## ICSE Living Science Physics for Class 8 Solutions Chapter 2 Physical Quantities and Measurement

Write true or false. Correct the false statements.
Question 1.
The density of irregular solids can be determined with help of a Eureka can.
True

Question 2.
The drop in the level of water in a measuring cylinder when an object is placed in it gives the volume of the object.
False.
Correct : The rise in the level of water in measuring cylinder when an object is placed in it gives the volume of the object.

Question 3.
A Eureka can is a container with a spout near its bottom.
False.
Correct : A Eureka can is a container with a spout near its top.

Question 1.
How can the knowledge of relative density help in verifying the purity of substances ?
Suppose you are given a sample of the substance and you need to verify its purity. First, find out the relative density
of the substance (r). Then, you should determine the relative density of the sample. Let us say it comes out to be r\ If r’ = r, then the substance is pure or else it is impure.

Fill in the blanks.

1. A wax candle …………. on water.
2. The ratio of density of a substance to the density of water at 4°C is known as the …………. of the substance.
3. A …………… is the instrument used to measure the relative density of liquids.
4. Kerosene will float over in water because its density is …………. than that of water.
5. The density of sea water is ………….. than the density of distilled water.

1. A wax candle floats on water.
2. The ratio of density of a substance to the density of water at 4°C is known as the relative density of the substance.
3. A hydrometer is the instrument used to measure the relative density of liquids.
4. Kerosene will float over in water because its density is less than that of water.
5. The density of sea water is more than the density of distilled water.

Question 1.
Why is the cooling unit of a refrigerator made at the top ?
Cooling unit of a refrigerator is at the top. This is because hot air rises and cold air sinks. The air near the cooling unit loses heat, so it sinks to the bottom. The less denser hotter air at the bottom moves up and gets cooled by the cooling unit. This then sinks to the bottom and the cycle repeats. So, the convection currents cool the entire refrigerator uniformly. If the cooling unit is placed at the bottom, then there will be no convection currents and the cooling of refrigerator will not be effective.

Question 2.
Why is an air-conditioner normally installed at a height in a room ?
An air conditioner is installed at a height in a room so that the convection currents can cool the entire room efficiently.

A. Tick the most appropriate answer.

Question 1.
An irregular solid which when completely immersed in water displaces 10 mL of water. The volume of the solid is –
a. 5 mL
b. 10 mL
c. 15 mL
d. 20 mL
b. 10 mL

Question 2.
a. kg
b. in
c. kg/m3
d. no unit
d. no unit

Question 3.
Relative density is also known as –
a. density.
b. gravity.
c. specific gravity.
d. mass.
c. specific gravity

Question 4.
A device used to measure the relative density of a liquid is known as –
a. thermometer.
b. hydrometer.
c. barometer.
d. spring balance.
b. hydrometer.

Question 5.
Which of the following substances has the greatest density?
a. aluminium
b. water
c. oxygen
d. air
a. aluminium

Question 6.
Which of the following liquids will float on water (density = 1 g/mL)?
a. sea water (density = 1.025 g/mL)
b. honey (density = 1.42 g/mL)
c. gasoline (density = 0.71 g/mL)
d. glycerine (density = 1.26 g/mL)
c. gasoline (density = 0.71 g/mL)

Question 7.
The markings on the hull of the ship that indicate the level to which the ship can be submerged in sea water are called –
a. horizontal lines.
b. vertical lines,
c. skew lines.
d. Plimsoll lines.
d. Plimsoll lines.

Question 8.
For a given rise in temperature, the increase in density is maximum in case of –
a. gases.
b. liquids,
c. solids.
d. none of these
a. gases.

Question 9.
When a substance is heated, its density –
a. decreases.
b. increases.
c. remains the same.
d. becomes zero.
a. decreases

Question 10.
In convection currents, hot air moves –
a. upwards.
b. downwards.
c. to the left.
d. to the right.
a. upwards.

Question 11.
Which of the following natural phenomena takes place due to convection currents?
a. land breeze
b. sea breeze
c. westerlies
d. all of these
d. ail of these

B. Fill in the blanks.

1. The ……….. of a substance is the ratio of its mass and volume.
2. A ………… is a container with a spout below its top.
3. A ……….. a small glass bottle with a stopper having a fine capillary tube, is used to determine the density of liquids.
4. An iron ball will …………. in water.
5. Both liquids and gases are together known as ………….
6. Icebergs ………….. in sea water.
7. A lactometer is an instrument used to test the purity of ………….
8. The relative density of a substance is the ratio of its density to the density of water at …………
9. When a substance is heated, its ………….. increases.

