The Amazing World of Solutes, Solvents, and Solutions — Class 8 Science (Curiosity)
"Sugar and sand are both solids. Why does sugar dissolve in water but not sand?" — the question Chapter 7 leaves open, answered by this one
1. About the Chapter
This is Chapter 9 of Curiosity (pages 134–151, Reprint 2026-27). It opens on a salt-gathering scene and runs in two connected halves.
| Section | Question |
|---|---|
| 9.1 | What are solute, solvent, and solution? |
| 9.2 | How much solute can a fixed amount of solvent dissolve? |
| 9.3 | Solubility of gases |
| 9.4 | Why do objects float or sink in water? |
| 9.5 | What is density? |
The hinge between the two halves is one sentence a student in the book says out loud: sawdust floats in water and sand sinks, yet both are non-uniform mixtures with it. Uniform-versus-non-uniform cannot explain that difference — so the chapter reaches for a new idea, density, to explain what everyone has already seen.
What this chapter is not. There is no formula for mass % or volume % concentration, no supersaturated solutions, no suspensions or colloids, and no Tyndall effect — those are Class 9–10. Solubility here is described only qualitatively, with two activities (salt, then baking soda across three temperatures) and no table of numeric solubility values.
2. Solute, Solvent and Solution
This is because when you add sugar and salt to water, they form a mixture in which the components are evenly distributed throughout ... a uniform mixture is formed. Chalk powder, sand or sawdust in water gives a non-uniform mixture instead.
A uniform mixture, such as that of salt or sugar, and water, is called a solution. Whenever a solid is mixed with a liquid to form a solution, the solid component is called the solute, and the liquid component is called the solvent. The solute dissolves in the solvent to form a solution.
Solute + Solvent → Solution
The two-liquid rule — used only when neither role is obvious:
When a solution is formed by mixing two liquids, it is not always clear which substance is dissolving the other. In such cases, the substance present in smaller amount is called the solute, while the one in larger amount is called the solvent.
Chashni looks like an exception and is not. The Chashni (sugar syrup) of the Indian sweet Gulab jamun is made of a large amount of sugar (solid) dissolved in a small amount of water (liquid). However, the water is still considered as the solvent and sugar as the solute! The two-liquid rule was only ever built for two liquids — a solid is always the solute, whatever the amounts.
A solution need not be liquid. Air qualifies: it is a uniform mixture because the gases dissolve evenly in water is said of dissolved gases, and the same logic makes air itself a solution of gases — which the chapter's own true/false exercise confirms.
3. How Much Solute Can a Fixed Amount of Solvent Dissolve?
Activity 9.1 — the salt that stops dissolving
Half-fill a tumbler with water. Add one spoon of salt, stir until dissolved. Add another spoonful, stir, and repeat, recording each attempt in Table 9.1 — salt dissolves or does not dissolve.
Initially, the salt completely dissolves in the water, forming a solution. After adding a few more spoons of salt, a stage comes when the added salt does not dissolve completely and the undissolved salt settles at the bottom.
Two design details carry the whole conclusion: one spoon at a time, so you know exactly which spoonful failed; stirring well every time, so poor mixing can never be blamed for the failure.
The four terms this gives you
| Term | Definition | Note |
|---|---|---|
| Unsaturated | More solute can be dissolved at a given temperature | |
| Saturated | The solute stops dissolving and begins to settle at the bottom, at that particular temperature | Meaningless without a stated temperature |
| Concentration | The amount of solute present in a fixed quantity of solution (or solvent) | |
| Dilute / Concentrated | Less / more solute — relative terms | Neither is an absolute measurement |
Worked comparison: which is more concentrated — 2 spoons of salt in 100 mL of water, or 4 spoons in 50 mL? Bring both to the same 100 mL: the first is 2 spoons per 100 mL; the second is equivalent to 8 spoons per 100 mL. The second is four times as concentrated — comparing salt alone (4 vs 2) or water alone would mislead.
The maximum amount of solute that dissolves in a fixed quantity of the solvent is called its solubility.
Concentration is how much is actually there; solubility is the maximum that could be. A solution is saturated exactly when its concentration reaches its solubility.
Stirring changes the rate, not the limit. A saturated solution stirred for another ten minutes will not take any more salt — because the limit is a fact about the solvent and temperature, not about how thoroughly you mix.
Activity 9.2 — temperature and solubility
50 mL of water at 20 °C. Add baking soda (sodium hydrogen carbonate) until some is left undissolved — saturated at 20 °C. Heat to 50 °C, stirring: you will observe that it has dissolved. Add more baking soda until some is again left over. Heat to 70 °C: the undissolved baking soda dissolves.
