05. Short and Long Answer Questions: Exploring Mixtures and Their Separation

Short Answer Type Questions

Ques 1: What is the difference between a homogeneous mixture and a heterogeneous mixture? Give two examples of each.

Ans: A homogeneous mixture has a uniform composition throughout – every part of the mixture looks and feels the same. Examples: sugar dissolved in water, vinegar (acetic acid in water).

A heterogeneous mixture does not have a uniform composition – different parts of the mixture may look different. Examples: sand and water, oil and water.

A solution is always homogeneous, whereas a suspension is always heterogeneous.

Ques 2: Define concentration of a solution. Write the formula for mass by mass percentage and calculate the mass by mass percentage of a solution prepared by dissolving 20 g of salt in 80 g of water.

Ans: The amount of solute dissolved in a given amount of solvent or solution is called the concentration of the solution.

$$\mathrm{\%}m\mathrm{/}m=\frac{\text{Mass of solute}}{\text{Mass of solution}}\times 100$$

Mass of solute (salt) $=20\text{ g}$

Mass of solvent (water) $=80\text{ g}$

Total mass of solution $=20+80=100\text{ g}$

$$\mathrm{\%}m\mathrm{/}m=\frac{20}{100}\times 100=20\mathrm{\%}m\mathrm{/}m$$

Ques 3: What is a saturated solution? How does temperature affect the solubility of solid solutes in liquids?

Ans: A solution that cannot dissolve any more solute at a given temperature is called a saturated solution. The maximum amount of solute that dissolves in 100 g (or 100 mL) of a solvent at a given temperature is called its solubility.

For solid solutes dissolved in liquids, solubility generally increases with an increase in temperature. This is why more sugar or salt dissolves in hot water than in cold water.

In contrast, for gases dissolved in liquids, solubility generally decreases with an increase in temperature.

Ques 4: What is crystallization? State the principle on which it is based and mention any two conditions required for it to occur successfully.

Ans: Crystallization is the process of forming pure crystals from a saturated solution by cooling it slowly. A crystal is a solid whose particles are arranged in a regular geometric pattern.

The process is based on the difference in solubility of a substance at different temperatures – more solute dissolves at higher temperature, so when the solution is cooled, the extra solute separates out as crystals.

Two conditions required:

1. The solution must be saturated at the higher temperature.
2. The solution should be cooled slowly and without disturbance so that large, well-formed crystals are obtained.

Ques 5: What is distillation? State the condition under which it can be used to separate two miscible liquids.

Ans: Distillation is the process of separating a homogeneous mixture of two miscible liquids by heating the mixture until the liquid with the lower boiling point vaporises, then cooling the vapour back into a liquid (distillate) using a condenser.

Condition: Distillation can be used when the boiling points of the two liquids differ by at least 25 °C. For example, acetone (boiling point 56 °C) and water (boiling point 100 °C) can be separated easily by distillation.

It can also be used to separate a liquid from a solution containing a dissolved solid.

Ques 6: What is paper chromatography? On what principle does it work? Give one real-life application of this technique.

Ans: Paper chromatography is a technique used to separate components of a mixture by taking advantage of differences in how fast each component travels on the paper when carried by a solvent.

Principle: The components of the mixture interact differently with the solvent and the paper. The solvent (mobile phase) carries the components up the paper; components that interact more strongly with the paper move slowly, while those interacting more with the solvent move faster, causing separation.

Application: Paper chromatography is used to separate different coloured pigments present in plant leaves (e.g., spinach extract) or to identify the dyes present in inks and food colours.

Ques 7: What is a separating funnel? Describe with the help of a brief procedure how you would use it to separate mustard oil from water.

Ans: A separating funnel is a glass apparatus used to separate two immiscible (non-mixing) liquids that form distinct layers because of their different densities. It has a stopcock at the bottom to drain out the lower layer.

