Floating, Sinking and What Density Is — "The Amazing World of Solutes, Solvents, and Solutions"

1. 13 marksCuriosity Grade 8, Chapter 9, page 140

   Give the chapter's everyday examples of floating and sinking, and state the common belief about why it happens.

   Hint. Rice washing is the example the chapter chooses, and it has both behaviours in one bowl.

   The examples. While washing rice, husk particles present in the rice float on the surface of water while rice sinks to the bottom of the container. And if you add oil to water, it floats on water.

   The common belief. Generally, it is believed that objects that float in a liquid are lighter and others that sink are heavier than the liquid.

   ✦ The rice bowl is a good example because it contains both outcomes at once. Same water, same bowl, same moment — and the husk goes up while the grain goes down. Since everything else is identical, the difference has to be a property of the two materials, not of the water or the circumstances. That is the reasoning that forces a new quantity into existence, exactly as the schoolbag straps did for pressure in Chapter 6.

   But notice the wording of the belief. Lighter and heavier are the words of everyday speech, and the chapter is about to show they are not precise enough — a large piece of wood is heavier than a small stone and still floats. What is needed is a comparison that does not depend on how much of the material you happen to have.

2. 23 marksCuriosity Grade 8, Chapter 9, page 140

   A wooden stick and an iron rod may be the same size, yet the iron feels much heavier. What property is being compared, and why is 'heavier' not enough?

   Hint. The chapter's phrasing — *may be of the same size* — is doing the work.

   The property is density.

   A wooden stick and an iron rod may be of the same size, yet the iron rod feels much heavier. When we say that iron is heavier than wood, we are referring to a special property known as density, which describes the heaviness of an object.

   Why 'heavier' alone will not do. A whole tree is heavier than an iron nail, and the tree still floats. Mass by itself tells you nothing about floating, because you can always take more or less of a material. Since the comparison only makes sense at the same size, the chapter builds the size into the quantity — and that is what density is.

   ✦ This is the same move the chapter made for concentration a few pages earlier. Two spoons of salt is not a concentration; two spoons per 100 mL is. One kilogram is not a density; one kilogram per cubic metre is. Because both quantities are amounts per fixed amount of something else, both let you compare things of different sizes fairly — and both are meaningless without the second half.

   The everyday analogies the chapter offers make the same point: a crowded bus is high density and the same bus with few people is low density; a forest with trees close together is dense, and one with trees far apart is not. In every case it is how much in how much space.

3. 33 marksCuriosity Grade 8, Chapter 9, pages 140-141

   Define density and give its formula. What is density independent of, and what does it depend on?

   Hint. Three properties are listed, and one of them is the reason the definition is useful.

   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. Pressure primarily affects the density of gases, while its effect on solids and liquids is negligible.

   Density does not depend on	Density does depend on
   Shape	Temperature
   Size (how much of it you have)	Pressure — mainly for gases

   ✦ Independence of shape and size is what makes density useful at all, and the chapter's exercise 10 tests exactly this: clay moulded into a cube and then flattened into a sheet has the same density both times, because neither its mass nor its volume changed. Since a property that changed every time you cut a piece off could never be used to compare materials, this independence is not a footnote — it is the whole reason for defining the quantity.

   And density does describe the material, not the object. Iron has a density; an iron rod merely has iron's density.

4. 44 marksCuriosity Grade 8, Chapter 9, page 141

   State the units of density and the conversion the chapter gives. Why are g/mL and g/cm³ used for liquids?

   Hint. Start from the SI units of mass and volume and let the formula do the rest.

   The SI unit follows from the formula. The SI units of mass and volume are kilogram (kg) and cubic metre (m³) ... Therefore, the SI unit of density is kilogram per cubic metre, abbreviated as kg/m³. You never memorise it separately — divide the unit of mass by the unit of volume.

   Other units for liquids: gram per millilitre (g/mL) and gram per cubic centimetre (g/cm³).

