TEACHGYAN · CLASS 10 · SCIENCE · CHAPTER 1
Chemical Reactions and Equations
Handwritten-style chapter notes · NCERT-aligned study support
1. What is a chemical reaction?
A chemical reaction is a process in which one or more substances are converted into new substances with different properties. The starting substances are called reactants, and the substances formed are called products.
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Chemical change
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Products
Example: 2Mg + O₂ → 2MgO
Example: When magnesium ribbon burns in air, it reacts with oxygen and forms magnesium oxide. A bright white flame is observed and a white powder is formed.
2Mg + O₂ → 2MgO
Magnesium ribbon
Silvery metal
Burning
Bright white flame
Magnesium oxide
White powder
Safety: This is an observation diagram for textbook learning, not a home experiment. Magnesium burns intensely; demonstrations require teacher supervision and proper eye protection.
2. Signs that a chemical reaction may have occurred
- Change in colour
- Evolution of a gas
- Change in temperature
- Formation of a precipitate (an insoluble solid)
- Change in state
3. Writing chemical equations
A word equation represents a reaction using names of substances. A chemical equation represents it using chemical symbols and formulae. For example:
Magnesium + Oxygen → Magnesium oxide
2Mg + O₂ → 2MgO
4. Why must chemical equations be balanced?
A balanced chemical equation contains the same number of atoms of each element on both sides. This follows the law of conservation of mass: matter is neither created nor destroyed in a chemical reaction.
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