The Amazing World of Solutes, Solvents, and Solutions – Important Questions and Answers
Complete premium revision post for CBSE Class 8 Science based on the chapter The Amazing World of Solutes, Solvents, and Solutions. Includes definitions, important keywords, exam-style questions, case studies, assertion-reason questions, HOTS, density numericals, diagram-based questions, FAQs, and an interactive quiz.
Short Introduction
We use solutions every day—ORS, tea, sugar syrup, saltwater, medicines, and many more. This chapter explains how substances mix, why some mixtures are uniform, what solutes and solvents are, how saturated and unsaturated solutions are formed, how temperature affects solubility, why gases dissolve in water, and how density helps explain floating and sinking.
Chapter Overview
Uniform and Non-uniform Mixtures
Salt and sugar in water form uniform mixtures, while sand, chalk powder, and sawdust in water form non-uniform mixtures.
Solute, Solvent and Solution
A solute dissolves in a solvent to form a solution. In saltwater, salt is the solute and water is the solvent.
Saturated and Unsaturated Solutions
A saturated solution cannot dissolve more solute at a given temperature, while an unsaturated solution can.
Solubility and Temperature
For most solids, solubility in water increases with temperature. For gases, solubility generally decreases on heating.
Density
Density is mass per unit volume. It helps explain why some objects float while others sink.
Measuring Mass and Volume
Mass is measured using a balance and volume is measured using measuring cylinders or displacement method.
Important Formulae
Example: Salt + Water → Salt solution
Common units: kg/m³, g/mL, g/cm³
Relative density has no unit.
Important Keywords with Meanings
| Keyword | Meaning |
|---|---|
| Mixture | A combination of two or more substances. |
| Uniform mixture | A mixture in which components are evenly distributed throughout. |
| Non-uniform mixture | A mixture in which components are not evenly distributed. |
| Solution | A uniform mixture formed when two or more substances mix evenly. |
| Solute | The substance that dissolves in a solvent to form a solution. |
| Solvent | The substance in which a solute dissolves. |
| ORS | Oral Rehydration Solution; a solution of water, sugar, and salt used to treat dehydration. |
| Unsaturated solution | A solution in which more solute can dissolve at a given temperature. |
| Saturated solution | A solution in which no more solute can dissolve at that temperature. |
| Concentration | The amount of solute present in a fixed quantity of solution or solvent. |
| Dilute solution | A solution containing less amount of solute in a fixed quantity of solution. |
| Concentrated solution | A solution containing more amount of solute in a fixed quantity of solution. |
| Solubility | The maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature. |
| Dissolved oxygen | Oxygen gas dissolved in water, which supports aquatic life. |
| Density | Mass present in unit volume of a substance. |
| Mass | The quantity of matter present in an object. |
| Volume | The space occupied by an object or substance. |
| Balance | An instrument used to measure mass. |
| Measuring cylinder | A narrow cylindrical apparatus used to measure liquid volume. |
| Meniscus | The curved surface formed by liquid in a measuring cylinder. |
| Displacement method | A method to find the volume of irregular solids by measuring the rise in water level. |
| Relative density | Ratio of density of a substance to density of water at the same temperature. |
| Floating | Remaining on the surface of a liquid. |
| Sinking | Going down to the bottom of a liquid. |
| Expansion | Increase in volume, usually due to heating or special structural changes such as ice formation. |
Very Short Answer Questions
1. What is a solution?
Answer: A solution is a uniform mixture of two or more substances.
2. What is a solute?
Answer: A solute is the substance that dissolves in a solvent.
3. What is a solvent?
Answer: A solvent is the substance in which a solute dissolves.
4. In saltwater, what is the solute?
Answer: Salt is the solute.
5. In sugar solution, what is the solvent?
Answer: Water is the solvent.
6. Write the word equation for formation of a solution.
Answer: Solute + Solvent → Solution.
7. Is ORS a uniform mixture?
Answer: Yes, ORS is a uniform mixture.
8. Is sand and water a solution?
Answer: No, sand and water form a non-uniform mixture.
9. What is a saturated solution?
Answer: A saturated solution is one in which no more solute can dissolve at a given temperature.
10. What is an unsaturated solution?
Answer: An unsaturated solution is one in which more solute can dissolve at a given temperature.
