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Solutions: Solutes and Solvents

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces the concept of solutions as uniform mixtures, explaining the roles of solute and solvent. It explores how much solute can dissolve in a solvent (saturation, solubility), how temperature affects solubility for solids and gases, and the concept of density as mass per unit volume. The big idea is that matter can form uniform or non-uniform mixtures, and the properties of these mixtures (like concentration and density) determine how substances behave, such as floating or sinking.
Key Concepts (15)
Solution
A uniform mixture formed when two or more substances mix evenly, such that the components are not visible separately.
e.g. When you mix sugar and salt in water to make ORS, every sip tastes the same because it is a solution.
Solute
The component of a solution that dissolves in the solvent. In a solid-liquid solution, the solid is the solute.
e.g. In sugar water, sugar is the solute.
Solvent
The component of a solution that dissolves the solute. In a solid-liquid solution, the liquid is the solvent.
e.g. In sugar water, water is the solvent.
Unsaturated solution
A solution in which more solute can be dissolved at a given temperature.
e.g. When you add one spoon of salt to a glass of water and it dissolves completely, the solution is unsaturated.
Saturated solution
A solution in which the maximum amount of solute has been dissolved at a given temperature, and no more solute can dissolve.
e.g. When you keep adding salt to water and eventually some salt settles at the bottom, the solution is saturated.
Concentration
The amount of solute present in a fixed quantity of solution or solvent.
e.g. A solution with 4 spoons of salt in 50 mL of water is more concentrated than 2 spoons of salt in 100 mL of water.
Dilute solution
A solution that contains a relatively small amount of solute compared to the solvent.
e.g. One spoon of salt in a glass of water makes a dilute solution compared to four spoons.
Concentrated solution
A solution that contains a relatively large amount of solute compared to the solvent.
e.g. Four spoons of salt in a glass of water makes a concentrated solution.
Solubility
The maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.
e.g. At 20°C, water can dissolve only a certain amount of baking soda; at 70°C, it can dissolve more.
Density
The mass present in a unit volume of a substance. Formula: Density = Mass / Volume.
e.g. If an aluminium block has mass 27 g and volume 10 cm³, its density is 2.7 g/cm³.
Relative density
The ratio of the density of a substance to the density of water at the same temperature. It has no units.
e.g. Aluminium has a relative density of 2.7, meaning it is 2.7 times denser than water.
Meniscus
The curved surface formed by a liquid in a measuring cylinder.
e.g. When measuring water in a measuring cylinder, you read the mark at the bottom of the meniscus.
Mass
The quantity of matter present in an object or substance. Measured in grams (g) or kilograms (kg).
e.g. A stone weighs 16.400 g on a digital balance.
Volume
The space occupied by an object or substance. SI unit is cubic metre (m³). For liquids, litres (L) and millilitres (mL) are used.
e.g. A tetra pack of buttermilk contains 200 mL.
Non-uniform mixture
A mixture in which the components are not evenly distributed and can be seen with the naked eye or a magnifying device.
e.g. Chalk powder mixed with water forms a non-uniform mixture.
How It Works
Formation of a solution
  1. Step 1: Take a solvent (e.g., water) in a container.
  2. Step 2: Add a solute (e.g., salt or sugar) to the solvent.
  3. Step 3: Stir the mixture until the solute dissolves completely.
  4. Step 4: The solute particles spread evenly throughout the solvent, forming a uniform mixture called a solution.
💡 In a solution, the solute particles are not visible separately; the mixture looks the same throughout.
Determining saturation point
  1. Step 1: Take a fixed amount of water in a glass.
  2. Step 2: Add one spoon of salt and stir until it dissolves.
  3. Step 3: Continue adding salt one spoon at a time, stirring after each addition.
  4. Step 4: Observe when the added salt stops dissolving and settles at the bottom.
  5. Step 5: At that point, the solution is saturated at that temperature.
💡 The saturation point depends on temperature; heating a saturated solution can allow more solute to dissolve.
Effect of temperature on solubility of solids
  1. Step 1: Take 50 mL of water at 20°C and add baking soda until no more dissolves (saturated solution).
  2. Step 2: Heat the solution to 50°C. The undissolved baking soda dissolves.
  3. Step 3: Add more baking soda until saturation at 50°C.
  4. Step 4: Heat further to 70°C. More baking soda dissolves.
  5. Step 5: Conclusion: For most solids, solubility increases with temperature.
💡 A saturated solution at a lower temperature becomes unsaturated when heated.
Effect of temperature on solubility of gases
  1. Step 1: Gases like oxygen dissolve in water.
  2. Step 2: As temperature increases, the solubility of gases decreases.
  3. Step 3: Cold water can hold more dissolved oxygen than warm water.
  4. Step 4: This is why aquatic life thrives in cold water — more oxygen is available.
💡 Solids dissolve more in hot water, but gases dissolve more in cold water.
Measuring volume of an irregular solid (displacement method)
