Class 8 Science Chapter 9 – The Amazing World of Solutes, Solvents, and Solutions from NCERT Curiosity with easy Notes and complete Book Solutions. Get simple explanations, important concepts, questions and answers, and revision material for effective learning, practice and exam preparation.
The Amazing World of Solutes, Solvents, and Solutions
Chapter 9 | Curiosity – Textbook of Science for Grade 8
Short Notes, Easy Concepts and Book Solutions
Probe and Ponder
What do you think is happening in the picture above?
What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?
Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Why are water bottles usually tall and cylindrical in shape instead of spherical?
Chapter 9 – Easy Notes and Concepts
1. What is a Solution?
A solution is a uniform mixture in which one substance dissolves in another.
Example: Salt + Water → Salt solution.
- Solute: The substance that dissolves.
- Solvent: The substance in which the solute dissolves.
- Solution: The uniform mixture formed.
Salt
Water
Salt water
2. Uniform and Non-uniform Mixtures
| Uniform Mixture | Non-uniform Mixture |
|---|---|
| Components are evenly distributed. | Components are not evenly distributed. |
| Components cannot usually be seen separately. | Components may be seen separately. |
| Example: Salt solution. | Example: Sand and water. |
3. Solute and Solvent in Two Liquids
When two liquids form a solution, the substance present in the smaller amount is called the solute and the substance present in the larger amount is called the solvent.
Example: In Gulab jamun syrup, sugar may be present in a large amount, but water is still considered the solvent and sugar the solute.
4. Saturated and Unsaturated Solutions
| Unsaturated Solution | Saturated Solution |
|---|---|
| More solute can still dissolve at that temperature. | No more solute can dissolve at that temperature. |
| Example: A small amount of salt dissolved in water. | Example: Salt solution with extra salt settled at the bottom. |
5. Concentration
Concentration tells us how much solute is present in a fixed amount of solution or solvent.
- Less solute → dilute solution
- More solute → concentrated solution
These are relative terms.
6. Solubility
Solubility is the maximum amount of solute that can dissolve in a fixed amount of solvent at a particular temperature.
For most solids, solubility increases when temperature increases.
7. Solubility of Gases
Many gases can dissolve in water. Oxygen dissolves in water and helps aquatic organisms survive.
Generally, the solubility of gases in liquids decreases when temperature increases.
Therefore, cold water can contain more dissolved oxygen than warm water.
8. Density
Density tells us how much mass is present in a unit volume of a substance.
Common units:
- kg/m³
- g/mL
- g/cm³
9. Mass and Volume
- Mass: Quantity of matter present in an object.
- Volume: Space occupied by an object.
- Mass is measured using a balance.
- Liquid volume is commonly measured using a measuring cylinder.
10. Measuring Volume of a Regular Solid
For a cuboid:
Example:
25 cm × 18 cm × 2 cm = 900 cm³
11. Measuring Volume of an Irregular Solid
We can use the water displacement method.
Volume of object = Final water level − Initial water level.
Example: 55 mL − 50 mL = 5 mL = 5 cm³
12. Effect of Temperature on Density
Generally, when a substance is heated, its particles move farther apart. Its volume increases while its mass remains the same.
Therefore:
Hot air is less dense than cool air, so hot air rises.
13. Effect of Pressure on Density
- For gases, increasing pressure brings particles closer together, so density increases.
- Pressure has a small effect on liquids.
- Pressure usually has a negligible effect on solids.
14. Why Does Ice Float?
Water has its highest density at about 4 °C. When water freezes, its particles arrange themselves in a structure that occupies more space.
Therefore, ice is less dense than liquid water and floats.
15. Why Do Some Objects Float or Sink?
Density is an important factor.
- If an object is less dense than the liquid, it tends to float.
- If an object is denser than the liquid, it tends to sink.
However, density is not the only factor involved in every floating situation.