   The chapter's conversion chain:

   1 kg/m³ = 1000 g/m³ = 1000 g/1000 L = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³

   Reading that the other way round gives the fact you will use constantly: 1 g/cm³ = 1000 kg/m³.

   ✦ Why the smaller units for liquids: convenience. The mass of 1 mL of water is close to 1 g at room temperature, so water comes out as almost exactly 1 g/mL — a number you can compare anything against at a glance. In SI units the same fact reads 1000 kg/m³, which is correct and far less useful in your head. Because floating and sinking are decided by comparison with water, the unit that makes water equal 1 is the one worth using.

   (And note the practical consequence the chapter draws: 10 mL of water is about 10 g, 100 mL about 100 g.)

5. 54 marksCuriosity Grade 8, Chapter 9, page 141

   An aluminium block has a mass of 27 g and a volume of 10 cm³. Find its density and its relative density with respect to water. What does the second number mean?

   Hint. Relative density is a comparison, so check what happens to the units.

   Density.

   Density = Mass ÷ Volume = 27 g ÷ 10 cm³ = 2.7 g/cm³

   Relative density.

   Relative density with respect to water = Density of that substance ÷ Density of water at that temperature

   = 2.7 g/cm³ ÷ 1 g/cm³ = 2.7

   From this, it can be said that aluminium is 2.7 times denser than water. We express this fact by saying that the relative density of aluminium with respect to water is 2.7. It is a number without any units.

   ✦ The units cancel, and that is the whole point of the quantity. A density in g/cm³ and the same density in kg/m³ are different numbers; the relative density is the same number either way, because it is a ratio of two densities and the units divide out. It says how many times denser than water something is, and nothing about which units you measured in.

   It is also the number that answers the floating question fastest. Relative density greater than 1 means denser than water; less than 1 means less dense — with the chapter's own caution that density is not the only factor.

6. 64 marksCuriosity Grade 8, Chapter 9, page 141

   A packet of oil is labelled 1 litre but only 910 grams. What does this tell you about the density of the oil, and is it more or less than water's?

   Hint. You have a mass and a volume, so you have a density.

   Calculate it.

   Density = 910 g ÷ 1 litre = 910 g ÷ 1000 mL = 0.91 g/mL

   Compare with water. The mass of 1 mL of water is close to 1 g at room temperature, so water is about 1 g/mL. 0.91 is less than 1, so the oil is less dense than water — its relative density with respect to water is 0.91.

   ✦ And that is exactly why oil floats on water, which the chapter noted on page 140 without yet being able to explain it. The label on a supermarket packet turns out to contain the explanation. Because the same volume of oil has less mass than that volume of water, the oil layer sits on top.

   Notice what the question is really teaching. A litre and 910 grams look like two unrelated pieces of packaging information. Put them together in the right order and they are a measurement of a physical property — and one you could check for any liquid in the kitchen.

   (The chapter labels this a Think like a scientist box, which is the invitation: the data was in front of you all along.)

7. 73 marksCuriosity Grade 8, Chapter 9, page 140

   The chapter twice warns that density is not the whole story about floating. Why is that warning necessary?

   Hint. Recall the buoyancy work from Chapter 5, and think about a steel ship.

   The two warnings, in the chapter's own words.

   However, the density of a substance is not the only factor that decides whether it will float or sink in a particular liquid. (page 140)

   In this chapter, you have learnt the concept of density and how it explains partially why some objects float while others sink. (page 148)

   Why the warning is needed. Iron is far denser than water and an iron nail sinks — yet ships built of steel float. The material's density has not changed; what differs is the shape, and with it the volume of water the object pushes aside. Chapter 5 established the missing idea: a submerged object experiences an upward push from the liquid, and how much it gets depends on how much liquid it displaces.

   ✦ So the honest statement is a two-part one. For a solid lump of a single material, comparing its density with the liquid's predicts floating well. For a hollow or shaped object it does not, because the object's overall density — including the air inside it — is what matters, not the density of the metal it is made from.

   The chapter's own exercise 8 lives on this distinction. An orange floats with its peel and sinks without it, and the flesh has not changed at all.

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.