Class 9 › Science › Exploration Exploring Mixtures and their Separation Class 9 Science · Exploration · Chapter 5
Summary Questions & answers Practise this chapter → This chapter looks closely at mixtures and the techniques used to separate them. A homogeneous mixture (a solution) has uniform composition throughout — every sip of sugar-water is equally sweet; examples include vinegar and soda. A heterogeneous mixture does not have uniform composition (e.g. sand in water). Mixtures are also classified by particle size into true solutions (particles too small to see or scatter light), colloids (medium-sized particles that scatter a beam of light — the Tyndall effect, as in milk or fog) and suspensions (large particles that settle on standing, like muddy water). A solution has a solvent (present in larger amount) and a solute; solubility depends on temperature and the nature of the substances, and a solution that can dissolve no more solute is saturated. Concentration can be expressed as mass percentage or volume percentage. The chapter then surveys separation methods chosen according to the property that differs between components: evaporation (recovering a dissolved solid), filtration (insoluble solid from liquid), sedimentation and decantation, centrifugation (separating denser particles by spinning, as in blood tests), separating-funnel (immiscible liquids by density), sublimation (solids that turn directly to vapour like camphor), chromatography (separating dissolved coloured components such as dyes in ink), and distillation and fractional distillation (separating miscible liquids by differences in boiling point). These techniques underpin everyday and industrial processes from making salt and sugar to medical diagnosis.
Homogeneous mixture (solution)
A mixture with uniform composition throughout, in which the components cannot be distinguished, e.g. sugar dissolved in water.
Tyndall effect
The scattering of a beam of light by the medium-sized particles of a colloid, making the light path visible (as in milk or fog).
Suspension
A heterogeneous mixture whose large particles are visible and settle down on standing, e.g. muddy water.
Saturated solution
A solution that has dissolved the maximum amount of solute possible at a given temperature.
Chromatography
A technique that separates dissolved components, such as the different dyes in ink, based on how differently they move through a medium.
Fractional distillation
A method to separate two or more miscible liquids by repeated evaporation and condensation using their different boiling points.
Q1 Differentiate homogeneous and heterogeneous mixtures. Answer Q2 What is the Tyndall effect and where is it seen? Answer Q3 How do a true solution, a colloid and a suspension differ? Answer Q4 What is the difference between a solvent and a solute? Answer Q5 What is a saturated solution? Answer Q6 How would you separate a mixture of salt and water? Answer Q7 Why is centrifugation used, and give one use. Answer Q8 How are two immiscible liquids separated? Answer Q9 Which method separates camphor from sand, and why? Answer Q10 How does distillation differ from fractional distillation? Answer Q1 Why do suspended particles settle in muddy water over time but not in milk? Answer Q2 How is evaporation different from boiling? Answer Q3 Why do you see bright rays of sunlight when it passes through small gaps between the leaves of a dense tree? Answer Q1 1. A common talcum powder contains 4 % m/m zinc oxide, which acts as an antiseptic. How much zinc oxide is present in 300 g of the talcum powder? Answer Q2 2. Your mother gives you a bottle of orange juice concentrate to mix with water and serve it to your visiting friends. She asks you to mix two tablespoons of the concentrate with water in a glass tumbler. If each tablespoon measures 15 mL and you make 150 mL of juice per person, what is the % v/v of orange juice concentrate in the mixture you prepared? Answer Q3 3. Vinegar, used as a food preservative and additive, contains 5 % v/v acetic acid. Glacial acetic acid is a liquid, i.e., 100% acetic acid. If you want to make vinegar from glacial acetic acid, how would you proceed? Answer Q1 Based on the information from the above graph, predict which of the two compounds, ‘A’ or ‘B’, will dissolve more in a given amount of water at a given temperature? Answer Q2 Observe Fig. 5.6 and fill in the blanks of the following statements: (i) The solubility of compound ‘A’ in water at 20 °C is ______________ (less than/more than/similar to) its solubility at 60 °C. Answer Q3 Observe Fig. 5.6 and fill in the blanks of the following statements: (ii) The solubility of compound ‘B’ at 20 °C is ____________ (less than/more than/similar to) its solubility at 60 °C. Answer Q4 Observe