Water at 70 °C dissolves more baking soda than water at 50 °C. The amount of baking soda dissolved in water at 20 °C is even lesser.
For most of the substances, the solubility increases with an increase in temperature ... a saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased.
Run the rule backwards, and you get a real kitchen fact: a saturated sugar solution made hot and left to cool will show sugar crystals settling out, because the water's capacity falls as it cools.
Our scientific heritage. Water has primarily been used as a solvent for the preparation of medicinal formulations in Ayurveda, Siddha, and other traditional systems of medicine — alongside oils, ghee, milk, and other substances as solvents.
Be a scientist. Asima Chatterjee ... used solvents and solutions extensively to extract and isolate important compounds from medicinal plants — developing anti-epileptic and anti-malarial drugs, and becoming the first woman to receive the Shanti Swarup Bhatnagar Award in chemical science.
4. Solubility of Gases
Many gases, including oxygen, dissolve in water. Oxygen dissolves in water only to a small extent ... it is this dissolved oxygen that sustains all aquatic life.
Small in amount, large in consequence — both statements are the chapter's, deliberately placed together.
The solubility of gases generally decreases as temperature increases. More oxygen can dissolve in cold water ... when water warms up, the solubility of oxygen decreases.
The two solubility rules point opposite ways, and the Snapshot states both at once: generally, in liquids, the solubility of solids increases and that of gases decreases with an increase in temperature.
The hinge
I observed that in some non-uniform mixtures, such as sawdust in water, the sawdust floats, whereas in the mixture of sand and water, the sand sinks. I wonder why that happens?
Both are non-uniform mixtures, and they behave in opposite ways. Uniform-versus-non-uniform describes distribution, and nothing about it predicts up or down. That gap is what the rest of the chapter exists to close.
5. Why Do Objects Float or Sink?
While washing rice, husk particles present in the rice float on the surface of water while rice sinks to the bottom. Oil added to water floats.
Generally, it is believed that objects that float in a liquid are lighter and others that sink are heavier than the liquid.
But 'heavier' cannot survive being made precise. A wooden stick and an iron rod may be of the same size, yet the iron rod feels much heavier — comparing two objects of the same size is what forces a new quantity into existence.
Density is defined as the mass present in a unit volume of that substance.
Density = Mass ÷ Volume
The density of a substance is independent of its shape or size. However, it is dependent on temperature and pressure.
Units and relative density
| Unit | Where used |
|---|---|
| kg/m³ | SI unit |
| g/mL, g/cm³ | Liquids, for convenience — water is about 1 g/mL at room temperature |
1 kg/m³ = 1000 g/m³ = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³
Worked example. Mass 27 g, volume 10 cm³ → density = 2.7 g/cm³.
Relative density = Density of substance ÷ Density of water at that temperature. For aluminium, 2.7 ÷ 1 = 2.7 — a number with no units.
The oil-packet check. A pack labelled 1 litre, 910 g → density = 910 g ÷ 1000 mL = 0.91 g/mL, less than water's ~1 g/mL — which is exactly why oil floats.
Density explains floating only partially, and the chapter says so twice: the density of a substance is not the only factor that decides whether it will float or sink (page 140), and you have learnt the concept of density and how it explains partially why some objects float while others sink (page 148). A steel ship floats despite iron being far denser than water — shape and trapped air matter too.
6. Measuring Mass and Volume
Activity 9.3 — mass
Tare the balance to zero. Place a watch glass, tare again. Place the object; the reading is its mass alone — say 16.400 g. The second taring removes the watch glass's own mass from the count.
Mass is the quantity of matter present in an object ... its units are gram (g) and kilogram (kg). Weight is the force by which the Earth attracts an object ... measured in newtons (N). Most balances ... actually measure weight, but their scales are marked in mass units.
Volume, and the meniscus
Volume of liquids is expressed in litres (L) which is equivalent to 1 dm³. A commonly used submultiple ... millilitre (mL) which is equivalent to 1 cm³.
Activity 9.4 — finding a cylinder's least count. For a 100 mL cylinder: the gap between 10 mL and 20 mL is 10 mL, with 10 divisions between them, so one small division can read 10 ÷ 10 = 1 mL.
| Capacity | Smallest reading |
|---|---|
| 10 or 25 mL | 0.1 mL |
| 100 mL | 1 mL |
| 250 mL | 2 mL |
| 500 mL | 5 mL |
Choosing a cylinder for 70 mL: a 50 mL cylinder needs two steps (measuring volume in more than one step is not convenient); a 250 mL or 500 mL cylinder does it in one step but reads more coarsely. Hence, a 100 mL measuring cylinder is the best choice.
Activity 9.5 — reading the meniscus. This curved surface is called the meniscus. For colourless liquids, read the bottom of the curve, eyes level with it; for coloured liquids, the top — since the bottom cannot be seen through them.