  1. Step 1: Fill a measuring cylinder with a known volume of water (e.g., 50 mL). Record as initial volume.
  2. Step 2: Tie the irregular solid (e.g., a stone) with a thread.
  3. Step 3: Gently lower the solid into the water until fully submerged.
  4. Step 4: Record the new water level (e.g., 55 mL). This is the final volume.
  5. Step 5: Subtract the initial volume from the final volume. The difference (5 mL) is the volume of the solid.
💡 The volume of water displaced equals the volume of the object. 1 mL = 1 cm³.
Calculating density
  1. Step 1: Measure the mass of the object using a balance (e.g., 16.400 g).
  2. Step 2: Measure the volume of the object (e.g., 5 cm³ using displacement method).
  3. Step 3: Apply the formula: Density = Mass / Volume.
  4. Step 4: Calculate: 16.400 g / 5 cm³ = 3.28 g/cm³.
💡 Density is independent of shape or size but depends on temperature and pressure.
Exam Traps — Don't Make These Mistakes
⚠ Confusing solute and solvent in liquid-liquid mixtures
✗ In a mixture of two liquids, the liquid that dissolves the other is always the solvent.
→ In liquid-liquid mixtures, the substance present in larger amount is the solvent, not necessarily the one that 'does the dissolving'.
✓ In a solution formed by mixing two liquids, the component present in larger amount is called the solvent, and the one in smaller amount is the solute.
⚠ Thinking saturated solution cannot dissolve anything at any temperature
✗ A saturated solution cannot dissolve any more solute at all.
→ A saturated solution is defined at a particular temperature. If you increase the temperature, it can dissolve more solute.
✓ A saturated solution cannot dissolve more solute at that specific temperature, but if the temperature is increased, it becomes unsaturated and can dissolve more.
⚠ Confusing mass and weight
✗ Mass and weight are the same thing.
→ Mass is the quantity of matter (measured in kg or g), while weight is the force of gravity on that mass (measured in newtons). Most balances measure weight but show mass units.
✓ Mass is the amount of matter in an object (unit: kg or g). Weight is the force with which the Earth attracts the object (unit: newton). They are different.
⚠ Misreading the meniscus for coloured liquids
✗ Always read the bottom of the meniscus for all liquids.
→ For colourless liquids like water, you read the bottom of the meniscus. For coloured liquids, you read the top of the meniscus.
✓ For colourless liquids, read the mark at the bottom of the meniscus. For coloured liquids, read the mark at the top of the meniscus.
⚠ Assuming all mixtures are solutions
✗ Any mixture of a solid and a liquid is a solution.
→ Only uniform mixtures are called solutions. Mixtures like sand in water or chalk powder in water are non-uniform and are not solutions.
✓ A solution is a uniform mixture. If the components are not evenly distributed (e.g., sand and water), it is a non-uniform mixture, not a solution.
Compare & Contrast
Ability to dissolve more solute at a given temperature
Saturated solution
Unsaturated solution
A saturated solution cannot dissolve any more solute at that temperature, while an unsaturated solution can still dissolve more solute.
Amount of solute relative to solvent
Dilute solution
Concentrated solution
A dilute solution has a small amount of solute compared to the solvent, while a concentrated solution has a large amount of solute.
Effect of temperature on solubility
Solubility of solids in liquids
Solubility of gases in liquids
For most solids, solubility increases with temperature. For gases, solubility decreases with temperature.
Exam Focus
Marks pattern: 2 MCQs (1 mark each) on definitions, 1 SA (3 marks) on solubility or density calculation, 1 LA (5 marks) on comparing saturated/unsaturated or density concepts
Most asked: Definition and identification of solute, solvent, and solution · Saturated vs unsaturated solutions · Factors affecting solubility (temperature for solids and gases) · Density formula and calculations · Volume measurement by displacement method
Don't skip: Numerical problems on density (Mass/Volume); Distinguishing between saturated and unsaturated solutions with examples; Explaining why ice floats on water using density
One-Line Revision (read aloud before the exam)
  1. A solution is a uniform mixture where the components are evenly distributed.
  2. In a solid-liquid solution, the solid is the solute and the liquid is the solvent.
  3. In a liquid-liquid solution, the component in larger amount is the solvent.
  4. An unsaturated solution can dissolve more solute at a given temperature.
  5. A saturated solution cannot dissolve more solute at that temperature.
  6. Solubility is the maximum amount of solute that dissolves in a fixed amount of solvent at a given temperature.
  7. For most solids, solubility increases with temperature.
  8. For gases, solubility decreases with temperature.
  9. Density is mass per unit volume: Density = Mass / Volume.
  10. The SI unit of density is kg/m³, but g/cm³ is also commonly used.
  11. Relative density has no units and is the ratio of a substance's density to that of water.
  12. Ice floats on water because its density is less than that of liquid water.
  13. The volume of an irregular solid can be found by the water displacement method.
  14. For colourless liquids, read the bottom of the meniscus; for coloured liquids, read the top.
  15. Mass is the quantity of matter; weight is the force of gravity on that mass.

