Temperature, Pressure, and Why Ice Floats — "The Amazing World of Solutes, Solvents, and Solutions"

1. 14 marksCuriosity Grade 8, Chapter 9, page 147

   How does temperature affect density? Explain using what you learnt in 'Particulate Nature of Matter'.

   Hint. The formula has two parts; work out what happens to each.

   Generally, the density of a substance decreases with heating and increases with cooling.

   The explanation, in the chapter's own chain.

   As temperature increases, the particles of a substance whether solid, liquid, or gas, 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.

   On heating
   Mass	Unchanged — no particles were added or removed
   Volume	Increases — the particles spread out
   Density = mass ÷ volume	Decreases

   ✦ The 'no change in mass' step is the one that carries the argument, and it is easy to skip. Heating does not add matter to the substance; the same particles are simply further apart. Because only the denominator of the formula changes, the direction of the result is certain — the density must fall.

   This is Chapter 7 put to work. There you learnt that heating increases the movement of particles and so the spacing between them. Here that spacing becomes a measurable volume, and the volume becomes a density.

2. 23 marksCuriosity Grade 8, Chapter 9, page 147

   Why does hot air rise? Explain how a hot air balloon works.

   Hint. Compare the hot air with the air around it, not with anything else.

   Because hot air is less dense than the cool air around it.

   This explains why hot air moves up as it is less dense than the cool air around it. The hot air balloon works on the same principle.

   The chain. The air inside the balloon is heated → its particles spread out → its volume increases while its mass stays the same → its density falls → it is now less dense than the surrounding air → so it rises, taking the balloon with it.

   ✦ Note that the comparison is with the surrounding air, not with any fixed standard. Floating and sinking are always relative to what the object is in — the same warm air that rises in a cold room would not rise in a hotter one. Since a hot air balloon relies entirely on the difference, letting the air inside cool brings it down, and reheating it makes it climb again. That is the whole of how such a balloon is steered up and down.

   And it is exactly the same physics as the oil on water and the sawdust in the tumbler — something less dense than its surroundings goes up. Three completely different situations, one rule.

3. 33 marksCuriosity Grade 8, Chapter 9, page 148

   How does pressure affect density? Why is the effect so different for the three states of matter?

   Hint. Recall from Chapter 7 how much empty space each state has.

   For gases, increasing pressure causes the particles to move closer together. As a result, the volume of the gas decreases and its density increases. In the case of liquids, pressure has a small effect because they are nearly incompressible ... Solids are even less affected by pressure than liquids, and changes in their density are usually negligible.

   State	Effect of pressure on density
   Gas	Large — volume decreases, density increases
   Liquid	Small — liquids are nearly incompressible
   Solid	Negligible

   ✦ The reason is interparticle spacing, straight from Chapter 7. A gas has a great deal of space between its particles, so pressure has something to squeeze out. A liquid's particles are already close together, and a solid's closer still — so there is almost nothing left to compress, whatever pressure you apply. The syringe activity of Chapter 7 is the demonstration: air compresses easily, water is practically incompressible.

   This is why the chapter's earlier statement was worded as it was: pressure primarily affects the density of gases, while its effect on solids and liquids is negligible. It is not that pressure behaves differently in different states — it is that the states leave it different amounts of room to work in.

4. 44 marksCuriosity Grade 8, Chapter 9, page 148

   Why does ice float on water? Explain the property of water that makes this happen.

   Hint. Something unusual happens to water's structure below 4 °C.

   Ice floats on water because it is lighter than liquid water. 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. As a result, ice becomes lighter than liquid water and floats on its surface.

   The chain: water cools below 4 °C → its particles rearrange into a structure that takes up more space → the volume increases while the mass stays the same → the density falls → ice is less dense than liquid water → it floats.

   ✦ This runs against the general rule of section 9.5.2, and that is exactly why the chapter calls it a special property. Generally, the density of a substance decreases with heating and increases with cooling — but between 4 °C and 0 °C water does the opposite, expanding as it cools. Because water is the exception rather than the illustration here, it must not be used as an example of the general rule.

   Chapter 7 flagged the same oddity from the other side: ice is an exception — its particles are farther apart than those in water. Two chapters, the same unusual fact, stated in the vocabulary each one had available.

5. 53 marksCuriosity Grade 8, Chapter 9, page 148

   Why is it important for aquatic life that ice floats rather than sinks?

   Hint. Think about where the ice ends up and what it does there.

   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, even in extremely cold weather.

   The floating ice acts as a lid. It forms at the surface and stays there, insulating the water below from the freezing air above. The water underneath stays liquid, and the fish and other creatures in it survive the winter.

   Imagine the alternative. If ice were denser than water it would sink as it formed, exposing fresh water at the surface to freeze in turn. A lake would freeze from the bottom upwards and could freeze solid, leaving nothing alive in it.

   ✦ So a single unusual property of one substance decides whether lakes can support life through a winter. The chapter puts this immediately after the mechanism for a reason: because the expansion at 0 °C looks like a small technical oddity, it is worth seeing at once how much depends on it.

   And it connects to section 9.3. Cold water holds more dissolved oxygen than warm water, so the water beneath the ice is both liquid and comparatively oxygen-rich — two separate facts from this chapter working together in favour of the fish.

6. 64 marksCuriosity Grade 8, Chapter 9, page 148

   A raw egg placed in a glass of tap water sinks. What change could you make to the set-up to make it float instead, and why would that work?

   Hint. You cannot change the egg, so change the liquid.

   Dissolve a good deal of salt in the water.

   Why it works. The egg sinks because it is denser than tap water. You cannot easily change the egg, but you can change the liquid: dissolving salt adds mass to the water without increasing its volume much, since the salt particles occupy the interparticle spaces that are already there (Chapter 7, Activity 7.7). So the density of the liquid rises, and once it exceeds the egg's density the egg floats.

   What to observe as you do it. Add salt a spoonful at a time, stirring. The egg will lift off the bottom at some point and then rise — and the concentration at which it does is a rough measure of the egg's own density.

   ✦ This makes the point that floating is a comparison, not a property of the object alone. The egg has not changed at all; it sinks in one liquid and floats in another. Since the same object can do both, 'this object floats' is as incomplete a statement as 'this solution is concentrated' — you have to say in what.

   (The chapter's Dead Sea project on page 151 is the same experiment at natural scale: a body of water so salty, and therefore so dense, that people float in it — and, as the project asks you to find out, so salty that aquatic life cannot live there.)

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.