🔹 Very Short Answer Questions (1 Mark Each)
Q1. Define pure substance.
Ans. A substance made up of only one kind of particles (atoms or molecules) is called a pure substance.
Q2. What is a mixture?
Ans. A substance composed of two or more pure substances physically mixed in any proportion.
Q3. Give two examples of elements.
Ans. Oxygen, Iron.
Q4. Give two examples of compounds.
Ans. Water (H₂O), Carbon dioxide (CO₂).
Q5. Give two examples of homogeneous mixtures.
Ans. Salt solution, Air.
Q6. Give two examples of heterogeneous mixtures.
Ans. Sand and water, Oil and water.
Q7. What are alloys?
Ans. Homogeneous mixtures of two or more metals, or a metal with a non-metal.
Q8. Define solution.
Ans. A homogeneous mixture of two or more substances.
Q9. What is the component present in larger amount in a solution called?
Ans. Solvent.
Q10. What is the component present in smaller amount in a solution called?
Ans. Solute.
🔹 Short Answer Questions (2–3 Marks Each)
Q11. Differentiate between homogeneous and heterogeneous mixtures.
Ans.
- Homogeneous: Uniform composition, cannot see separate components (e.g., sugar solution).
- Heterogeneous: Non-uniform composition, components can be seen (e.g., oil in water).
Q12. What are suspensions? Give an example.
Ans. A heterogeneous mixture in which solid particles are dispersed in a liquid or gas, and they settle on standing. Example: Muddy water.
Q13. Define colloid with one example.
Ans. A heterogeneous mixture where particle size is between true solution and suspension, particles do not settle. Example: Milk.
Q14. Differentiate between suspension and colloid (two points).
Ans.
- Suspension: Particles settle, unstable.
- Colloid: Particles do not settle, stable.
Q15. State two properties of a true solution.
Ans.
- Homogeneous mixture.
- Particles are very small and cannot be seen with naked eye.
Q16. What is Tyndall effect?
Ans. Scattering of light by colloidal particles.
Q17. Name one mixture that shows Tyndall effect and one that does not.
Ans. Shows: Milk (colloid).
Does not: Salt solution (true solution).
Q18. What is chromatography used for?
Ans. For separation of different coloured pigments from a mixture.
Q19. Name two methods of separating volatile and non-volatile components of a mixture.
Ans. Evaporation and Distillation.
Q20. Name a method to separate cream from milk.
Ans. Centrifugation.
🔹 Long Answer Questions (4–5 Marks Each)
Q21. Differentiate between elements, compounds and mixtures.
Ans.
- Element: Pure substance made of one type of atom (e.g., O₂).
- Compound: Pure substance made of two or more elements chemically combined (e.g., H₂O).
- Mixture: Physical combination of two or more substances (e.g., air).
Q22. Explain how you would separate salt from salt solution.
Ans. By evaporation: On heating, water evaporates leaving salt behind.
Q23. Explain the process of separating dyes in black ink using chromatography.
Ans. Place a drop of ink on filter paper and dip in water. Different dyes rise at different levels due to varying solubility.
Q24. State the principle of separation by distillation.
Ans. Distillation separates components based on difference in their boiling points.
Q25. What is fractional distillation? Give one use.
Ans. Fractional distillation is separation of miscible liquids having different boiling points. Example: Separation of petroleum into fractions.
Q26. How will you separate a mixture of sand and salt?
Ans.
- Add water – salt dissolves, sand remains.
- Filter – sand separates.
- Evaporate filtrate – salt is obtained.
Q27. Why is air considered a mixture and not a compound?
Ans.
- Variable composition.
- Components retain their properties.
- Can be separated by physical means (liquid air distillation).
Q28. Differentiate between physical and chemical changes (two examples each).
Ans.
- Physical: No new substance formed (e.g., melting ice, boiling water).
- Chemical: New substance formed (e.g., rusting, burning).
Q29. Write properties of colloids (any four).
Ans.
- Show Tyndall effect.
- Do not settle on standing.
- Cannot be filtered by ordinary filter paper.
- Stable in nature.
Q30. Explain crystallisation with an example.
Ans. Process of obtaining pure crystals of a substance from its saturated solution by cooling. Example: Crystals of copper sulphate.
🔹 Case Study / Application-Based Questions
Q31. Why is brass called an alloy? What are its components?
Ans. Brass is a homogeneous mixture of copper and zinc, hence an alloy.
Q32. Why can we see beam of light in a dusty room?
Ans. Due to scattering of light by dust particles, which is similar to Tyndall effect.
Q33. Why is smoke considered a colloid?
Ans. Because small solid particles are dispersed in air (gas).
Q34. Why is milk considered a colloid?
Ans. Because fat particles are dispersed in water.
Q35. Why is water a compound and not a mixture?
Ans. Because hydrogen and oxygen are chemically combined in fixed ratio.
Q36. Why is seawater called a mixture?
Ans. Because it contains salts, water and gases mixed physically, not chemically.
Q37. A student tries to separate petrol and kerosene. Which method should he use and why?
Ans. Fractional distillation, because both are miscible liquids with different boiling points.
Q38. Why is evaporation used to separate salt from seawater?
Ans. Because salt is non-volatile while water is volatile and evaporates easily.
Q39. Why is crystallisation considered a better method than evaporation for separating solids?
Ans. Because it removes impurities and gives pure crystals, while evaporation leaves impurities behind.
Q40. Why is Tyndall effect not shown in true solutions?
Ans. Because particles are too small to scatter light.
🔹 Additional Very Important Questions
Q41. Give two differences between solution and colloid.
Ans.
- Solution: Homogeneous, does not show Tyndall effect.
- Colloid: Heterogeneous, shows Tyndall effect.
Q42. Why can a solution not be separated by filtration?
Ans. Because solute particles are very small and pass through filter paper.
Q43. What type of mixture is air?
Ans. A homogeneous mixture of gases.
Q44. Give two examples of compounds used in daily life.
Ans. Common salt (NaCl), Sugar (C₆H₁₂O₆).
Q45. Give two examples of mixtures used in daily life.
Ans. Seawater, Soil.
Q46. Why can we separate mixtures by physical methods but not compounds?
Ans. Because components in mixtures are not chemically combined, while in compounds they are chemically bonded.
Q47. What is meant by “saturated solution”?
Ans. A solution in which no more solute can be dissolved at a given temperature.
Q48. What is the principle of centrifugation?
Ans. Separation based on difference in densities of components using centrifugal force.
Q49. Why is ink not a pure substance?
Ans. Because it is a mixture of different dyes.
Q50. Why is lemonade a mixture?
Ans. Because it contains water, sugar, salt, and lemon juice mixed physically, not chemically.