9.1 What Are Solute, Solvent, and Solution?
Can you predict whether this mixture is uniform or not (Fig. 9.1)? What happens when chalk powder is mixed with water—does it form a uniform mixture?
We know air is a mixture. Would a mixture of gases also be considered a solution?
9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?
Activity 9.1: Let us investigate
How many spoons of salt were you able to dissolve before some of it remained undissolved?
What does this indicate about the capacity of water to dissolve salt?
What will happen if we keep on adding more salt in a given amount of water?
Can you now reflect — which solution is more concentrated; 2 spoons of salt in 100 mL of water or 4 spoons of salt in 50 mL of water?
9.2.1 How does temperature affect the solubility of a solute?
What happens to the undissolved baking soda?
Again, heat the contents further to 70 °C while continuing to stir. What do you observe?
What do you infer from this experiment?
Be a Scientist
What inspired Asima Chatterjee to work on medicinal plants?
9.3 Solubility of Gases
Do gases also dissolve in water?
Is the mixture of gases in water a uniform or non-uniform mixture?
Does temperature affect the solubility of gases in liquids also, if so, how?
I wonder why that happens?
9.4 Why Do Objects Float or Sink in Water?
Why does this happen?
9.5 What Is Density?
What does this tell us about the density of the oil, and is it less or more than that of water?
Important Formula
For example, if mass = 27 g and volume = 10 cm³:
Relative Density
Relative density has no unit.
9.5.1 Determination of Density
What is the maximum volume it can measure?
What is the smallest volume it can measure?
How much is the volume difference indicated between the two bigger marks (for example, between 10 mL and 20 mL)?
How many smaller divisions are there between the two bigger marks?
How much volume does one small division indicate?
The smallest volume that the measuring cylinder can read is__________.
Why are measuring cylinders always designed narrow and tall instead of wider and short like a beaker?
9.5.2 Effect of Temperature on Density
Easy Concept
When most substances are heated, their particles spread farther apart. Their volume increases while mass remains nearly the same.
This is why hot air rises.
9.5.3 Effect of Pressure on Density
Easy Concept
- Increasing pressure on a gas generally increases its density.
- Liquids are only slightly affected by pressure.
- Solids are usually affected very little by pressure.
Why does ice float on water?
Why does ice float on water?
Think Like a Scientist
What change can you make to this setup to make the egg float in water instead of sinking?
Snapshots – Quick Revision
- A solution is formed when two or more substances mix to form a uniform mixture.
- In a solution formed by dissolving a solid in a liquid, the solid is the solute and the liquid is the solvent.
- In a solution of two liquids, the substance present in smaller quantity is the solute.
- A saturated solution cannot dissolve more solute at that temperature.
- An unsaturated solution can dissolve more solute at that temperature.
- Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.
- Generally, the solubility of solids increases with temperature.
- Generally, the solubility of gases decreases with temperature.
- Mass is the amount of matter present in an object.
- Volume is the space occupied by an object.
- Mass is measured using a balance.
- Liquid volume is measured using a measuring cylinder.
- Density = Mass/Volume.
- Generally, density decreases when temperature increases.
- Pressure affects density differently for solids, liquids and gases.
Keep the Curiosity Alive – Book Questions
1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s).
(i) Oxygen gas is more soluble in hot water rather than in cold water.
(ii) A mixture of sand and water is a solution.
(iii) The amount of space occupied by any object is called its mass.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
(v) The presence of different gases in the atmosphere is also a uniform mixture.
2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is __________ in water and the ____________ in water level is measured.
(ii) The maximum amount of _______________ dissolved in _______________ at a particular temperature is called solubility at that temperature.
(iii) Generally, the density ____________ with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a __________ solution of glucose.
3. You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
(i) Oil is denser than water
(ii) Water is denser than oil
(iii) Oil and water have the same density
(iv) Oil dissolves in water
4. A stone sculpture weighs 225 g and has a volume of 90 cm3. Calculate its density and predict whether it will float or sink in water.
