1) What is meant by a substance?
Answer: A substance is a form of matter that has a uniform and definite composition. It can be either an element or a compound, where all the constituent particles have the same chemical nature.
2) List the points of differences between homogeneous and heterogeneous mixtures.
• Homogeneous Mixtures:
1. Have a uniform composition throughout.
2. The different components are not visually distinguishable.
3. Examples: Salt dissolved in water, air.
• Heterogeneous Mixtures:
1. Do not have a uniform composition.
2. The different components are visually distinguishable.
3. Examples: Mixture of sand and iron filings, oil and water.
3) How are sol, solution, and suspension different from each other?
• Solution:
1. A homogeneous mixture with very small particle size (less than 1 nanometer).
2. Particles are not visible to the naked eye.
3. Does not scatter light, and the particles do not settle.
4. Example: Salt dissolved in water.
• Sol:
1. A type of colloid where solid particles are dispersed in a liquid.
2. Particles are larger than in a solution but smaller than in a suspension.
3. Scatters light (Tyndall effect) and is stable.
4. Example: Milk of magnesia, muddy water.
• Suspension:
1. A heterogeneous mixture with large particle size.
2. Particles are visible to the naked eye and can settle over time.
3. Scatters light and can be separated by filtration.
4. Example: Sand in water.
4) 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.
Answer: The concentration of the solution can be calculated as follows:
Mass of solution = Mass of solute + Mass of solvent = 36 g + 100 g = 136 g
Concentration = (Mass of solute / Mass of solution) × 100
= (36 g / 136 g) × 100 = 26.47%
So, the concentration of the sodium chloride solution is 26.47%.
5) 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
• Burning of paper and wood: Chemical change
6) Try segregating the things around you as pure substances or mixtures.
• Pure Substances:
o Water (distilled)
o Iron
o Gold
• Mixtures:
o Air
o Soil
o Tea
Exercise Questions
1. Which separation techniques will you apply for the separation of the following?
(a) Sodium chloride from its solution in water: Evaporation
(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 using a magnet
(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: Using a magnet
(i) Wheat grains from husk: Winnowing
(j) Fine mud particles suspended in water: Decantation and filtration
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate, and residue.
Answer: To make tea, first, boil water, which acts as the solvent. Add tea leaves, the solute, into the water. The tea leaves dissolve, creating a solution. However, tea leaves are insoluble, so they do not completely dissolve. After boiling, filter the solution to remove the tea leaves, which are the residue, leaving behind the filtrate, which is the tea.
3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).
(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?
Answer: If 62 grams dissolve in 100 grams of water, then in 50 grams of water: You would need (62 grams / 2) = 31 grams of potassium nitrate.
So, you would need 31 grams of potassium nitrate to produce a saturated solution in 50 grams of water at 313 K.
(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.
Answer: As the solution cools, some of the potassium chloride will start to crystallize out because the solubility decreases with temperature. This happens because a saturated solution at a higher temperature can hold more solute than at a lower temperature.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
Answer: At 293 K:
Potassium nitrate: 32 g/100 g water
Sodium chloride: 36 g/100 g water
Potassium chloride: 35 g/100 g water
Ammonium chloride: 37 g/100 g water
Ammonium chloride has the highest solubility at this temperature.
(d) What is the effect of change of temperature on the solubility of a salt?
Answer: Generally, the solubility of salts increases with an increase in temperature. This means that more salt can be dissolved in water at higher temperatures.
4. Explain the following giving examples.
(a) Saturated solution: A saturated solution is one that contains the maximum amount of solute that can dissolve at a given temperature. For example, if you add sugar to water until no more can dissolve, you have a saturated solution.
(b) Pure substance: A pure substance consists of only one type of particle and has a uniform composition. Examples include elements like gold and compounds like water.
(c) Colloid: A colloid is a mixture where very small particles of one substance are dispersed in another. These particles do not settle down and cannot be separated by filtration. Examples include milk and fog.
(d) Suspension: A suspension is a heterogeneous mixture in which the particles of the solute are large enough to be seen and may settle down upon standing. An example is muddy water.
5. Classify each of the following as a homogeneous or heterogeneous mixture.
1. Soda water: Homogeneous
2. Wood: Heterogeneous
3. Air: Homogeneous
4. Soil: Heterogeneous
5. Vinegar: Homogeneous
6. Filtered tea: Homogeneous
6. How would you confirm that a colorless liquid given to you is pure water?
Answer: To confirm if the liquid is pure water, check its boiling point. Pure water boils at 100°C (at 1 atmospheric pressure). If the liquid boils at this temperature, it is likely to be pure water.
7. Which of the following materials fall in the category of a “pure substance”?
(a) Ice: Pure substance
(b) Milk: Mixture
(c) Iron: Pure substance
(d) Hydrochloric acid: Pure substance
(e) Calcium oxide: Pure substance
(f) Mercury: Pure substance
(g) Brick: Mixture
(h) Wood: Mixture
(i) Air: Mixture
8. Identify the solutions among the following mixtures.
(a) Soil: Not a solution
(b) Sea water: Solution
(c) Air: Solution
(d) Coal: Not a solution
(e) Soda water: Solution
9. Which of the following will show “Tyndall effect”?
(a) Salt solution: Will not show Tyndall effect
(b) Milk: Will show Tyndall effect
(c) Copper sulphate solution: Will not show Tyndall effect
(d) Starch solution: Will show Tyndall effect
10. Classify the following into elements, compounds, and mixtures.
(a) Sodium: Element
(b) Soil: Mixture
(c) Sugar solution: Mixture
(d) Silver: Element
(e) Calcium carbonate: Compound
(f) Tin: Element
(g) Silicon: Element
(h) Coal: Mixture
(i) Air: Mixture
(j) Soap: Compound
(k) Methane: Compound
(l) Carbon dioxide: Compound
(m) Blood: Mixture
11. Which of the following are chemical changes?
(a) Growth of a plant: Chemical change
(b) Rusting of iron: Chemical change
(c) Mixing of iron filings and sand: Physical change
(d) Cooking of food: Chemical change
(e) Digestion of food: Chemical change
(f) Freezing of water: Physical change
(g) Burning of a candle: Both physical (melting) and chemical (burning) changes