Fig. 5.6 and fill in the blanks of the following statements: (iii) The solubility of _____________ increases more than that of ______________ with an increase in the temperature. Answer Q1 If a hot, saturated solution of copper sulfate is cooled rapidly in ice-cold water, smaller and less well-formed crystals will form than if it is cooled slowly at room temperature. How would you design and perform an experiment to test this hypothesis? Hint: Prepare a hot saturated solution of copper sulfate and divide it into two equal parts. Answer Q1 4. Refer to the solubility curves given in Activity 5.2. If equal masses of hot, saturated solutions of compounds ‘A’ and ‘B’ are cooled from 80 °C to 60 °C, which solution is likely to deposit more solid? Answer Q2 5. Will there be any change in the size of common salt crystals if the rate of evaporation is increased or decreased? Explain. Answer Q1 Observe Fig. 5.9, it shows how salt crystals are obtained from seawater. Can you describe the process in your own words? Answer Q1 6. State whether the following statements are True or False. Also, correct the False statements. (i) Salt can be separated from a salt solution by evaporation or distillation. Answer Q2 6. State whether the following statements are True or False. Also, correct the False statements. (ii) Distillation can be used for separation of two liquids even when these have the same boiling point. Answer Q3 6. State whether the following statements are True or False. Also, correct the False statements. (iii) In paper chromatography, the solvent level should be above the sample spot at the beginning of the experiment. Answer Q4 6. State whether the following statements are True or False. Also, correct the False statements. (iv) Evaporation and crystallization are the same processes. Answer Q1 7. Why do immiscible liquids form two separate layers in a separating funnel? Answer Q2 8. Is sublimation different from evaporation? Justify. Answer Q1 9. Clouds are made up of tiny water droplets or ice crystals floating in the air. Based on what you know about solutions, suspensions and colloids, what type of mixture do you think clouds are and why? Answer Q2 10. Why do cities with a lot of smoke and dust in the air often look hazy? Answer Q1 1. Which of the following mixtures are correctly classified as homogeneous (Hm) and heterogeneous (Ht)? Choose the correct option. (i) Air — Hm, Milk — Ht, Sugar solution — Hm, Smoke — Hm (ii) Brass — Ht, Fog — Ht, Vinegar — Ht, Muddy water — Hm (iii) Copper sulfate solution — Hm, Salt solution — Hm, Milk — Hm, Bronze — Hm (iv) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm Answer Q2 2. Choose the correct options, and explain the reason for the correct and incorrect options. Which among the following mixtures show the Tyndall Effect? A mixture of: (a) air and dust particles (b) copper sulfate and water (c) starch and water (d) acetone and water (i) a and b (ii) b and d (iii) a and c (iv) c and d Answer Q3 3. A mixture can be categorised as a solution, a suspension, or a colloid, each possessing distinct properties. Utilise the words or phrases provided in the box to fill in the Table 5.2. Words and Phrases Large-sized particles; Particles remain evenly distributed; Small-sized particles (less than 1 nm diameter); Moderate-sized particles (1–1000 nm); Settles down when left undisturbed (more than 1000 nm in diameter); Does not settle down; Scatters light; Separates by filtration; Transparent; Salt solution; Milk; Sand in water; Smoke; Heterogeneous mixture; Cannot be separated by filtration; Mud; Butter; Brass. Complete the Table 5.2. - Solution Properties Answer Q4 3. A mixture can be categorised as a solution, a suspension, or a colloid, each possessing distinct properties. Utilise the words or phrases provided in the box to fill in the Table 5.2. Words and Phrases Large-sized particles; Particles remain evenly distributed; Small-sized particles (less than 1 nm diameter); Moderate-sized particles (1–1000 nm); Settles down when left undisturbed (more than 1000 nm in diameter); Does not settle down; Scatters light; Separates by filtration; Transparent; Salt solution; Milk; Sand in water; Smoke; Heterogeneous mixture; Cannot be separated by filtration; Mud; Butter; Brass. Complete the Table 5.2. - Suspension Properties Answer Q5 3. A mixture can be categorised as a solution, a suspension, or a colloid, each possessing distinct properties. Utilise the words or phrases provided in the box to fill in the Table 5.2. Words and Phrases Large-sized particles; Particles remain evenly distributed; Small-sized particles (less than 1 nm diameter); Moderate-sized particles (1–1000 nm); Settles down when left undisturbed (more than 1000 nm in diameter); Does not settle down; Scatters