The Amazing World of Solutes, Solvents, and Solutions – Class 8 Science Chapter 9 Notes & NCERT PDF

Class 8ScienceChapter 9NCERT book: Curiosity - Science Class 8

Notes and a simple summary of Chapter 9 of the NCERT Class 8 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 9: The Amazing World of Solutes, Solvents, and Solutions

Probe and Ponder

What happens when you add so much sugar to tea that it stops dissolving?

Why do sugar and salt dissolve in water but not in oil?

Why are water bottles usually tall and cylindrical instead of spherical?

Introduction

Every sip of homemade ORS tastes the same because the sugar and salt are evenly distributed in the water: it is a uniform mixture. Chalk powder, sand or sawdust in water are non-uniform. This chapter explores solutions and then asks why some things float and others sink, which leads to density.

Key Concepts

9.1 Solute, solvent and solution

A uniform mixture (e.g. salt or sugar in water) is a solution. The solid is the solute, the liquid is the solvent.

Solute + Solvent → Solution

When two liquids mix, the one in smaller amount is the solute and the one in larger amount is the solvent.

Chashni (sugar syrup for gulab jamun) has more sugar than water, but water is still the solvent and sugar the solute.

9.2 How much solute can dissolve?

Activity 9.1: Keep adding spoons of salt to half a tumbler of water. After a few spoons, salt stops dissolving and settles at the bottom.

Unsaturated solution: more solute can still dissolve at that temperature.

Saturated solution: no more solute dissolves at that temperature; the extra settles.

Concentration: the amount of solute in a fixed quantity of solution (or solvent).

Dilute (less solute) and concentrated (more solute) are relative terms.

Solubility: the maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature.

Worked comparison: 2 spoons of salt in 100 mL, or 4 spoons in 50 mL? 4 spoons in 50 mL is 8 spoons per 100 mL, so it is more concentrated.

#### Temperature and solubility (Activity 9.2)

Baking soda (sodium hydrogen carbonate) is added to 50 mL of water at 20 °C until some stays undissolved.

Heating to 50 °C dissolves it; more is added until some stays undissolved again.

Heating to 70 °C dissolves it again.

For most substances, solubility increases with temperature. A saturated solution becomes unsaturated when heated.

Our scientific heritage: Ayurveda, Siddha and other Indian systems used water, hydro-alcoholic extracts, oils, ghee and milk as solvents for medicines.

Be a scientist: Asima Chatterjee

Developed anti-epileptic and anti-malarial drugs using solvents to extract compounds from medicinal plants.

Was the second Indian woman to earn a Doctorate of Science, after Janaki Ammal.

Was the first woman to receive the Shanti Swarup Bhatnagar Award in chemical science.

Was honoured with the Padma Bhushan.

9.3 Solubility of gases

Oxygen dissolves in water only slightly, but this dissolved oxygen sustains all aquatic life.

Gases in water form a uniform mixture.

Solubility of gases generally decreases as temperature increases, so cold water holds more oxygen for aquatic life.

9.4 Why do objects float or sink?

Rice husk floats while rice sinks; oil floats on water.

An iron rod "feels heavier" than a same-sized wooden stick. This property is density.

Density is not the only factor that decides floating or sinking.

9.5 What is density?

A crowded bus has a high density of people; a forest with trees close together is "dense".

Density = Mass ÷ Volume, the mass present in a unit volume of a substance.

It is independent of shape or size, but depends on temperature and pressure (pressure matters mainly for gases).

SI unit: kg/m³. For liquids, g/mL or g/cm³ are also used.

Conversion: 1 kg/m³ = 1000 g/m³ = 1000 g/1000 L = 1 g/L.

1 mL of water has a mass of about 1 g at room temperature.

Relative density = density of substance ÷ density of water (no units).

Worked example: An aluminium block of mass 27 g and volume 10 cm³ has density 2.7 g/cm³. It is 2.7 times denser than water (relative density 2.7).

Think like a scientist: An oil pack says 1 litre but weighs about 910 g, so oil is less dense than water.

#### 9.5.1 Measuring mass and volume

Mass: a digital weighing balance. Switch it on, press tare to zero, tare again with the watch glass on the pan, then place the object. Example stone: 16.400 g.

Mass vs weight: mass is the quantity of matter (g, kg); weight is the Earth's pull on it (newtons). Most balances actually measure weight but show mass units.

Volume units: 1 L = 1 dm³, 1 mL = 1 cm³ = 1 cc.

Measuring cylinder (Activity 9.4): in a 100 mL cylinder, 10 mL between big marks ÷ 10 divisions = 1 mL per division.

- Typical least counts: 0.1 mL (10 or 25 mL cylinders), 1 mL (100 mL), 2 mL (250 mL), 5 mL (500 mL).

- To measure 70 mL, a 100 mL cylinder is best: one step with good accuracy.

Meniscus (Activity 9.5): read at the bottom of the meniscus for water and colourless liquids, at the top for coloured liquids. Keep your eyes level with it.

Regular solid (Activity 9.6): Volume = l × w × h. Notebook: 25 cm × 18 cm × 2 cm = 900 cm³.

Irregular solid, by displacement (Activity 9.7): water at 50 mL rises to 55 mL, so the stone's volume is 5 cm³.

Worked example: stone density = 16.400 g ÷ 5 cm³ = 3.28 g/cm³.

Let us dig deeper: Earth's layers (crust, upper mantle, lower mantle, outer core (liquid), inner core (solid)) increase in density towards the centre.

