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Why does every sip of homemade ORS taste the same? Why does a fish tank need cool water? Chapter 9 of the new Class 8 science book answers both, and then moves on to a second idea, density, that explains why some things float. This post goes through both halves of the chapter, works the sums, lists the common slips and ends with a practice plan.
Which chapter is this?
The contents page of Curiosity, Grade 8 (Reprint 2026-27) lists Chapter 9 as "The Amazing World of Solutes, Solvents, and Solutions". It follows Chapter 8 on elements, compounds and mixtures. You can read the Chapter 9 PDF free on the NCERT site.
What are a solute, a solvent and a solution?
The chapter begins with ORS. Sugar and salt spread evenly through the water, so the mixture is uniform. Chalk powder, sand or sawdust in water do not spread evenly, so those are non-uniform mixtures.
A solution is a uniform mixture.
When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent.
When two liquids mix, the one present in the smaller amount is called the solute.
The book adds a twist: in the sugar syrup (chashni) for gulab jamun, there is far more sugar than water, yet water is still the solvent, because it is the liquid. It also asks you to think about whether air, a uniform mixture of gases, counts as a solution.
What makes a solution saturated?
In Activity 9.1 you keep adding spoons of salt to half a glass of water. At first it all dissolves: the solution is unsaturated. Eventually salt stays at the bottom: the solution is saturated at that temperature. The maximum amount of solute that dissolves in a fixed amount of solvent is its solubility.
"Dilute" and "concentrated" are relative words. The book asks which is more concentrated: 2 spoons of salt in 100 mL of water, or 4 spoons in 50 mL.
Worked example. Compare spoons per mL.
First: 2 ÷ 100 = 0.02 spoon per mL.
Second: 4 ÷ 50 = 0.08 spoon per mL.
0.08 ÷ 0.02 = 4, so the second solution is four times as concentrated.
How does temperature change solubility?
In the baking soda demonstration, water at 50 °C dissolves soda that would not dissolve at 20 °C, and water at 70 °C dissolves even more. The book concludes that for most substances, solubility of a solid increases as temperature rises.
Gases behave the opposite way: they dissolve less in warmer water. The chapter uses this to explain why cold water holds enough oxygen for fish. The US Environmental Protection Agency says the same in its guide to dissolved oxygen: high temperatures reduce the solubility of oxygen, so warm water holds less dissolved oxygen than cold water.
The chapter's "Be a scientist" box profiles Asima Chatterjee, who used solvents to extract compounds from medicinal plants. You can read more in our post on Asima Chatterjee.
What is density, and how do you measure it?
Density is the mass in a unit volume of a substance:
Density = Mass ÷ Volume
The SI unit is kg/m³, but for small samples g/cm³ or g/mL is easier. The book notes that 1 mL equals 1 cm³, and that 1 mL of water has a mass close to 1 g at room temperature.
To find the density of a stone, the chapter weighs it on a digital balance (16.400 g), then measures its volume by displacement: water in a measuring cylinder rises from 50 mL to 55 mL, so the stone's volume is 5 cm³.
Worked example 1. Density = 16.400 ÷ 5 = 3.28 g/cm³. That is 3.28 times the density of water, so the stone sinks.
Worked example 2: the oil packet. A 1 litre packet of oil has a mass of 910 g (the book's example). 1 litre = 1,000 mL, so density = 910 ÷ 1,000 = 0.91 g/mL. That is less than water's 1 g/mL, which is why oil floats.
Worked example 3: finding volume. An iron block has mass 600 g and density 7.9 g/cm³ (a chapter exercise). Volume = mass ÷ density = 600 ÷ 7.9 ≈ 75.9 cm³.
When reading a measuring cylinder, keep your eye level with the liquid and read the bottom of the curved surface (the meniscus) for water and other colourless liquids, and the top for coloured ones.
What mistakes do students make in this chapter?
Calling sand in water a solution. It is a non-uniform mixture; the sand settles.
Thinking heat always helps things dissolve. True for most solids, false for gases.
Changing density by changing shape. Flattening a 120 g, 60 cm³ lump of clay into a sheet keeps the same mass and volume, so the density stays 120 ÷ 60 = 2 g/cm³.
Dividing the wrong way. Density is mass on top, volume below. Check units: g ÷ cm³ gives g/cm³.
Reading the wrong part of the meniscus, or reading from above, which makes the level look different.
Treating density as the whole story of floating. The book's own note says density is not the only factor that decides whether something floats or sinks.
A practice plan for the week
Repeat Activity 9.1 at home with salt and count the spoons before it saturates.
Try the egg activity: add salt to the water a spoon at a time and note when the egg rises.
Solve exercises 4, 7, 9 and 11 from the chapter, writing units at every step.
Measure the volume of a key or a stone by displacement in a measuring jug.
Explain to someone at home why fish need cool water, using the word solubility.
If a term such as solubility or density still feels slippery, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing. For more experiments that use kitchen materials, see our list of kitchen science experiments.
Start now: weigh a stone on a kitchen scale, find its volume in a measuring jug, and calculate its density.