11. What is concentration?
Answer: Concentration is the amount of solute present in a fixed quantity of solution or solvent.
12. What is a dilute solution?
Answer: A dilute solution contains less amount of solute in a fixed quantity of solution.
13. What is a concentrated solution?
Answer: A concentrated solution contains more amount of solute in a fixed quantity of solution.
14. What is solubility?
Answer: Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature.
15. How does temperature affect the solubility of most solids in water?
Answer: The solubility of most solids in water increases with increase in temperature.
16. How does temperature affect the solubility of gases in water?
Answer: The solubility of gases in water generally decreases with increase in temperature.
17. Why is dissolved oxygen important?
Answer: Dissolved oxygen supports aquatic life such as fish, plants, and other organisms.
18. What is density?
Answer: Density is mass per unit volume of a substance.
19. Write the formula for density.
Answer: Density = Mass ÷ Volume.
20. What is the SI unit of density?
Answer: The SI unit of density is kg/m³.
21. Which instrument is used to measure mass?
Answer: A balance is used to measure mass.
22. Which apparatus is used to measure volume of a liquid?
Answer: A measuring cylinder is used to measure liquid volume.
23. What is meniscus?
Answer: Meniscus is the curved surface of a liquid in a measuring cylinder.
24. Why does ice float on water?
Answer: Ice floats because it is less dense than liquid water.
Short Answer Questions
1. Why does every sip of ORS taste the same?
Answer: ORS is a uniform mixture. Sugar and salt dissolve evenly in water, so every part of the solution has the same composition and taste.
2. Why is sand and water not called a solution?
Answer: Sand does not dissolve in water. Its particles are not evenly distributed and settle at the bottom, so sand and water form a non-uniform mixture, not a solution.
3. How do we identify solute and solvent in a solid-liquid solution?
Answer: In a solid-liquid solution, the solid that dissolves is called the solute and the liquid in which it dissolves is called the solvent.
4. In a liquid-liquid solution, how do we identify solute and solvent?
Answer: In a solution formed by mixing two liquids, the liquid present in smaller amount is called the solute and the one present in larger amount is called the solvent.
5. Why is water still called the solvent in sugar syrup?
Answer: In sugar syrup, sugar is a solid dissolved in water. Even if sugar is present in large amount, water is still considered the solvent because it dissolves the sugar.
6. What happens when too much salt is added to a fixed amount of water?
Answer: Initially, salt dissolves completely. After a limit, no more salt dissolves and the extra salt settles at the bottom. The solution becomes saturated at that temperature.
7. How can a saturated solution become unsaturated?
Answer: For most solid solutes, increasing the temperature allows more solute to dissolve. Thus, a saturated solution at one temperature may behave as an unsaturated solution when heated.
8. Why does baking soda dissolve more in hot water?
Answer: For most solids, solubility increases with temperature. Therefore, hot water can dissolve more baking soda than cold water.
9. Why is cold water better for aquatic life than warm water?
Answer: More oxygen dissolves in cold water than in warm water. This dissolved oxygen is needed by fish and other aquatic organisms for survival.
10. Why does oil float on water?
Answer: Oil floats on water because oil is less dense than water and does not dissolve in water.
11. What is the difference between mass and volume?
Answer: Mass is the quantity of matter present in an object, measured in gram or kilogram. Volume is the space occupied by an object, measured in m³, cm³, litre, or millilitre.
12. Why are measuring cylinders narrow and tall?
Answer: Narrow and tall measuring cylinders give more accurate readings because a small change in volume produces a clearly visible change in liquid level.
13. How should we read the level of water in a measuring cylinder?
Answer: For water and other colourless liquids, the reading should be taken at the bottom of the meniscus, with eyes at the same level as the meniscus.
14. How is the volume of an irregular solid measured?
Answer: The irregular solid is lowered into water in a measuring cylinder. The rise in water level gives the volume of the object by displacement method.
15. Why does density generally decrease on heating?
Answer: On heating, particles move apart and volume increases while mass remains the same. Since density = mass ÷ volume, density decreases when volume increases.
Long Answer Questions
1. Explain solute, solvent and solution with examples.
Answer:
- A solute is the substance that dissolves.