1. The density of a substance is the ratio of its mass and volume.
2. A Eureka can is a container with a spout below its top.
3. A relative density bottle, a small glass bottle with a stopper having a fine capillary tube, is used to determine the density of liquids.
4. An iron ball will sink in water.
5. Both liquids and gases are together known as fluids.
6. Icebergs float in sea water.
7. A lactometer is an instrument used to test the purity of milk.
8. The relative density of a substance is the ratio of its density to the density of water at 4 °C.
9. When a substance is heated, its volume increases.

C. Write true or false. Correct the false statements.

Question 1.
The density of gases is more than that of solids.
False.
Correct : Density of solids is more than that of solids.

Question 2.
The volume of any liquid completely filled in a relative density bottle remains the same.
True

Question 3.
We need to know only the mass of the substance to determine its relative density.
False.
Correct : We need to know only the mass of the substance and mass of the same volume of water 4 °C to determine its relative density.

Question 4.
Kerosene is lighter than water.
True

Question 5.
The density of a substance is the ratio of its density to the density of water at 4°C.
False.
Correct : The relative density of a substance is the ratio of its density to the density of water at 4°C.

Question 6.
On multiplying the relative density by 1000, one gets the density of a substance in kg/m3.
True

Question 7.
A body of density 3.5 g/cm3 will float on water.
False.
Correct: A body of density 3.5 g/cm3 will sink in water.

Question 8.
The base of the bulb of the hydrometer is filled with lead so that the hydrometer can float in a horizontal position.
False.
Correct: The base of the bulb of the hydrometer is filled with lead so that the hydrometer can float in a vertical position.

Question 9.
Convection currents occur due to differences in density in different sections of a liquid or gas.
True

Question 10.
A land breeze arises due to the differential heating between land and water surfaces.
True

D. Match the columns.

 1. Mass a. relative density bottle 2. Volume b. lactometer 3. Sea breeze c. Eureka can 4. Density of milk d. physical balance 5. Relative density e. convection currents

 1. Mass d. physical balance 2. Volume c. Eureka can 3. Sea breeze e. convection currents 4. Density of milk b. lactometer 5. Relative density a. relative density bottle

E. Circle (Italic) the odd one out.

Question 1.
kg/L 100 g/cm3 50 mg/mm3 6 N 0.1 kg/mL –
6N

Question 2.
temperature mass relative density weight density –
relative density

Question 3.
iron nail cork plastic paper thermocol –
iron nail

Question 4.
safety pin empty polythene bag metal key metal lock stone –
empty polythene bag

F. Answer the following in short.

Question 1.
How can we determine the volume of a liquid with a meniscus that curves upwards?
If a liquid has a meniscus that curves upwards, the volume of the liquid can be determined by reading the top of the meniscus.

Question 2.
Write the order in which the density decreases in three states of matter.
The order in which the density decreases in the three states of matter is
Solids > Liquids > Gases

Question 3.
Explain briefly the construction of a hydrometer.
A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. It usually contains a paper scale inside the stem, to read the relative density directly.

Question 4.
Define the relative density of a substance. Why does relative density have no units?
Relative density of a substance is defined as the ratio of the density of the substance to the density of water at 4 °C. As relative density is a ratio of the same physical quantities, therefore, it has no unit.

Question 5.
Why is a Plimsoll line a safety requirement for ships?
A Plimsoll line indicates the level up to which a ship should be submerged safely in a particular sea or river. If a ship is loaded beyond its Plimsoll line, it runs the risk of sinking.

Question 6.
Write the factors that determine whether a solid will float or sink in a liquid.
The factors that determine whether a solid will float or sink in a liquid are the densities of both the solid and the liquid.

Question 7.
Explain how the density of a substance varies with change in temperature.
In case of solids, there is no significant change in their densities with change in temperature. In case of liquids and gases, the densities increase with decrease in temperature and decrease with increase in temperature.

G. Answer the following in detail.

Question 1.
How can you determine the density of an irregular solid with the help of a measuring cylinder?
To measure the density of an irregular solid, both its mass and volume have to be measured. Its mass (m) can be determined with the help of a physical balance. Its volume can be measured with the help of a measuring cylinder as follows.

Step 1 : Pour some water in the measuring cylinder and record the level of water in the cylinder from the graduations marked on its walls.

