Solubility of Gases — "The Amazing World of Solutes, Solvents, and Solutions"

1. 13 marksCuriosity Grade 8, Chapter 9, page 139

   Do gases dissolve in water? What does the chapter say about oxygen in particular?

   Hint. Note the two apparently opposite statements the chapter makes in one breath.

   Many gases, including oxygen, dissolve in water. Oxygen dissolves in water only to a small extent. Even though present in minute quantities, it is this dissolved oxygen that sustains all aquatic life, including plants, fishes, and other organisms.

   Two statements that must be held together. The amount is small — oxygen is not very soluble in water. And the amount matters enormously — every fish, every water plant and every aquatic organism depends on it.

   ✦ A small quantity is not the same as an unimportant one, and that is the point of putting both sentences side by side. Fish do not breathe the oxygen that is chemically part of the water molecule; they take the dissolved oxygen out of the water with their gills. Since there is so little of it to begin with, anything that reduces it further has consequences out of all proportion to the amount involved.

   Is it a solution? Yes: it is a uniform mixture because the gases dissolve evenly in water. Dissolved oxygen is a solute, water the solvent.

2. 23 marksCuriosity Grade 8, Chapter 9, page 139

   How does temperature affect the solubility of gases? How does this differ from solids?

   Hint. The two rules run in opposite directions, which is why the chapter states both.

   It has been observed that the solubility of gases generally decreases as temperature increases. More oxygen can dissolve in cold water, ensuring sufficient oxygen for aquatic life. On the other hand, when water warms up, the solubility of oxygen decreases.

   	Solids in liquids	Gases in liquids
   As temperature rises	Solubility generally increases	Solubility generally decreases
   The chapter's evidence	Activity 9.2 — baking soda at 20, 50, 70 °C	Observed: more oxygen dissolves in cold water

   The Snapshot states both in one line: Generally, in liquids, the solubility of solids increases and that of gases decreases with an increase in temperature.

   ✦ Note the word generally on both sides. These are tendencies with exceptions, and the chapter is careful never to state either as an absolute law. Because the two rules point opposite ways, this is one of the easiest pairs to write down backwards under exam pressure — the fix is to anchor the gas rule to something you cannot get wrong: warm water holds less oxygen, which is why fish struggle in a warm pond.

3. 34 marksCuriosity Grade 8, Chapter 9, page 139

   Why does the fall in oxygen solubility with temperature matter for aquatic life? Give a consequence.

   Hint. Follow the chain from a warmer pond to the fish in it.

   The chain of reasoning. Water warms → the solubility of oxygen decreases → less dissolved oxygen is available → the fish, plants and other organisms that depend on it have less to draw on. In a badly warmed or shallow body of water this can leave too little oxygen for aquatic life to survive.

   Two situations where it shows up:

   • A pond in high summer holds less dissolved oxygen than the same pond in winter, so fish are more likely to be in difficulty in hot weather even though nothing has been added to the water.
   • Warm water discharged into a river from any process reduces the oxygen the river can hold downstream — a change in temperature alone, with no substance added at all.

   ✦ The chapter's own framing is worth keeping: more oxygen can dissolve in cold water, ensuring sufficient oxygen for aquatic life. Cold water is not an inconvenience for a pond; it is part of what makes the pond habitable. Since the oxygen present is in minute quantities to begin with, a modest fall in solubility is a large fall in what is actually available.

   (This connects directly to Fig. 9.8 and to the Dead Sea project on page 151, which asks why one particular body of water supports no aquatic life at all.)

4. 43 marksCuriosity Grade 8, Chapter 9, page 139

   Summarise what the chapter's student says about uniform and non-uniform mixtures at the end of section 9.3.

   Hint. This is a deliberate recap, placed exactly where the chapter changes subject.

   Now I understand that the mixtures we use can be of two types — uniform and non-uniform. Uniform mixtures are called solutions, and their components are not visible separately. In non-uniform mixtures, the components can be seen either with the naked eye or with a magnifying device.

   	Uniform	Non-uniform
   Also called	Solutions	—
   Components	Not visible separately	Visible to the eye or a magnifying device
   Examples	ORS, seawater, air, dissolved oxygen in a pond	Sand in water, sawdust in water, chalk in water

   ✦ The recap is placed here because the chapter is about to change subject, and it needs one loose end from the earlier pages before it can. The student's next remark is the hinge: 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? Two mixtures of the same type behaving in opposite ways is a question the first half of the chapter cannot answer — and sections 9.4 and 9.5 exist to answer it.

   So the whole second half of this chapter grows out of one observation about non-uniform mixtures. Knowing that is the easiest way to remember why a chapter about solutions spends nine pages on density.

5. 53 marksCuriosity Grade 8, Chapter 9, page 139

   Both sand-in-water and sawdust-in-water are non-uniform mixtures, yet one settles and the other floats. What does this tell you the classification does not capture?

   Hint. Being able to name a mixture is not the same as being able to predict what it will do.

   It tells you that uniform-versus-non-uniform describes how the components are distributed, and nothing else. Neither the sand nor the sawdust dissolves, so both mixtures are non-uniform. But which way the undissolved material moves — down or up — is a completely separate question that the classification does not address.

   What decides it is a property of the material itself. Sand sinks and sawdust floats, and the chapter says the property involved is density: generally, it is believed that objects that float in a liquid are lighter and others that sink are heavier than the liquid.

   ✦ The chapter attaches a warning to that sentence immediately, and it should be reproduced: However, the density of a substance is not the only factor that decides whether it will float or sink in a particular liquid. Because there is more to floating than density alone, an answer that says 'sawdust floats because it is less dense than water' is right as far as it goes and should say so — that it goes only part of the way.

   The chapter says the same thing again at the end of section 9.5: you have learnt the concept of density and how it explains partially why some objects float while others sink. The word partially is the book's own.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 9 (hecu109.pdf), Reprint 2026-27, pages 134-151. Questions are referenced from the NCERT textbook for identification.