- A solvent is the substance in which the solute dissolves.
- A solution is the uniform mixture formed when solute dissolves in solvent.
- Example: In saltwater, salt is solute, water is solvent, and saltwater is solution.
- In ORS, sugar and salt act as solutes, while water acts as solvent.
2. Differentiate between uniform and non-uniform mixtures.
| Uniform Mixture | Non-uniform Mixture |
|---|---|
| Components are evenly distributed. | Components are not evenly distributed. |
| Components are not visible separately. | Components may be visible separately. |
| Also called a solution. | Not called a solution. |
| Examples: ORS, sugar solution, salt solution. | Examples: sand-water, chalk-water, sawdust-water. |
3. Explain saturated and unsaturated solutions.
Answer: An unsaturated solution is a solution in which more solute can still dissolve at a given temperature. A saturated solution is a solution in which the maximum amount of solute has dissolved and no more solute can dissolve at that temperature. If more solute is added to a saturated solution, it remains undissolved and settles at the bottom.
4. Explain concentration, dilute solution and concentrated solution.
Answer: Concentration is the amount of solute present in a fixed quantity of solution or solvent. A dilute solution contains a smaller amount of solute, while a concentrated solution contains a larger amount of solute. These are relative terms. For example, a solution with one spoon of salt in water is dilute compared to a solution with two or more spoons of salt in the same amount of water.
5. How does temperature affect solubility of solids and gases?
Answer:
- For most solid solutes, solubility increases when temperature increases.
- For example, water at 70 °C dissolves more baking soda than water at 50 °C or 20 °C.
- A saturated solution at a lower temperature may become unsaturated when heated.
- For gases, solubility generally decreases when temperature increases.
- This is why cold water can contain more dissolved oxygen than warm water.
6. Why is dissolved oxygen important for aquatic life?
Answer: Many gases dissolve in water, including oxygen. Oxygen dissolves only to a small extent, but even this small amount is very important for aquatic organisms. Fish, aquatic plants, and other organisms depend on dissolved oxygen for survival. Cold water can hold more oxygen than warm water, so temperature affects aquatic life.
7. Explain density with formula, units and example.
Answer: Density is the mass present in unit volume of a substance.
- Formula: Density = Mass ÷ Volume.
- SI unit: kg/m³.
- Other convenient units are g/mL and g/cm³.
- If an aluminium block has mass 27 g and volume 10 cm³, its density = 27 ÷ 10 = 2.7 g/cm³.
- This means aluminium is 2.7 times denser than water.
8. How do we measure mass and volume in the laboratory?
Answer: Mass is measured using a balance, such as a digital weighing balance. Before measuring, the balance should be set to zero using the tare or reset button. Volume of liquids is measured using a measuring cylinder. The liquid level should be read at the meniscus. Volume of regular solids can be calculated using formulae such as length × width × height. Volume of irregular solids can be measured by displacement of water.
9. Describe the displacement method for finding volume of an irregular solid.
Answer:
- Take water in a measuring cylinder and note the initial volume.
- Tie the irregular object with a thread and lower it into the water.
- Note the final water level.
- The rise in water level gives the volume of water displaced.
- The volume of displaced water is equal to the volume of the object.
- Example: If initial level is 50 mL and final level is 55 mL, volume of object = 5 mL = 5 cm³.
10. Explain the effects of temperature and pressure on density.
Answer: Generally, density decreases with heating because particles move apart, volume increases, and mass remains the same. Hot air becomes less dense than cool air and rises, which is why hot air balloons rise. Pressure affects gases strongly because gas particles can be pushed closer, decreasing volume and increasing density. Liquids are nearly incompressible, so pressure has little effect on them. Solids are even less affected by pressure.
Case-Study Based Questions
Case Study 1: ORS Solution
Riya prepares ORS at home by mixing sugar and salt in water. Every sip tastes almost the same.
Q1. Why does every sip taste the same?
Answer: Sugar and salt are evenly distributed in water, so ORS is a uniform mixture.
Q2. Identify the solute and solvent.
Answer: Sugar and salt are solutes, and water is the solvent.
Q3. What is the mixture called?
Answer: It is called a solution.
