CLASS 8 SCIENCE · CHAPTER 9 · STUDY NOTES

The Amazing World of Solutes, Solvents, and Solutions

Understand the amazing world of solutes, solvents, and solutions through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.

EXPLORE

A solution has a solute and a solvent

Explain what dissolves and what it dissolves in.

Dissolving is distribution

Orange symbols represent the solute. This before/after model shows spreading through the water, not a chemical conversion into water particles.

Solution: A uniform mixture in which a solute is dissolved in a solvent.
Solute: The substance being dissolved.
Solvent: The substance in which the solute dissolves.

In sugar water, sugar is the solute and water is the solvent. The sugar is still present even though its crystals are no longer visible. A solution can contain a dissolved solid, liquid or gas.

For two mixed liquids, the larger component is usually called the solvent. But do not memorise “the solvent is always heavier”: water can still be the solvent in a concentrated sugar syrup.

Take a moment. What do you think?

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Question

In salt water, which component is the solute?

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NCERT reference: chapter PDF pages 2, 3.

EXPLORE

A solution can reach a dissolving limit

Keep concentration and saturation separate.

Extra solute need not all dissolve

Compare the same solute, solvent and temperature. Orange symbols are dissolved; dark bottom symbols are excess undissolved solid. The dot counts are schematic, not a measured solubility value.

Saturated solution: A solution holding the maximum amount of a particular solute that can dissolve under the stated conditions.
Solubility: The maximum quantity of a solute that dissolves in a stated quantity of solvent at a specified temperature.
Concentration: How much solute is present in a given amount of solution or solvent, with the basis stated.

An unsaturated solution can dissolve more of that solute under the same conditions. In a saturated solution, adding more solute leaves some undissolved after enough time and mixing.

“Concentrated” means relatively more solute compared with another solution; it does not automatically mean saturated. Always state the temperature and the amount of solvent when comparing solubility.

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Question

A solution has more sugar than another but can still dissolve more. Is it necessarily saturated?

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NCERT reference: chapter PDF pages 3, 4.

EXPLORE

Temperature affects different solutes differently

Separate speed of dissolving from how much can dissolve.

Do not use one temperature rule for everything

This is a qualitative comparison on warming, not experimental data. The upward blue arrow represents gas leaving a solution; it does not mean every gas instantly escapes or all solids follow the same rule.

Dissolving rate: How quickly a solute dissolves; it is different from the final dissolving limit.

For many solid solutes, warmer solvent can dissolve more, but this is not a rule for every substance. Stirring or crushing can increase the dissolving rate without increasing the final solubility at the same temperature.

Gases usually become less soluble in water as temperature rises. Dissolved oxygen matters for many aquatic organisms; warm water may hold less of it under comparable conditions.

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Question

You stir a saturated salt solution without changing its temperature. Must it now dissolve unlimited extra salt?

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NCERT reference: chapter PDF pages 4, 5, 6.

EXPLORE

Density compares mass with volume

Explain why a larger object is not automatically denser.

Same material, same mass per unit volume

The block widths/heights illustrate a larger sample, not exact three-dimensional scale. The written mass and volume values establish density; colour alone is not evidence.

Density: Mass divided by volume: density = mass ÷ volume.
Relative density: A substance’s density divided by the density of water at the stated conditions; the ratio has no unit.

Mass tells how much matter is in a sample; volume tells how much space it occupies. A large piece can have more mass than a small one while both have the same density.

State units clearly. 1 mL = 1 cm³, so g/mL and g/cm³ have the same numerical value. 1 g/cm³ = 1000 kg/m³. For ordinary comparisons, water is approximately 1 g/mL near room temperature.

WORKED IDEA

Use the same sample

ρ = m ÷ V = 36 g ÷ 18 cm³ = 2 g/cm³

Divide mass by volume. The symbol ρ (rho) represents density.

WORKED IDEA

Convert the unit

2 g/cm³ = 2000 kg/m³

1 g = 0.001 kg and 1 cm³ = 0.000001 m³, so their quotient gives a factor of 1000.

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Question

A solid has mass 36 g and volume 18 cm³. What is its density?