Why narrow and tall? The same small change in volume produces a far larger, more readable change in height than in a wide, shallow container — which is also the answer to the chapter's opening question about water-bottle shape.
7. Volume of Solids, and Calculating Density
Activity 9.6 — regular solids. Volume = l × w × h. Notebook example: 25 cm × 18 cm × 2 cm = 900 cm³.
Activity 9.7 — irregular solids, by displacement. Fill a cylinder to an initial volume (say 50 mL). Lower the object in on a thread, slowly. Read the final volume (say 55 mL).
Subtract the initial volume from the final volume ... This is the volume of the object. 55 − 50 = 5 mL = 5 cm³ — since 1 mL = 1 cm³ exactly, no numeric conversion is needed, only a relabelling.
Putting the two activities together:
Density = Mass ÷ Volume = 16.400 g ÷ 5 cm³ = 3.28 g/cm³
If the stone were dropped rather than lowered on a thread, a splash would leave the final level too low, making the calculated volume too small — and, mass being unaffected, the calculated density comes out too large.
Let us dig deeper. Earth's layers — crust, upper mantle, lower mantle, outer core, inner core — increase in density toward the centre, as both the pressure and the temperature rise significantly, making the materials heavier and more compact.
8. Temperature, Pressure, and Why Ice Floats
As temperature increases, the particles of a substance ... tend to move away and spread. This results in an increase in volume but there is no change in mass. Since Density = Mass/Volume, upon heating, the volume increases and the density decreases.
This is why hot air rises, and why a hot air balloon works — heated air expands, its mass is unchanged, its density falls below the surrounding cooler air's, and it rises.
For gases, increasing pressure causes the particles to move closer together ... its density increases. In the case of liquids, pressure has a small effect because they are nearly incompressible ... Solids are even less affected.
The reason is Chapter 7's interparticle spacing. A gas has plenty of room to compress; a liquid and a solid have almost none.
Why ice floats
Water has a special property that its density is highest at 4 °C. As the temperature drops, and water turns into ice at 0 °C, it undergoes a change in structure — the particles arrange themselves in a way that takes up more space. This process is called expansion. Because the same amount of water now occupies a larger volume, its density decreases.
This runs against the general cooling rule, and the chapter calls it out as an exception — do not use water as an example of "density increases on cooling."
This is important for animals living in lakes and oceans because ice floats, it forms a layer on top, keeping the water underneath warm enough for fish and other creatures to survive.
Making a sunk egg float: dissolve salt into the water. The salt raises the water's density (Chapter 7's interparticle spaces again) without adding much volume, until it exceeds the egg's own density.
9. The Traps
Applying the two-liquid rule to a solid-liquid mixture. A solid is always the solute — chashni is the chapter's own check on this.
Saying "saturated" or "unsaturated" with no temperature. Both words are incomplete without one.
Solubility of gases rising with temperature like a solid's. It falls — warm water holds less dissolved oxygen.
Comparing two densities using only mass or only volume. Density is mass per volume; both numbers must enter together (Object A: 200 g/40 cm³ = 5 g/cm³ beats Object B: 240 g/60 cm³ = 4 g/cm³, despite B's larger mass).
"Less dense, therefore floats" as the whole story. The chapter states twice that density only partially explains floating — an unpeeled orange floats, the same flesh peeled sinks.
Confusing mass with weight. Mass (g, kg) is quantity of matter; weight (N) is the pull of gravity. Most balances measure weight and display it as mass.
Reading a coloured liquid's meniscus at the bottom. Read the top for coloured liquids; only colourless ones are read at the bottom.
Treating water's cooling behaviour as typical. Between 4 °C and 0 °C it expands rather than contracts — the stated exception behind ice floating.
Importing Class 9–10. Mass %, volume %, supersaturated solutions, suspensions, colloids and the Tyndall effect are not in this chapter.
10. What to Carry Forward
- Solute + Solvent → Solution. Solid + liquid: solid is always the solute. Two liquids only: smaller amount is the solute.
- Saturated / unsaturated / dilute / concentrated — all meaningless without a stated temperature (the first two) or a stated comparison (the last two).
- Solubility = maximum solute per fixed quantity of solvent, at a temperature. Solids: usually rises with heat. Gases: usually falls.
- Density = Mass ÷ Volume. Independent of shape and size; depends on temperature and pressure.
- Relative density = density ÷ density of water — unitless.
- 1 mL = 1 cm³. Displacement gives a solid's volume as a level rise.
- Density explains floating only partially — shape and trapped air matter too.
- Water is densest at 4 °C; ice, expanding below that, floats — keeping lakes liquid beneath the surface for aquatic life.