Case Study 2: Salt Stops Dissolving
A student keeps adding salt to a glass of water. After some time, extra salt settles at the bottom.
Q1. What type of solution has been formed?
Answer: A saturated solution has been formed.
Q2. Why does salt settle at the bottom?
Answer: Water has reached its limit to dissolve salt at that temperature.
Q3. How can more salt be dissolved?
Answer: Heating the water can dissolve more salt for most solid solutes.
Case Study 3: Baking Soda in Hot Water
Baking soda is added to water at 20 °C until no more dissolves. On heating the water to 50 °C and then 70 °C, more baking soda dissolves.
Q1. What does this show?
Answer: It shows that solubility of baking soda increases with temperature.
Q2. What happens to a saturated solution on heating?
Answer: It may behave like an unsaturated solution and dissolve more solute.
Q3. Is this trend true for most solid solutes?
Answer: Yes, for most solids, solubility increases with temperature.
Case Study 4: Fish in Warm Water
In a pond, water temperature rises during summer. Fish seem less active due to reduced oxygen availability.
Q1. What happens to oxygen solubility when water warms?
Answer: Oxygen solubility decreases as temperature increases.
Q2. Why is dissolved oxygen important?
Answer: Aquatic organisms need dissolved oxygen for survival.
Q3. Which water contains more dissolved oxygen: cold or warm?
Answer: Cold water contains more dissolved oxygen.
Case Study 5: Floating Egg
A raw egg sinks in plain water. When salt is added and dissolved, the egg starts floating.
Q1. Why does the egg sink in plain water?
Answer: The egg is denser than plain water, so it sinks.
Q2. Why does it float in saltwater?
Answer: Adding salt increases the density of water, making it easier for the egg to float.
Q3. Which concept explains this?
Answer: Density explains floating and sinking.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation.
B. Both Assertion and Reason are true but Reason is not the correct explanation.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
1. Assertion: ORS tastes the same in every sip. Reason: Sugar and salt are evenly distributed in water.
Answer: A
2. Assertion: Sand and water form a solution. Reason: Sand dissolves uniformly in water.
Answer: Both Assertion and Reason are false. Sand and water form a non-uniform mixture.
3. Assertion: A saturated solution cannot dissolve more solute at the same temperature. Reason: It already contains the maximum amount of dissolved solute at that temperature.
Answer: A
4. Assertion: A dilute solution has less solute than a concentrated solution. Reason: Concentration depends on the amount of solute in a fixed quantity of solution.
Answer: A
5. Assertion: Solubility of most solids increases with temperature. Reason: More baking soda dissolves in hot water than in cold water.
Answer: A
6. Assertion: Warm water contains more dissolved oxygen than cold water. Reason: Solubility of gases generally increases with temperature.
Answer: Both Assertion and Reason are false. Solubility of gases generally decreases with increase in temperature.
7. Assertion: Oil floats on water. Reason: Oil is less dense than water.
Answer: A
8. Assertion: Density is mass per unit volume. Reason: Density can be calculated by dividing mass by volume.
Answer: A
9. Assertion: Hot air rises. Reason: Hot air is less dense than cool air.
Answer: A
10. Assertion: Pressure affects gases more than solids and liquids. Reason: Gas particles can be pushed closer together more easily.
Answer: A
Competency-Based and Critical Thinking / HOTS Questions
1. Which is more concentrated: 2 spoons of salt in 100 mL water or 4 spoons of salt in 50 mL water?
Answer: 4 spoons of salt in 50 mL water is more concentrated because more solute is present in a smaller quantity of solvent.
2. A stone sculpture weighs 225 g and has volume 90 cm³. Calculate density and predict floating or sinking in water.
Answer: Density = 225 ÷ 90 = 2.5 g/cm³. Since this is greater than water’s density, the stone will sink.
3. An object has mass 400 g and volume 40 cm³. Find its density.
Answer: Density = Mass ÷ Volume = 400 ÷ 40 = 10 g/cm³.
4. Object A has mass 200 g and volume 40 cm³. Object B has mass 240 g and volume 60 cm³. Which is denser?
Answer: Density of A = 200 ÷ 40 = 5 g/cm³. Density of B = 240 ÷ 60 = 4 g/cm³. Object A is denser.
