Question 1
(a) Define a chemical reaction.
(b) What is a chemical equation?
(c) Why do we need to balance chemical equations?
(a) Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction and the product formed, in the form of a chemical equation.
(c) A chemical equation needs to be balanced so as to make the number of atoms of the reactants equal to the number of atoms of the products because a chemical reaction is just a rearrangement of atoms. The atoms themselves are neither created nor destroyed during the course of a chemical reaction.
ICSE Class VII Chemistry Chapter 4 12 Marks
Language of Chemistry — Quick Study Guide
Chemical symbols, formulae, and equations are the language chemists use to describe substances and reactions.
Key Concepts
- Chemical symbol: Shorthand for an element (first letter capital, second lowercase)
- Chemical formula: Shows types and numbers of atoms in a molecule
- Cross-multiplication: Write symbols → valencies → cross → simplify
- Balanced equation: Same number of each atom on both sides
- Combination reaction: A + B → AB
- Decomposition reaction: AB → A + B
Latin-Origin Symbols to Memorise
Na (Sodium), Fe (Iron), Au (Gold), Ag (Silver), K (Potassium), Cu (Copper), Hg (Mercury), Pb (Lead), Sn (Tin)
Exam Tips
- Never change subscripts when balancing — only change coefficients
- Use brackets for polyatomic ions when subscript > 1
- Practice writing formulae using cross-multiplication daily
- CO ≠ Co (carbon monoxide vs cobalt)
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