MCQs — 10 Questions with Answers

Q1. When sugar is mixed with water to form a uniform mixture, the sugar is called the _____ and water is called the _____.

  • A. solute; solvent ✓
  • B. solvent; solute
  • C. solution; mixture
  • D. mixture; solution

Answer: A — By definition, the substance being dissolved is the solute and the substance doing the dissolving is the solvent.

Q2. Which of the following is an example of a non-uniform mixture?

  • A. Salt dissolved in water
  • B. Sugar dissolved in water
  • C. Sand mixed with water ✓
  • D. Salt and sugar dissolved in water

Answer: C — Sand does not dissolve in water and remains visible; solutions (A, B, D) are uniform mixtures where components are evenly distributed.

Q3. A solution that cannot dissolve any more solute at a given temperature is called a _____ solution.

  • A. dilute
  • B. concentrated
  • C. saturated ✓
  • D. unsaturated

Answer: C — A saturated solution is one where no more solute can dissolve at that particular temperature, and excess solute settles at the bottom.

Q4. The maximum amount of solute that dissolves in a fixed quantity of solvent is called its _____.

  • A. concentration
  • B. solubility ✓
  • C. saturation
  • D. density

Answer: B — Solubility is defined as the maximum amount of solute that can dissolve in a fixed amount of solvent at a given temperature.

Q5. In Activity 9.2 with baking soda, when water temperature was increased from 20°C to 70°C, what happened to the undissolved baking soda?

  • A. It separated from water
  • B. It dissolved in the water ✓
  • C. It became heavier
  • D. It turned into a gas

Answer: B — The experiment demonstrates that solubility of most solids increases with temperature; more baking soda dissolved at higher temperatures.

Q6. Which of the following correctly relates density to floating and sinking in water?