Procedure:

1. Pour the mixture of mustard oil and water into the separating funnel and allow it to stand undisturbed.
2. Two distinct layers will form – the denser water settles at the bottom and the lighter mustard oil floats on top.
3. Open the stopcock slowly to drain out the lower water layer into a container; close it when the oil layer begins to flow.
4. Collect the oil layer separately by opening the stopcock again.

Ques 8: What is sublimation? Explain how it can be used to separate camphor from sand.

Ans: Sublimation is the process in which a solid directly changes into the vapour state upon heating without passing through the liquid state. When the vapour cools back into a solid without becoming a liquid, the process is called deposition.

Separation of camphor from sand:

1. Place the camphor-sand mixture in a china dish on a wire gauze over a burner.
2. Invert a glass funnel (with a cotton plug at the nozzle) over the china dish.
3. Heat the dish gently. Camphor sublimes and its vapour rises up into the funnel.
4. On cooling, camphor vapour deposits as pure white solid on the inner walls of the funnel, while sand remains in the china dish.

Ques 9: What is centrifugation? Explain the principle behind it and state any two practical applications.

Ans: Centrifugation is the process of separating the components of a heterogeneous solid-liquid mixture by spinning the mixture at high speed in a tube.

Principle: When the mixture is spun at high speed, the tubes become horizontal and the centrifugal force (outward force on a body in circular motion) causes the heavier particles to move outward and settle at the bottom of the tube, while the lighter liquid remains at the top.

Applications:

1. In laboratories, centrifugation is used to separate the components of blood, such as red blood cells, platelets, white blood cells, and plasma.
2. A hand-powered device called a paperfuge uses the same principle to help detect diseases like malaria and anaemia in remote areas without electricity.

Ques 10: What is coagulation? How is alum used to purify muddy water? Name the process by which the coagulated particles are finally removed.

Ans: Coagulation is the process by which fine suspended particles in a liquid are made to clump together into larger masses by adding a substance called a coagulant.

When powdered alum (fitkari) is added to muddy water, it acts as a coagulant. The alum causes the fine suspended mud particles to clump together into larger, heavier masses. These larger clumps then settle to the bottom by gravity (sedimentation).

The settled impurities can then be removed from the clear water above by decantation or filtration. The formation of cheese (paneer) from milk is also an example of coagulation, where acid (lemon juice or vinegar) acts as the coagulant.

Ques 11: What is a colloid? How is it different from a solution and a suspension in terms of particle size? Give two examples of colloids found in daily life.

Ans: A colloid is a type of mixture that is neither a true solution nor a true suspension. In a colloid, the dispersed particles are uniformly spread throughout the mixture and do not settle over time, unlike particles in a suspension.

Comparison of particle size:

1. Solution: Particle size is less than 1 nm in diameter – particles are not visible even under a microscope.
2. Colloid: Particle size is between 1 nm and 1000 nm in diameter.
3. Suspension: Particle size is more than 1000 nm – particles are visible to the naked eye and settle over time.

Examples of colloids: milk, tomato sauce, ice cream, fog (water droplets in air).

Ques 12: What is the Tyndall effect? Why is this effect observed in a colloid or a suspension but not in a true solution? Give one everyday example where this effect can be seen.

Ans: The Tyndall effect is the scattering of a beam of light by the particles in a colloid or a suspension, making the path of the light beam visible when viewed from the side. It was first explained by the scientist John Tyndall.

Why not in a solution? In a true solution, the dissolved particles (solute) are extremely small (less than 1 nm). These particles are too small to scatter light, so the beam of light passes through without becoming visible. In colloids and suspensions, the particles are larger and scatter the light beam, making it visible.

Everyday example: When a fine beam of light enters a dark room through a small hole, the path of the beam becomes visible due to scattering of light by dust particles in the air. This is the Tyndall effect. The same can be seen in the floodlights of a sports stadium.

