| Class 10 |
After completing this chapter, students should be able to:
In our daily lives, many changes occur around us. Milk turns sour, iron rusts, and food gets digested in our bodies. All these are examples of chemical changes. This chapter explains how to represent these chemical changes using mathematical-style chemical equations, how to balance them to obey the Law of Conservation of Mass, and how to classify different types of reactions based on how atoms rearrange themselves.
A chemical reaction is a process in which one or more substances (reactants) are converted to one or more different substances (products).
How do we know a chemical reaction has taken place?Look for these signs:
1. Change in state (e.g., gas to liquid)
2. Change in colour (e.g., iron turning reddish-brown)
3. Evolution of a gas (e.g., bubbles forming)
4. Change in temperature (e.g., a flask getting hot)
Instead of writing long sentences like "Magnesium reacts with oxygen to form magnesium oxide", we write a chemical equation:
Mg + O₂ → MgOAccording to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. This means the number of atoms of each element must be the same on both sides of the arrow.
Unbalanced (Skeletal): Fe + H₂O → Fe₃O₄ + H₂ Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂| Term | Meaning |
| Reactants | Substances that take part in a chemical reaction (left side). |
| Products | New substances formed as a result of the reaction (right side). |
| Exothermic | Reactions in which heat is released along with the products. |
| Endothermic | Reactions that require energy (heat, light, or electricity) to proceed. |
| Catalyst | A substance that speeds up a reaction without being consumed. |
A reaction where two or more substances combine to form a single product.
Example: Burning of coal. C(s) + O₂(g) → CO₂(g)A single reactant breaks down into two or more simpler products.
Example: Heating limestone. CaCO₃(s) → CaO(s) + CO₂(g)A more reactive element displaces a less reactive element from its compound.
Example: Iron nail in copper sulphate solution. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)Reactions in which there is an exchange of ions between the reactants.
Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Note: BaSO₄ forms a white precipitate).When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode.
Example: The black coating on silver and the green coating on copper. Rusting of iron is the most common example.When fats and oils are oxidized, they become rancid, and their smell and taste change.
Prevention: Storing food in airtight containers or flushing bags of chips with nitrogen gas.1. Identify the products when Magnesium burns in the air.
2. What is meant by a skeletal chemical equation?
3. Why should a magnesium ribbon be cleaned before burning in air?
4. Write the balanced equation for: Hydrogen + Chlorine → Hydrogen chloride.
5. A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.
6. Why is respiration considered an exothermic reaction? Explain.
A. Both A and R are true and R is the correct explanation of A.
B. Both are true but R is NOT the correct explanation.
C. A is true but R is false.
D. A is false but R is true.
Assertion (A): The balancing of chemical equations is based on law of conservation of mass. Reason (R): Total mass of reactants is equal to total mass of products. Answer: AA) Dilution of sulphuric acid
B) Sublimation of dry ice
C) Condensation of water vapours
D) Respiration in human beings
Correct Answer: B. Sublimation requires heat. 2. What type of reaction is: Pb + CuCl₂ → PbCl₂ + Cu?A) Combination
B) Decomposition
C) Displacement
D) Double Displacement
Correct Answer: C. Lead displaces Copper.1. Magnesium Oxide (MgO).
2. An unbalanced chemical equation.
3. To remove the protective layer of basic magnesium carbonate so it burns readily.
4. H₂ + Cl₂ → 2HCl.
5. X is Copper (Cu). The black compound is Copper(II) oxide (CuO).
6. During respiration, glucose breaks down combining with oxygen to release energy, hence it is exothermic.