5. A block of iron has mass 600 g and density 7.9 g/cm³. Find its volume.
Answer: Volume = Mass ÷ Density = 600 ÷ 7.9 = 75.95 cm³ approximately.
6. A bottle has volume 2 litres. It contains 500 mL water. How much more water can it hold?
Answer: 2 litres = 2000 mL. Remaining capacity = 2000 − 500 = 1500 mL.
7. Reema moulds 120 g clay into a cube of volume 60 cm³ and then flattens it into a sheet. What happens to density?
Answer: Density does not change if mass and volume remain the same. Changing shape alone does not change density.
8. Why does an unpeeled orange float while a peeled orange may sink?
Answer: The peel contains air spaces that reduce the overall density of the unpeeled orange. After peeling, the density becomes greater, so it may sink.
9. Why does ice floating on lakes help aquatic life survive in cold regions?
Answer: Ice is less dense than water, so it floats and forms a layer on top. Water below remains liquid and warm enough for fish and other organisms to survive.
10. Why is water called a versatile solvent?
Answer: Water can dissolve many substances such as salt, sugar, and gases like oxygen. This makes it useful in daily life, medicine, food, and living systems.
Diagram-Based Questions
Diagram 1: Formation of a Solution
Question: In sugar solution, identify solute and solvent.
Answer: Sugar is the solute and water is the solvent.
Diagram 2: Saturated and Unsaturated Solution
Unsaturated: More solute can dissolve
Saturated: Extra solute remains undissolved
Question: What is seen at the bottom of a saturated salt solution?
Answer: Undissolved salt may settle at the bottom.
Diagram 3: Measuring Cylinder and Meniscus
Question: For colourless liquids, where should the reading be taken?
Answer: The reading should be taken at the bottom of the meniscus at eye level.
Diagram 4: Displacement Method
Question: What is the volume of the stone?
Answer: Volume = 55 − 50 = 5 mL = 5 cm³.
Diagram 5: Floating and Sinking
Question: Why does oil float on water?
Answer: Oil floats because it is less dense than water.
Quick Revision Box
- A solution is a uniform mixture.
- Solute dissolves in solvent to form solution.
- In saltwater, salt is solute and water is solvent.
- Sand-water and sawdust-water are non-uniform mixtures.
- Saturated solution cannot dissolve more solute at that temperature.
- Unsaturated solution can dissolve more solute.
- Concentration is amount of solute in fixed quantity of solution.
- Solubility is maximum amount of solute dissolved in fixed quantity of solvent at a temperature.
- Solubility of most solids increases with temperature.
- Solubility of gases generally decreases with temperature.
- Dissolved oxygen supports aquatic life.
- Density = Mass ÷ Volume.
- Mass is measured by balance.
- Volume is measured by measuring cylinder.
- Irregular solid volume can be measured by displacement method.
- Density generally decreases with heating.
Important Exam Tips
Interactive Quiz
Choose the correct answer and click submit.
Frequently Asked Questions
1. What is a solution?
A solution is a uniform mixture formed when two or more substances mix evenly.
2. What is the difference between solute and solvent?
The solute dissolves, while the solvent is the substance in which the solute dissolves.
3. Why is ORS a solution?
ORS is a solution because sugar and salt are evenly distributed in water.
4. What is a saturated solution?
A saturated solution contains the maximum amount of dissolved solute at a given temperature.
5. What is solubility?
Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.
6. Why does heating help dissolve more baking soda?
For most solid solutes, solubility increases with temperature, so hot water dissolves more baking soda.
7. Why is dissolved oxygen important?
Dissolved oxygen is needed by fish, aquatic plants, and other aquatic organisms for survival.
8. What is density?
Density is mass per unit volume of a substance.
9. How is the volume of an irregular solid measured?
It is measured by displacement method using a measuring cylinder and water.
10. Why does ice float on water?
Ice floats because it is less dense than liquid water.
Final Conclusion
The chapter The Amazing World of Solutes, Solvents, and Solutions helps us understand common mixtures around us scientifically. It explains how solutions form, why saturation depends on temperature, why dissolved oxygen is important for aquatic life, and how density helps us understand floating and sinking. These concepts are useful in daily life, laboratory work, environment, medicine, and higher science.
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