Long Answer Type Questions

Ques 1: Explain the three methods of expressing the concentration of a solution in terms of percentage. Write the formula for each method and give a solved numerical example for mass by volume percentage.

Ans: The concentration of a solution tells us the amount of solute dissolved in a given amount of solvent or solution. It can be expressed as a percentage in three main ways:

(A) Mass by Mass Percentage (% m/m or % w/w):
This method is used to express the concentration of homogeneous mixtures and is also used on food labels. It tells us how many grams of solute are present in 100 g of the total solution.

$$\mathrm{\%}m\mathrm{/}m=\frac{\text{Mass of solute}}{\text{Mass of solution}}\times 100$$

(B) Mass by Volume Percentage (% m/v or % w/v):
This method is used in medicines and laboratories where measuring the volume of a liquid is easier than weighing it. It tells us how many grams of solute are present in 100 mL of the solution. Example: 5% glucose solution used in hospitals.

$$\mathrm{\%}m\mathrm{/}v=\frac{\text{Mass of solute}}{\text{Volume of solution}}\times 100$$

(C) Volume by Volume Percentage (% v/v):
This method is used when two miscible liquids are mixed, such as in perfumes, cosmetics, and vinegar. It tells us how many millilitres of solute are present in 100 mL of the solution.

$$\mathrm{\%}v\mathrm{/}v=\frac{\text{Volume of solute}}{\text{Volume of solution}}\times 100$$

Solved Example (% m/v): If 15 g of glucose is dissolved in water to make 300 mL of solution, calculate its concentration in mass by volume percentage.

Mass of glucose (solute) $=15\text{ g}$

Volume of solution $=300\text{ mL}$

$$\mathrm{\%}m\mathrm{/}v=\frac{15}{300}\times 100=5\mathrm{\%}m\mathrm{/}v$$

Note: % m/m and % w/w are numerically equal and used interchangeably in industries.

Ques 2: What is fractional distillation? How is it different from simple distillation? Explain with the help of the industrial separation of crude petroleum as an example.

Ans: Fractional distillation is the process of separating components of a mixture with relatively small differences (less than 25 °C) in their boiling points. It is an extension of simple distillation and uses a fractionating column to achieve separation.

Difference from Simple Distillation:

1. Simple distillation works when the boiling points differ by at least 25 °C. Fractional distillation is used when the difference is less than 25 °C.
2. Simple distillation separates one liquid from another or from a dissolved solid. Fractional distillation separates a mixture into several different fractions at different temperatures.
3. Fractional distillation requires a fractionating column, whereas simple distillation does not.

Industrial Separation of Crude Petroleum:
Crude petroleum (crude oil) is a mixture of several useful liquids such as petroleum gas, petrol, kerosene, diesel, lubricating oil, and bitumen – each having a different boiling point. The crude oil is fed into a furnace and heated. The vapours rise up a fractionating tower (a tall column that is hotter at the bottom and cooler at the top).

As the vapours rise, they cool at different heights and condense into liquids at the temperature corresponding to their boiling points. The fractions are collected at different levels:

1. Petroleum gas – collected at the top (lowest boiling point)
2. Petrol and aviation fuel (kerosene) – middle levels
3. Diesel and lubricating oil – lower levels
4. Bitumen – remains at the bottom (highest boiling point)

The gaseous fraction is filled in steel cylinders under high pressure (liquefied) and used as LPG (Liquefied Petroleum Gas) for domestic fuel.

Ques 3: Compare suspensions, colloids, and solutions on the basis of: (i) nature of mixture, (ii) particle size, (iii) visibility of particles, (iv) settling behaviour, and (v) Tyndall effect. Give one example of each.

Ans: The three types of mixtures – solutions, colloids, and suspensions – can be compared as follows:

(i) Nature of Mixture:
Solution – Homogeneous (uniform composition throughout).
Colloid – Appears homogeneous but is actually heterogeneous at the microscopic level.
Suspension – Heterogeneous (non-uniform composition).