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NCERT reference: chapter PDF pages 7, 8, 14.

EXPLORE

Measure the liquid, not its container

Use a tare and read a water meniscus at eye level.

Read the bottom of the water curve

The bottom of the water meniscus lines up with 30 mL. Each small interval represents 5 mL. Read at eye level. Different liquids and instruments can require different reading rules.

Tare: Setting a balance to zero with the empty container on it.
Meniscus: The curved surface of a liquid in a container.
Least count: The smallest change shown by the scale divisions of an instrument.

Subtract the empty container’s mass from the filled mass, or tare before adding liquid. For a water measuring cylinder, read the bottom of the concave meniscus at eye level.

Work out what each interval represents: subtract two labelled readings and divide by the number of equal spaces between them. A container’s capacity alone does not tell its precision.

1
Weigh the empty cup: 28 g.
2
Weigh cup plus liquid: 76 g.
3
Subtract: liquid mass = 48 g.
4
Measure the liquid volume using a suitable cylinder before calculating density.

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Question

An empty cup is 28 g. With a liquid it is 76 g. What mass belongs to the liquid?

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NCERT reference: chapter PDF pages 8, 9, 10, 11, 12.

EXPLORE

Volume can come from dimensions or displacement

Choose the method that suits the shape.

Subtract the readings

The second cylinder contains the fully submerged stone. The rise is the displaced volume, not the total water-plus-stone reading. Levels and stone shape are schematic.

Displaced volume: The volume of liquid pushed aside by an immersed object.

For a rectangular block, multiply length × width × height using consistent units. For a suitable irregular solid, subtract the starting water reading from the reading after full immersion.

The displacement method assumes the solid does not dissolve, absorb significant water or trap air bubbles, and is fully submerged. Floating objects need a different setup.

WORKED IDEA

Regular block

V = 4 cm × 3 cm × 2 cm = 24 cm³

Multiply all three dimensions, not only the area of one face.

WORKED IDEA

Irregular solid

V = 57 − 42 = 15 cm³

Subtract the initial reading. If its mass is 45 g, density = 45 ÷ 15 = 3 g/cm³.

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Question

A stone raises water from 42 mL to 57 mL. What is its volume?

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NCERT reference: chapter PDF pages 12, 13.

EXPLORE

Compare average density to explain floating

Connect floating with material, shape and temperature.

Material density is not the whole-object density

The lump is underwater; the boat has an air-filled cavity and displaces water. The arrows compare upward buoyancy and downward weight qualitatively; their lengths are not measurements.

Average density: Total mass of an object divided by the total volume it occupies, including enclosed spaces where relevant.

An object less dense on average than a liquid can float partly above its surface; a denser solid usually sinks if not supported. At rest, a floating object’s upward buoyant force balances its weight.

Heating often expands a substance without adding mass, reducing its density. Water is unusual: ice is less dense than liquid water, and liquid water has its greatest density near 4 °C. A sealed hollow metal boat can float because enclosed air lowers its average density.

Let’s look a little closer.

Go deeper: Adding salt changes the liquid too

Dissolved salt can increase water’s density, helping an egg float under suitable conditions. The egg need not become lighter. Avoid general claims that highly salty water contains no life; some organisms tolerate extreme salinity.

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Question

A metal lump sinks, but a watertight boat made from the same metal floats. Did the metal’s own density change?

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NCERT reference: chapter PDF pages 7, 14, 15.

The Amazing World of Solutes, Solvents, and Solutions Class 8 Notes

Chapter notes

The Amazing World of Solutes, Solvents, and Solutions

A solution has a solute and a solvent

Dissolving is distribution

A solution can reach a dissolving limit

Extra solute need not all dissolve

Temperature affects different solutes differently

Do not use one temperature rule for everything

Density compares mass with volume

Same material, same mass per unit volume

Use the same sample

Convert the unit

Measure the liquid, not its container

Read the bottom of the water curve

Volume can come from dimensions or displacement

Subtract the readings

Regular block

Irregular solid

Compare average density to explain floating

Material density is not the whole-object density

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