NCERT Solutions Curiosity Chapter 9 .4 Why Do Objects Float or Sink in Water? • 9.5 What Is Density? — In-text Questions
Book page 1409 Updated on2026-09-05
Q1.
You may have noticed that, while washing rice, husk particles present in the rice float on the surface of water while rice sinks to the bottom of the container. Why does this happen?
Answer
The husk is less dense than water and the rice grain is denser, so the husk stays on the surface while the rice settles.
Why it happens: Husk is the dry papery outer covering of the grain. It is thin, hollow and traps air, so it occupies a fair volume while carrying almost no mass — its density comes out below 1 g/cm³. The rice grain inside is packed solid with starch and has no air spaces, so the same volume carries much more mass and its density is above 1 g/cm³. Water simply sorts them by density.
Did you know? Farmers have used this for centuries. Winnowing separates husk from grain in air for the same reason, and floating off the husk in water is the wet version of the same trick.
Q2.
How do scientists define density?
Answer
Density is the mass present in a unit volume of a substance.
Density = Mass / Volume
SI unit: kilogram per cubic metre, kg/m³
Convenient units for liquids and small solids: g/mL and g/cm³
Two properties of density are worth fixing in your mind straight away:
It does not depend on the shape or the size of the piece you have. A gram of iron and a kilogram of iron have exactly the same density.
It does depend on temperature and pressure. Pressure matters mainly for gases; for solids and liquids its effect is negligible.
Why it is defined this way: Saying "iron is heavier than wood" is loose talk — a nail is lighter than a log. Fixing the volume at one unit removes the size of the piece from the comparison and leaves only the property of the material itself. That is what makes density a fair way to compare an iron rod with a wooden stick of the same size.
Tip: The chapter's own picture is a good one to remember — a crowded bus is high density, the same bus with a few passengers is low density. The bus (volume) has not changed; the number of people (mass) has.