(ii) Particle Size:
Solution – Less than 1 nm in diameter.
Colloid – Between 1 nm and 1000 nm in diameter.
Suspension – More than 1000 nm in diameter.

(iii) Visibility of Particles:
Solution – Particles not visible even under a microscope.
Colloid – Particles not visible to the naked eye but can be detected by the Tyndall effect.
Suspension – Particles visible to the naked eye.

(iv) Settling Behaviour:
Solution – Particles do not settle; remain uniformly distributed.
Colloid – Particles do not settle over time (unlike suspensions).
Suspension – Particles settle down when left undisturbed.

(v) Tyndall Effect:
Solution – Does not show the Tyndall effect (particles too small to scatter light).
Colloid – Shows the Tyndall effect.
Suspension – Shows the Tyndall effect.

Examples:
Solution – salt solution (salt dissolved in water).
Colloid – milk (fat droplets dispersed in water).
Suspension – muddy water (sand particles suspended in water).

Ques 4: What is an alloy? Why are physical separation methods ineffective for alloys? Give three examples of commonly used alloys, stating the metals present in each and one property that makes them useful.

Ans: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal, formed by melting the metals together at high temperatures and allowing the hot mixture to cool and solidify. The resulting material appears to be a single metal.

Why physical methods cannot separate alloys:
Since an alloy is a homogeneous mixture in the solid state – the atoms of the component metals are uniformly mixed at the atomic level – their particles cannot be separated by physical methods such as filtration, distillation, or a separating funnel. The components can only be separated by chemical means or by extremely advanced industrial techniques (e.g., electrolysis).

Three examples of alloys:

1. Brass – approximately 80% copper and 20% zinc. It is harder and more corrosion-resistant than pure copper, and is used in making musical instruments, taps, and fittings.
2. Bronze – approximately 80% copper and 20% tin. It is stronger than copper and is used in making coins, statues, and medals.
3. Stainless Steel – mainly iron with carbon (0.03-0.8%), chromium (16-18%), nickel (10-14%), and molybdenum. It is highly resistant to corrosion (rusting) and is used in making utensils, surgical instruments, and cutlery.

Alloys are generally prepared to produce materials that are stronger, more rigid, or more corrosion-resistant than the individual metals.

Ques 5: A student has a mixture of sand, common salt, and naphthalene. Describe step-by-step the correct sequence of separation techniques to obtain each component in pure form. Give a reason for choosing each technique.

Ans: The mixture contains three components: sand (insoluble in water), common salt (soluble in water), and naphthalene (sublimes on heating). The separation is done in three steps:

Step 1 – Sublimation (to separate naphthalene):
Heat the mixture gently in a china dish using an inverted glass funnel over it (with a cotton plug at the nozzle). Naphthalene sublimes (changes directly from solid to vapour) and deposits as pure solid on the inner walls of the funnel. Sand and salt remain in the china dish.
Reason for choosing this technique: Naphthalene is a sublimable substance, so sublimation separates it from non-sublimable sand and salt without affecting them.

Step 2 – Filtration (to separate sand from salt solution):
Dissolve the remaining sand-salt mixture in water. Filter the mixture through a filter paper set in a funnel over a beaker. Sand particles (insoluble) are retained on the filter paper as residue, while salt solution (filtrate) passes through.
Reason: Sand does not dissolve in water whereas salt does, so filtration effectively separates the two.

Step 3 – Evaporation or Crystallization (to obtain pure salt):
Heat the filtrate (salt solution) in an evaporating dish until all the water evaporates, leaving behind pure common salt crystals. Alternatively, allow the solution to cool slowly (crystallization) to obtain larger, well-formed salt crystals.
Reason: Evaporation removes the solvent (water) to give the dissolved solute (salt) in the solid form.

Thus, all three components – naphthalene, sand, and common salt – are obtained in their pure forms by this sequence.