Density = Mass ÷ Volume
= 225 ÷ 90
= 2.5 g/cm³
Density of water ≈ 1 g/cm³.
Since 2.5 g/cm³ is greater than 1 g/cm³, the stone sculpture will sink.
5. Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
(i) A saturated solution can still dissolve more solute at a given temperature.
(ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature.
(iii) No more solute can be dissolved into the saturated solution at that temperature.
(iv) A saturated solution forms only at high temperatures.
(i) is wrong because a saturated solution cannot dissolve more solute at that temperature.
(ii) is wrong because an unsaturated solution can dissolve more solute.
(iv) is wrong because saturation can occur at different temperatures.
6. You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?
2 L = 2000 mL
Water already present = 500 mL
Remaining capacity = 2000 − 500 = 1500 mL or 1.5 L.
7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Density = Mass ÷ Volume
= 400 ÷ 40
= 10 g/cm³
8. Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.
9. Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?
Object A density = 200 ÷ 40 = 5 g/cm³
Object B density = 240 ÷ 60 = 4 g/cm³
Therefore, Object A is denser.
Why?
When?
How long?
Where? Why not?
Prepare some questions based on your learnings so far …
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
10. Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm3. Later, she flattens it into a thin sheet. Predict what will happen to its density.
11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Density = Mass ÷ Volume
Therefore, Volume = Mass ÷ Density
= 600 ÷ 7.9
≈ 75.95 cm³
So, the volume is approximately 76 cm³.
12. You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70 °C), the water level in the glass tube rises. How does it affect the density?
Discover, Design, and Debate
Research project on Dead Sea: Why is there no aquatic life in the Dead Sea? Try to find out if there are any other similar water bodies.
Investigate how well common salt dissolves in different solvents, such as water, vinegar, and oil. Compare the solubility of salt in each solvent and record your observations.
Debate in class—Is water truly the most versatile solvent?
Society Science – Interdisciplinary Projects
Our Scientific Heritage
Ningel village in Manipur’s Thoubal district has a traditional practice of producing salt from salty water. The salt solution is collected and boiled until water evaporates and salt crystals form. The salt is then shaped into traditional salt cakes.
Learning Point: This is a real-life example of using evaporation to separate a dissolved solid from a solution.
Reflect on the Questions Framed by Your Friends
Reflect on the questions framed by your friends and try to answer …
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
………………………………………………………………………….
I think …
But we thought … Maybe …
Shouldn’t it be …
Quick Formula Revision
One-Minute Revision
| Term | Easy Meaning |
|---|---|
| Solute | Substance that dissolves. |
| Solvent | Substance in which the solute dissolves. |
| Solution | Uniform mixture of solute and solvent. |
| Unsaturated | More solute can dissolve. |
| Saturated | No more solute can dissolve at that temperature. |
| Solubility | Maximum amount of solute that can dissolve at a particular temperature. |
| Mass | Amount of matter. |
| Volume | Space occupied by matter. |
| Density | Mass per unit volume. |
| Meniscus | Curved surface of a liquid in a measuring cylinder. |
Prepare with this NCERT Curiosity worksheet for Chapter 9. The worksheet includes important questions and revision exercises to help students understand key concepts, practise what they have learned, and prepare effectively for class tests and exams.
Class 8 Science Chapter 9, Class 8 Science Curiosity Chapter 9, The Amazing World of Solutes Solvents and Solutions, Class 8 Science Chapter 9 Notes, Class 8 Science Chapter 9 Book Solutions, Solutes Solvents and Solutions Notes and Solutions, Class 8 Curiosity Chapter 9 Notes, NCERT Class 8 Science Curiosity, NCERT Class 8 Science Notes, CBSE Class 8 Science, Class 8 Science Questions and Answers, Class 8 Science Study Material, Class 8 Science Revision Notes