  • A. Objects that are larger always float
  • B. Objects with density less than water float, and those with density greater than water sink ✓
  • C. Only liquids can determine if objects float or sink
  • D. Floating depends only on the mass of the object

Answer: B — Density (mass per unit volume) is the key property that determines whether an object floats or sinks compared to the liquid's density.

Q7. Why does husk float on water while rice sinks when washing rice?

  • A. Husk is larger than rice
  • B. Husk has lower density than water; rice has higher density than water ✓
  • C. Rice is heavier because it has more mass
  • D. Water pushes rice down but not husk

Answer: B — Objects float when their density is less than water's density and sink when their density is greater; husk is less dense than water while rice is denser.

Q8. A packet of oil is labelled 1 litre volume but weighs only 910 grams. What does this tell us about the density of oil compared to water (density of water = 1000 kg/m³)?

  • A. Oil has the same density as water
  • B. Oil is denser than water
  • C. Oil has lower density than water and will float on water ✓
  • D. Oil has higher mass than water

Answer: C — 1 litre of water weighs 1000 grams; oil weighs only 910 grams for the same volume, so oil is less dense and floats on water.

Q9. In an Indian household, the Chashni (sugar syrup) for Gulab jamuns contains a large amount of sugar dissolved in a small amount of water. In this solution, which is the solvent?

  • A. Sugar, because it is present in larger amount
  • B. Water, because it is the dissolving medium even though present in smaller amount ✓
  • C. Both sugar and water equally
  • D. Neither; it is a suspension

Answer: B — When liquids mix, the substance in larger amount is usually the solvent, but when a solid dissolves in a liquid, the liquid is always the solvent regardless of amounts.

Q10. Why is dissolved oxygen more abundant in cold water than in warm water according to the chapter?

  • A. Cold water has more space for oxygen molecules
  • B. Oxygen cannot dissolve in warm water
  • C. Solubility of gases decreases with increase in temperature; therefore, cold water holds more dissolved oxygen ✓
  • D. Warm water repels oxygen molecules

Answer: C — The chapter states that solubility of gases generally decreases as temperature increases, so cold water can dissolve and hold more oxygen than warm water.

Flashcards

What is a solution in chemistry?

A uniform mixture formed when a solute dissolves evenly in a solvent so components are not visible separately.

Define solute and solvent.

Solute is the substance being dissolved (usually solid), and solvent is the substance dissolving it (usually liquid).

What is the difference between a saturated and unsaturated solution?

Unsaturated solution can dissolve more solute at that temperature, while saturated solution cannot dissolve any more solute.

How does temperature affect the solubility of solid solutes in water?

For most solids, solubility increases with increase in temperature.

How does temperature affect the solubility of gases in water?

Solubility of gases decreases as temperature increases; cold water dissolves more oxygen than warm water.

State the formula for density.

Density equals mass divided by volume, or Density = Mass ÷ Volume.

Why do some objects float and others sink in water?

Objects with density less than water float, and objects with density greater than water sink.

What does concentration of a solution mean?

Concentration is the amount of solute present in a fixed quantity of solution or solvent.

Give an example of a non-uniform mixture from the chapter.

Sand and water or sawdust and water are non-uniform mixtures where components are not evenly distributed.

Why is dissolved oxygen important in water?

Dissolved oxygen in water sustains aquatic life including fish, plants, and other organisms.

Important Board Questions

What is a solution? Give one example from your daily life. [1 mark]

Solution is a uniform mixture where solute dissolves completely in solvent. Example: salt or sugar in water, ORS, tea, milk, juice.

Explain the difference between a saturated solution and an unsaturated solution with the help of an example. [2 marks]

Unsaturated: more solute can dissolve at that temperature (e.g., 1 spoon salt in water can dissolve more salt). Saturated: no more solute dissolves, excess settles (e.g., after adding 5 spoons of salt, more salt does not dissolve).

