Physical Quantities and Measurement
ICSE Class 8 · Physics · Chapter 2
What this chapter covers
- Density: a quick revision
- Measuring the volume of an irregular solid with a measuring cylinder
- Measuring volume with a eureka can
- Density of an irregular solid
- Density of a liquid
- Floating and sinking
- Comparing the densities of solids, liquids and gases
Key definitions
Density is the mass of a substance contained in a unit volume of that substance.
Displacement is the pushing aside of a liquid by an object placed in it.
Floating is the resting of an object on or near the surface of a liquid without going down to the bottom.
Sinking is the movement of an object down through a liquid until it settles at the bottom.
Key points
- Density is the mass of a substance in a unit volume of it: density = mass ÷ volume, or D = M/V.
- The SI unit of density is kg/m3; in the laboratory g/cm3 is common, and 1 g/cm3 = 1000 kg/m3.
- Every pure substance has its own density, whatever the size of the piece.
- The volume of an irregular solid equals the volume of water it displaces when fully under water.
- With a measuring cylinder, volume of solid = final reading − first reading; read the bottom of the meniscus at eye level.
- A eureka can, filled to its spout, lets out water equal in volume to the solid lowered into it.
- To find the density of an irregular solid, measure its mass first, then its volume by displacement, then divide.
- The mass of a liquid is the mass of the container with liquid minus the mass of the empty container.
- A solid floats in a liquid that is denser than it and sinks in a liquid that is less dense than it.
- A ship floats because its average density, steel and air together, is less than that of water.
- Solids are usually densest, liquids less dense, and gases least dense, because their particles are packed less and less closely.
Questions answered
Define density and write its SI unit.
Density is the mass of a substance contained in a unit volume of it. Its SI unit is kg/m3.
Convert 2.5 g/cm3 into kg/m3.
2.5 × 1000 = 2500 kg/m3.
Why is the volume of an irregular solid found by displacement and not with a ruler?
An irregular solid has no definite length, breadth and height, so its volume cannot be calculated from ruler measurements. The water it displaces has the same volume as the solid.
The water level in a measuring cylinder rises from 35 mL to 47 mL when a key is lowered into it. What is the volume of the key?
47 - 35 = 12 mL, which is 12 cm3.
Why must a eureka can be filled until water drips from the spout before the solid is put in?
So that the water starts exactly at the level of the spout. Then all the water displaced by the solid overflows into the cylinder and none is lost in raising the level inside the can.
Why should you find the mass of a stone before finding its volume by displacement?
After displacement the stone is wet, and the water on it would add to its mass and make the density too large.
An empty beaker has a mass of 50 g. With 40 mL of a liquid in it, the mass is 82 g. Find the density of the liquid.
Mass of liquid = 82 - 50 = 32 g. Density = 32 ÷ 40 = 0.8 g/cm3.
A solid of density 1.1 g/cm3 is placed in water and then in a salt solution of density 1.2 g/cm3. What happens in each?
It sinks in water, because it is denser than water, and floats in the salt solution, because it is less dense than the solution.