletal chemical equation.

Consider: $\text{Mg} + \text{O}_2 \longrightarrow \text{MgO}$

• On the LHS: $1$ atom of $\text{Mg}$ and $2$ atoms of $\text{O}$.
• On the RHS: $1$ atom of $\text{Mg}$ and $1$ atom of $\text{O}$.

The number of oxygen atoms is not equal on both sides. Hence, the mass is not balanced. This is an unbalanced or skeletal chemical equation for the burning of magnesium in air.

3. Making Chemical Equations More Informative

A bare skeletal equation does not indicate whether a substance is a solid, liquid, gas, or in water solution, nor does it specify reaction temperature or catalysts. To convey complete scientific information, standard notations are added.

1. Physical State Notations

The physical states of reactants and products are written in parentheses next to their chemical formulas:

Enriched Example: $\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \longrightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$

2. Specialized Directional Symbols

• Gas Evolution ($\uparrow$): A vertical upward arrow indicates that a gaseous product escapes into the atmosphere: $\text{CaCO}_3(s) + 2\text{HCl}(aq) \longrightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \uparrow$
• Precipitate Formation ($\downarrow$): A vertical downward arrow indicates that an insoluble solid precipitate settles out of solution: $\text{AgNO}_3(aq) + \text{NaCl}(aq) \longrightarrow \text{AgCl}(s) \downarrow + \text{NaNO}_3(aq)$

3. Reaction Conditions (Written on the Arrow)

Conditions such as temperature, pressure, heat input, or catalysts are placed above and/or below the reaction arrow:

1. Heat Input ($\Delta$): Thermal decomposition of limestone: $\text{CaCO}_3(s) \xrightarrow{\quad \Delta \quad} \text{CaO}(s) + \text{CO}_2(g)$
2. Pressure Conditions: Industrial synthesis of methanol: $\text{CO}(g) + 2\text{H}_2(g) \xrightarrow{\quad 340\text{ atm} \quad} \text{CH}_3\text{OH}(l)$
3. Sunlight and Chlorophyll: Photosynthesis in green plants: $6\text{CO}_2(aq) + 12\text{H}_2\text{O}(l) \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \text{C}_6\text{H}_{12}\text{O}_6(aq) + 6\text{O}_2(aq) + 6\text{H}_2\text{O}(l)$

4. Solved Board Exam Questions

Solved Example 1: Translating Word Descriptions to Chemical Equations

Problem: Write a balanced chemical equation with state symbols for the following reaction: "Solutions of barium chloride and sodium sulphate in water react to give an insoluble barium sulphate precipitate and a solution of sodium chloride."

Solution:

1. Identify reactants and products with formulas:
   • Barium chloride solution $\implies \text{BaCl}_2(aq)$
   • Sodium sulphate solution $\implies \text{Na}_2\text{SO}_4(aq)$
   • Insoluble barium sulphate $\implies \text{BaSO}_4(s)$
   • Sodium chloride solution $\implies \text{NaCl}(aq)$
2. Write skeletal equation: $\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \longrightarrow \text{BaSO}_4(s) + \text{NaCl}(aq)$
3. Balance atom counts (putting 2 before $\text{NaCl}$): $\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \longrightarrow \text{BaSO}_4(s) \downarrow + 2\text{NaCl}(aq)$

Solved Example 2: Reaction with Specific Temperature Conditions

Problem: Write the chemical equation for the slaking of lime, including physical states and heat release.

Solution:

1. Quicklime is solid calcium oxide: $\text{CaO}(s)$.
2. Water is liquid: $\text{H}_2\text{O}(l)$.
3. Slaked lime formed in water is aqueous calcium hydroxide: $\text{Ca(OH)}_2(aq)$.
4. The reaction is strongly exothermic: $\text{CaO}(s) + \text{H}_2\text{O}(l) \longrightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$

5. Summary and Examination Tips

Exam Tip: In board exam questions stating "Write a balanced chemical equation with state symbols", failing to include $(s), (l), (g),$ or $(aq)$ will lead to deduction of half a mark per equation!

Common Mistake: Writing $(l)$ for aqueous salt solutions. Liquid $(l)$ is used only for pure liquids like water or molten salts. For solutions prepared in water, always use $(aq)$.