How does temperature affect the solubility of (a) solid solutes like salt, and (b) gases like oxygen in water? Explain with examples and give reasons why aquatic animals thrive better in cold water. [3 marks]

(a) For solids: higher temperature → higher solubility (heating dissolves undissolved baking soda in Activity 9.2). (b) For gases: higher temperature → lower solubility (cold water holds more oxygen). Reason: fish and aquatic organisms need dissolved oxygen; cold water has more oxygen, supporting aquatic life better.

Define density and explain with mathematical formula. Show why objects float or sink based on density using two examples: (1) husk floating and rice sinking in water, (2) oil floating on water. Draw a labelled diagram showing how particles are packed differently in dense and less dense objects. [5 marks]

Density = Mass ÷ Volume. Object floats if density < liquid density; sinks if density > liquid density. Diagram: show husk particles loosely packed (low density) vs rice particles tightly packed (high density). Explain: 1 litre oil weighs 910 g while 1 litre water weighs 1000 g, so oil is less dense and floats. Include labeled axes: mass on y-axis, volume on x-axis, or show particle arrangement differences.

Assertion-Reason Questions for Solutions: Solutes and Solvents

Q1. Assertion (A):

A saturated solution can become unsaturated if the temperature is increased.

Reason (R):

For most substances, solubility increases with an increase in temperature, allowing more solute to dissolve at higher temperatures.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason directly explains why the assertion is correct. Activity 9.2 demonstrates this principle with baking soda—what was saturated at 20°C becomes unsaturated at 50°C and again at 70°C.

Q2. Assertion (A):

Water is considered a good solvent.

Reason (R):

Water is colorless and transparent, which makes it easy to see when solutes dissolve.

Answer: C — A is true but R is false — The assertion is true—water is indeed a good solvent because it can dissolve many substances like salt and sugar. However, the reason is false; a good solvent's quality is determined by its ability to dissolve many substances, not by its optical properties like being colorless or transparent.

Q3. Assertion (A):

Chalk powder and water form a uniform mixture called a solution.

Reason (R):

Chalk powder dissolves completely in water and distributes evenly throughout, similar to how salt dissolves in water.

Answer: D — A is false but R is true — Both the assertion and reason are false. The textbook clearly states that chalk powder mixed with water forms a non-uniform mixture (Figure 9.2), not a solution. Chalk powder does not dissolve in water; it remains as visible particles.

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Case-Based Questions (CBQ) for Solutions: Solutes and Solvents

Case Study:

Ravi prepared ORS (Oral Rehydration Solution) at home to treat his sister's dehydration. He mixed sugar and salt thoroughly with water. When he tasted different portions of the ORS, every sip tasted the same—neither too sweet nor too salty. Later, he tried adding more salt to the same ORS and found that after a certain point, the salt stopped dissolving and settled at the bottom of the glass. He kept the ORS at room temperature and then heated it gently, after which the undissolved salt completely dissolved.

Q1. What type of mixture is the ORS that Ravi prepared initially?

  • A. A non-uniform mixture
  • B. A uniform solution ✓
  • C. A saturated solution
  • D. A concentrated suspension

Answer: B

Q2. When Ravi added more salt and found it did not dissolve at room temperature, what state had the solution reached?

  • A. Unsaturated solution
  • B. Dilute solution
  • C. Saturated solution ✓
  • D. Supersaturated solution

Answer: C

Q3. Why did the undissolved salt dissolve when Ravi heated the ORS?

  • A. Heating makes salt lighter, so it floats and dissolves
  • B. Higher temperature increases the solubility of salt, allowing the saturated solution to become unsaturated ✓
  • C. Heat breaks salt into smaller particles that dissolve faster
  • D. Heating removes water, concentrating the solution

Answer: B

Q4. If Ravi had used only 50 mL of water instead of the original amount to make ORS with the same amounts of sugar and salt, what would most likely happen?

  • A. More sugar and salt would dissolve
  • B. The temperature would decrease, preventing dissolution
  • C. The solution would be more dilute
  • D. Less solute could dissolve, and some sugar or salt might remain undissolved ✓

Answer: D

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