light; Separates by filtration; Transparent; Salt solution; Milk; Sand in water; Smoke; Heterogeneous mixture; Cannot be separated by filtration; Mud; Butter; Brass. Complete the Table 5.2. - Colloid Properties Answer Q6 4. Solve the following problems: (i) A cake recipe uses dry ingredients, namely 75 g of sugar for 420 g of all-purpose flour and 5 g of sodium hydrogencarbonate. Express the concentration of each component in the mixture using an appropriate method. Answer Q7 4. Solve the following problems: (ii) A brass alloy contains 70% copper by mass. Calculate the quantities of copper and zinc present in 120 g of brass. Answer Q8 5. The label on a cooking oil pack says one litre (910 g). If this oil is mixed with water, will it form a separate layer? If so, which substance will be on top? How will you separate the two layers? Also, draw the diagram of the apparatus used. Answer Q9 6. Assertion (A): Solutions do not exhibit the Tyndall effect. Reason (R): The particles in solutions are larger than 100 nm, so they cannot scatter light. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true. Answer Q10 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Mud from muddy water Answer Q11 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Plasma from other components in the blood sample Answer Q12 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Naphthalene and sand Answer Q13 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Chalk powder and common salt Answer Q14 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Common salt and water Answer Q15 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Oil from water Answer Q16 7. How would you separate the mixtures given in Table 5.3? Mention the reason for choosing your method. If a mixture cannot be separated, explain why. - Pigments of the flower Answer Q17 8. Two miscible liquids, A and B, are present in a mixture. The boiling point of A is 60 °C and the boiling point of B is 90 °C. Suggest a method to separate them. Also, draw a labelled diagram of the method suggested. Answer Q18 9. Compare evaporation, crystallization and distillation. In which situation, would you prefer each of these over the others? Answer Q19 10. Blood is an example of a colloidal mixture. (i) What would happen if blood behaved like a true suspension inside the body? Answer Q20 10. Blood is an example of a colloidal mixture. (ii) In a blood sample, identify the dispersed phase and the dispersion medium. Answer Q21 11. You are given a mixture of sand, common salt and naphthalene (Fig. 5.25a). The Fig. 5.25b depicts various steps used to separate the components of this mixture. Identify and write down the correct sequence of separation techniques. Answer Q22 12. Why is distillation an effective method for separating a mixture of water and acetone? Answer Q23 13. Answer the following questions with the help of the data given in Table 5.4. (i) What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40 °C? Answer Q24 13. Answer the following questions with the help of the data given in Table 5.4. (ii) A student makes a saturated solution of potassium chloride in water at 80 °C and leaves the solution to cool at room temperature (25 °C). What would she observe as the solution cools? Explain. Answer Q25 13. Answer the following questions with the help of the data given in Table 5.4. (iii) What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10 °C to 80 °C. Answer Q26 14. Three students, A, B and C, are preparing sugar solutions for an experiment: Student A dissolves 20 g of sugar in 80 g of water. Student B dissolves 20 g of sugar in 100 g of water. Student C dissolves 30 g of sugar in 80 g of water. (i) Calculate the mass percentage (% m/m) concentration of sugar in each student’s solution. Answer Q27 14. Three students, A, B and C, are preparing sugar solutions for an experiment: Student A dissolves 20 g of sugar in 80 g of water. Student B dissolves 20 g of sugar in 100 g of water. Student C dissolves 30 g of sugar in 80 g of water. (ii) Whose solution is the most concentrated? Explain why. Answer Q28 15. Examine Fig. 5.26. (i) Identify the separation technique marked as ‘S’. Answer Q29 15. Examine Fig. 5.26. (ii) Label the apparatus A, B and C. Answer Q30 15. Examine Fig. 5.26. (iii) Which of the following mixtures can be separated by the technique identified above? Use the data given in Table 5.5. Mixtures: (a) water — acetone (b) water — salt (c) acetone — alcohol (d) sand — salt (e) alcohol — chloroform (f) alcohol — benzene Answer
A mixture with uniform composition throughout, such as sugar dissolved in water, is called:
A A heterogeneous mixture B A suspension C A homogeneous mixture D A colloid
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