Science Questions and Answers

Basic Concepts
Classification & Mixtures
Separation Techniques
Solutions & Solubility

Basic Concepts

1. What is meant by a substance?

A substance is a matter that has a definite composition and distinct properties. It can be an element or a compound that cannot be separated into other kinds of matter by any physical process.

2. List the points of differences between homogeneous and heterogeneous mixtures.
  • Homogeneous mixtures have uniform composition throughout, while heterogeneous mixtures have non-uniform composition.
  • In homogeneous mixtures, components are not visible to naked eye, whereas in heterogeneous mixtures, components are usually visible.
  • Homogeneous mixtures have only one phase, while heterogeneous mixtures have more than one phase.
  • Examples of homogeneous mixtures: salt solution, sugar solution. Examples of heterogeneous mixtures: mixture of sand and iron filings, mixture of oil and water.
3. Classify the following as chemical or physical changes:
  • Cutting of trees - Physical change
  • Melting of butter in a pan - Physical change
  • Rusting of almirah - Chemical change
  • Boiling of water to form steam - Physical change
  • Passing of electric current through water and the water breaking down into hydrogen and oxygen gases - Chemical change
  • Dissolving common salt in water - Physical change
  • Making a fruit salad with raw fruits - Physical change

Classification & Mixtures

1. Differentiate between homogeneous and heterogeneous mixtures with examples.

Homogeneous mixtures: These are mixtures that have uniform composition and properties throughout. The components are not visible to the naked eye and cannot be separated by filtration. Examples: Salt solution, sugar solution, air.

Heterogeneous mixtures: These are mixtures that do not have uniform composition. The components are visible to the naked eye and can be separated by simple physical methods. Examples: Mixture of sand and salt, mixture of oil and water, concrete.

2. How are sol, solution and suspension different from each other?
Property Solution Sol (Colloid) Suspension
Particle Size Less than 1 nm 1 nm to 100 nm More than 100 nm
Visibility Particles not visible Particles not visible with naked eye Particles visible with naked eye
Tyndall Effect Does not show Shows Shows
Stability Stable Stable Unstable
Separation Cannot be separated by filtration Cannot be separated by filtration Can be separated by filtration
Examples Sugar solution, salt solution Milk, blood, ink Muddy water, chalk in water
7. Which of the following materials fall in the category of a "pure substance"?

Pure substances: (a) Ice, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f) Mercury

Not pure substances: (b) Milk, (g) Brick, (h) Wood, (i) Air (these are mixtures)

8. Identify the solutions among the following mixtures.

Solutions: (b) Sea water, (c) Air, (e) Soda water

Not solutions: (a) Soil, (d) Coal

9. Which of the following will show "Tyndall effect"?

Will show Tyndall effect: (b) Milk, (d) Starch solution

Will not show Tyndall effect: (a) Salt solution, (c) Copper sulphate solution

10. Classify the following into elements, compounds and mixtures.

Elements: (a) Sodium, (d) Silver, (f) Tin, (g) Silicon

Compounds: (e) Calcium carbonate, (k) Methane, (l) Carbon dioxide

Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) Blood

11. Which of the following are chemical changes?

Chemical changes: (a) Growth of a plant, (b) Rusting of iron, (d) Cooking of food, (e) Digestion of food, (g) Burning of a candle

Physical changes: (c) Mixing of iron filings and sand, (f) Freezing of water

Separation Techniques

1. Which separation techniques will you apply for the separation of the following?
  • (a) Sodium chloride from its solution in water - Evaporation or crystallization
  • (b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride - Sublimation
  • (c) Small pieces of metal in the engine oil of a car - Filtration or sedimentation and decantation
  • (d) Different pigments from an extract of flower petals - Chromatography
  • (e) Butter from curd - Centrifugation
  • (f) Oil from water - Separating funnel
  • (g) Tea leaves from tea - Filtration
  • (h) Iron pins from sand - Magnetic separation
  • (i) Wheat grains from husk - Winnowing
  • (j) Fine mud particles suspended in water - Centrifugation or filtration
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Steps for making tea:

  1. Take water in a pan. Water acts as the solvent.
  2. Add tea leaves to the water. Tea leaves contain soluble components that will dissolve in water.
  3. Heat the mixture. The soluble components of tea leaves become the solute that dissolves in the solvent to form a solution.
  4. Add sugar which is also soluble and dissolves in the solution.
  5. Add milk which mixes with the solution.
  6. Filter the tea using a strainer. The liquid that passes through is the filtrate, which is our tea.
  7. The solid tea leaves left in the strainer form the residue as they are insoluble in water.

Solutions & Solubility

3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.

Mass of solute (sodium chloride) = 36 g

Mass of solvent (water) = 100 g

Mass of solution = Mass of solute + Mass of solvent = 36 g + 100 g = 136 g

Concentration = (Mass of solute / Mass of solution) × 100

Concentration = (36 / 136) × 100 = 26.47%

Therefore, the concentration of the saturated solution is 26.47%.

3. Pragya tested the solubility of three different substances at different temperatures...

(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?

From the table, at 313 K, 62 g of potassium nitrate dissolves in 100 g of water to form a saturated solution.

For 50 g of water, mass of potassium nitrate needed = (62/100) × 50 = 31 g

(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.

As the solution cools, she would observe crystals of potassium chloride forming and settling at the bottom. This happens because the solubility of potassium chloride decreases with decrease in temperature. At room temperature (around 293 K), the solubility is only 35 g per 100 g of water, so the excess potassium chloride crystallizes out.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

At 293 K:

  • Potassium nitrate: 32 g/100g water
  • Sodium chloride: 36 g/100g water
  • Potassium chloride: 35 g/100g water
  • Ammonium chloride: 37 g/100g water

Ammonium chloride has the highest solubility (37 g/100g water) at 293 K.

(d) What is the effect of change of temperature on the solubility of a salt?

The solubility of most salts increases with increase in temperature. However, the extent of increase varies for different salts. For example, potassium nitrate shows a large increase in solubility with temperature, while sodium chloride shows only a small increase.

4. Explain the following giving examples.

(a) Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution. Example: When sugar stops dissolving in water and settles at the bottom, the solution is saturated.

(b) Pure substance: A substance that has a fixed composition and distinct properties. It contains only one type of particles. Examples: Elements like gold, copper; compounds like water, sodium chloride.

(c) Colloid: A heterogeneous mixture where the particle size is between 1 nm to 100 nm. The particles are not visible to naked eye but scatter light (show Tyndall effect). Examples: Milk, blood, fog.

(d) Suspension: A heterogeneous mixture where the particle size is more than 100 nm. The particles are visible to naked eye, settle down on standing, and can be separated by filtration. Examples: Muddy water, chalk in water.

5. Classify each of the following as a homogeneous or heterogeneous mixture.
  • Soda water - Homogeneous mixture
  • Wood - Heterogeneous mixture
  • Air - Homogeneous mixture
  • Soil - Heterogeneous mixture
  • Vinegar - Homogeneous mixture
  • Filtered tea - Homogeneous mixture
6. How would you confirm that a colourless liquid given to you is pure water?

To confirm that a colourless liquid is pure water:

  1. Check its boiling point - pure water boils at 100°C at standard atmospheric pressure.
  2. Check its freezing point - pure water freezes at 0°C.
  3. Measure its density - pure water has a density of 1 g/mL at 4°C.
  4. Test for purity using evaporation - pure water should leave no residue after evaporation.
  5. Check electrical conductivity - pure water is a poor conductor of electricity.