Ever heard of: light, hollow bamboo rafts and hollowed wooden logs were used as boats long before large ships.

#### 9.5.2 and 9.5.3 Temperature and pressure

On heating, particles spread, so volume increases while mass stays the same, and density decreases. Hot air rises; hot-air balloons work on this idea.

Pressure raises the density of gases a lot, of liquids a little, and of solids negligibly.

Why ice floats: water is densest at 4 °C. On freezing at 0 °C its particles take up more space (expansion), so ice is less dense and floats. This keeps water below warm enough for fish.

Our scientific heritage: Ningel village (Thoubal, Manipur) makes salt from brine wells, one lined with a 100-year-old tree trunk. The brine is boiled in metal pans, and the salt is shaped into cakes with banana leaves and wrapped in a phanek.

Activities in the Chapter

9.1: Salt in water until saturated. Solutions have a limit.

9.2: Baking soda dissolving at 20 °C, 50 °C and 70 °C. Solubility rises with temperature.

9.3: Weigh a stone on a digital balance, using tare correctly.

9.4: Find the least count of a measuring cylinder (1 mL for 100 mL).

9.5: Measure 50 mL of water, reading the meniscus correctly.

9.6: Volume of a cuboid by l × w × h.

9.7: Volume of an irregular solid by water displacement.

Key Terms

Solution (विलयन): a uniform mixture

Solute (विलेय): the substance that dissolves (less in amount)

Solvent (विलायक): the substance that dissolves it (more in amount)

Saturated solution (संतृप्त विलयन): no more solute can dissolve at that temperature

Unsaturated solution (असंतृप्त विलयन): more solute can dissolve

Concentration (सांद्रता): amount of solute in a fixed quantity of solution

Solubility (विलेयता): maximum solute that dissolves at a given temperature

Density (घनत्व): mass per unit volume

Relative density (आपेक्षिक घनत्व): density compared with water

Mass (द्रव्यमान): quantity of matter

Volume (आयतन): space occupied

Meniscus (नवचंद्रक): curved liquid surface in a cylinder

Displacement (विस्थापन): rise in water level equal to an object's volume

Real-life Connections

Hot tea dissolves sugar faster and more than cold water.

Fish struggle in warm water because it holds less dissolved oxygen.

Oil packs weigh less than their litre label suggests (oil is less dense).

Hot-air balloons and floating icebergs are both density at work.

Common Misconceptions

❌ "In chashni, sugar is the solvent because there is more of it." ✅ Water remains the solvent.

❌ "Gases dissolve better in hot water." ✅ Gas solubility decreases as temperature rises.

❌ "Flattening clay changes its density." ✅ Density does not depend on shape.

❌ "Heavier objects always sink." ✅ Density, not just weight, matters, and even density is not the only factor.

❌ "Read the top of the meniscus for water." ✅ Read the bottom for colourless liquids.

Keep the Curiosity Alive (hints)

Q1: All five statements are false as written. (i) oxygen is more soluble in cold water; (ii) sand and water is not a solution; (iii) space occupied is volume; (iv) a saturated solution has more solute dissolved; (v) the atmosphere is a uniform mixture.

Q3: (ii) Water is denser than oil.

Q4: 225 g ÷ 90 cm³ = 2.5 g/cm³, more than water, so it sinks.

Q6: 2 L = 2000 mL, and 2000 − 500 = 1500 mL more.

Q7: 400 ÷ 40 = 10 g/cm³.

Q9: A = 200 ÷ 40 = 5 g/cm³ and B = 240 ÷ 60 = 4 g/cm³, so A is denser.

Q10: 120 ÷ 60 = 2 g/cm³, and the density stays the same when it is flattened.

Q11: Volume = 600 ÷ 7.9 ≈ 75.9 cm³.

💡 Key Learning Points

✓A solution is a uniform mixture of solute and solvent; the component in larger amount is the solvent (water in chashni).
✓Saturated solutions can dissolve no more solute at that temperature; solubility of most solids rises with temperature, but that of gases falls.
✓Density = Mass ÷ Volume; SI unit kg/m³ (g/cm³ or g/mL also used); relative density compares a substance with water.
✓Mass is measured with a tared balance; volume with a measuring cylinder (reading the meniscus correctly), by l × w × h, or by displacement.
✓Heating lowers density (hot air rises); water is densest at 4 °C and ice floats because it expands on freezing.

👨‍🏫 Teaching Tips

→Run Activity 9.1 as a class race: groups count the spoons of salt before saturation and compare results.
→Demonstrate Activity 9.2 with a thermometer so students see baking soda dissolve again at 50 °C and 70 °C.
→Let every student find the least count of a measuring cylinder and practise reading the meniscus at eye level.
→Do the stone displacement plus digital balance activity and have students calculate the density themselves.
→Use the egg-in-water challenge (add salt to make it float) to discuss density and solutions together.
→Work through the chapter's numerical questions (sculpture, objects A and B, iron block) as guided practice.
→Share the stories of Asima Chatterjee and the Ningel salt-makers to link the science to people and heritage.

📋 Assessment Questions

Can the student define and distinguish saturated and unsaturated, and dilute and concentrated, solutions?
Can the student explain how temperature affects the solubility of solids and of gases in water?
Can the student calculate density from mass and volume and predict floating or sinking in water?
Can the student determine the volume of an irregular solid by displacement and read a measuring cylinder correctly?
Can the student explain why ice floats and why hot air rises?

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