Purity and Mixtures – Long Answer Questions (CBSE Class 10 Science - Chemistry)
Medium Level (Application & Explanation)
Q1. Differentiate between the common and scientific meanings of “pure.” Use daily-life examples.
Answer:
- In common use, pure means no adulteration or harmful mixing.
- In science, pure means only one type of particle with a uniform chemical nature.
- So, “pure milk” in the market means milk is not mixed with water or urea.
- But scientifically, milk is a mixture of water, fats, proteins, and sugar.
- Distilled water is a scientific pure substance because it is only H2O.
- Mineral water is a mixture of water and minerals, not a pure substance.
Q2. Explain why milk is a mixture even if it looks uniform to the eye.
Answer:
- Milk contains water, fats, proteins, lactose, and minerals.
- These are different kinds of particles. So milk is a mixture.
- It may look uniform, but the tiny fat and protein particles are spread in water.
- On standing, cream can separate. This shows more than one component.
- Its properties can vary from cow milk to buffalo milk.
- Hence, milk is not a pure substance in the scientific sense.
Q3. How would you judge the “purity” of common salt bought from the market?
Answer:
- First, read the label. Iodized salt has iodine and anti-caking agents.
- So, it is safe for health, but it is a mixture, not a pure substance.
- Scientifically pure salt means only sodium chloride (NaCl) particles.
- Dissolve a sample in water. Insoluble dirt shows visible impurity.
- Evaporate the solution slowly. Residue or different crystals show mixed components.
- A pure substance shows uniform crystals and consistent properties.
Q4. Why do we say sea water, minerals, and soil are mixtures?
Answer:
- Sea water has water, salts, gases, and organic matter.
- Minerals are often mixed with other minerals and rocks.
- Soil has sand, clay, humus, water, and air.
- These have two or more components present together.
- Their composition varies from place to place.
- So, they are mixtures, not single pure substances.
Q5. What is a pure substance in science? Give suitable examples and features.
Answer:
- A pure substance has only one kind of particle.
- It has fixed properties, like a sharp melting and boiling point.
- It shows uniform composition throughout.
- It cannot be separated by physical methods into other substances.
- Examples: Distilled water, oxygen gas, pure copper, pure sulfur.
- These are different from daily items like milk or juice, which are mixtures.
High Complexity (Analysis & Scenario-Based)
Q6. A label says “100% Pure Fruit Juice.” Is it a pure substance scientifically? Explain.
Answer:
- “100% pure” on the label means no adulteration or no added sugar/preservatives.
- But fruit juice has water, sugars, acids, vitamins, and flavors.
- These are different particles, so juice is a mixture.
- It is uniform to the eye but not a single kind of particle.
- So, it is not a pure substance in the scientific sense.
- The label is about safety and quality, not chemical purity.
Q7. Compare “Pure Cow Ghee” and “Vanaspati Blended Ghee.” Which is pure scientifically?
Answer:
- “Pure cow ghee” means ghee is not adulterated with cheap oils.
- “Vanaspati blended ghee” is a mixture of ghee and vanaspati (hydrogenated oils).
- Chemically, both contain many fatty molecules.
- So, both are mixtures, not pure substances.
- The word “pure” here refers to quality, not single-particle chemistry.
- In science, neither sample is a pure substance like pure water or pure copper.
Q8. A shopkeeper adds water to milk. Explain the effect on “purity” in common and scientific views.
Answer:
- In common view, this is clear adulteration. The milk becomes unsafe or low quality.
- In science, milk is already a mixture. Adding water makes it a more diluted mixture.
- Its density, taste, and nutrient content change.
- The freezing point rises and the boiling behavior changes.
- Tools like a lactometer may detect dilution.
- So, common purity goes down; scientific class (mixture) remains the same.
Q9. A student checks purity using melting/boiling points. Explain why this works, and apply it to water samples.
Answer:
- Pure substances have sharp melting/boiling points.
- Mixtures show a range or a shift in these temperatures.
- Distilled water boils at about 100°C at 1 atm pressure.
- Mineral water has dissolved salts; it may boil at a slightly higher temperature.
- This small shift shows it is a mixture, not pure H2O.
- So, fixed points help judge scientific purity, not market claims.
Q10. You need chemically pure water from sea water. Propose a method and justify scientifically.
Answer:
- Sea water is a mixture of water and salts.
- Use distillation to separate pure water (H2O) from salts.
- Heat sea water. Steam forms and leaves salts behind.
- Cool the steam to get distilled water.
- Distilled water has only one type of particle and fixed properties.
- This method converts a mixture into a pure substance scientifically.