Step 2 : Tie the string around the stone and lower it into the cylinder gently without touching its walls or the bottom, until it is completely immersed.

Step 3 : The level of water in the cylinder rises. Note down the level of water in the cylinder again.

Step 4 : If initial level of water is V1 and final level of water is V2, then the volume of the solid is calculated to be V2– V1The density of the solid can be determined from the formula,
Density = $$\overli∩e{Q}$$

Question 2.
Explain why solids have greater densities than liquids and liquids have greater densities than gases.
Density in Solids : The density of a substance is affected by the concentration of molecules in a given volume of the substance. In solids, the intermolecular spaces forces are the strongest and the intermolecular spaces very small. This implies that the molecules of solids are very closely packed in a small volume. Hence, solids have high density due to the large number of molecules in a small volume.

Density in Liquids : In liquids, the intermolecular forces are weaker as compared to that in solids and the intermolecular spaces are larger than that in solids. This implies that the molecules of liquid are less tightly packed in a small volume as compared to solids. Hence, liquids have less density than solids.

Density in Gases : In gases, the intermolecular forces are negligible and the intermolecular spaces are much larger than that in liquids. This implies that tfieTnMecules of gases are very less tightly packed in a small volume as compared to liquids. Hence, gases have less density than liquids and solids.

Question 3.
How does the knowledge of relative density help in determining whether a substance floats or sinks in water?

• The relative density of a substance helps us to know whether it will float or sink in water.
• Substances having a relative density less than 1 will float on the surface of water. For example, wood, cork, kerosene, cooking oils, etc.
• Substances having relative density more than 1 sink in water. For example, gold, iron, aluminium, copper, etc.

Question 4.
What do you understand by convection currents?
Fluids expand on heating. On expansion, the density of the heated portion of the fluid decreases, and it becomes light. Warm fluid, being less dense and lighter, rises upwards. The cooler portion of the fluid, being comparatively denser and heavier, moves downwards and occupies the space created by the upward movement of the hot fluid. If the heat is supplied continuously, the entire cycle of hot fluid expanding and rising upwards, followed by cold fluid moving downwards to take its place continues. This constant upward and downward circulation of fluid forms currents in the medium called convection currents. Convection currents distribute the heat throughout the mass of the fluid.

Question 5.
Explain the procedure to determine the density of an unknown liquid using a relative density bottle.
The density of an unknown liquid can be determined using a relative density bottle as follows.
Step 1 : Wash the RD bottle with distilled water. Dry it well.
Step 2 : Insert the stopper and find its mass using the balance. Note down the mass of the empty RD bottle.
Step 3 : Fill the bottle with distilled water. Insert the stopper. Some water will drain out through the stopper. Dry the bottle well from outside.
Step 4 : Find the mass of the bottle filled with distilled water.
Step 5 : Empty the bottle and dry it well. Fill it with the liquid whose relative density is to be determined and insert the stopper. Some liquid will drain out through the stopper. Dry the bottle well from outside. Find the mass of the bottle filled with liquid.
Step 6 : Do the calculations as follows.
Mass of the empty bottle = m1
Mass of bottle + distilled water = m2
Mass of bottle + liquid = m3
Mass of distilled water – (m2 – m1)
Relative density of the liquid = $$\frac{m_{3}-m_{1}}{m_{2}-m_{1}}$$
Mass of liquid = (m3 – m1)
As volume of both the liquid and water are the same,
Density of the liquid = $$\frac{m_{3}-m_{1}}{m_{2}-m_{1}}$$ x 1 g /cm3

H. Solve the following numerical problems.

Question 1.
A piece of a solid has a mass of 10 g. When it is completely immersed in water, the water level rises by 5 mL. Determine the density of the solid.
Mass of the piece of solid, m = 10 g
Rise in level of water when the solid is immersed in it, V = 5mL
Volume of the solid = V = 5 mL
Density of the solid = $$\frac {m}{v}$$ = $$\frac{10 \mathrm{~g}}{5 \mathrm{~mL}}$$ = 2g/mL

Question 2.
A rectangular piece of wood measures 4 cm by 6 cm by 8 cm and has a mass of 20 g. What is the density of the piece of wood? Will this piece float in water or not?
Mass of the piece of wood, m = 20 g
Dimensions of the piece are 4 cm by 6 cm by 8 cm.
Volume of the piece of wood, V = 4 cm x 6 cm x 8 cm = 192 cm3
Density of the piece of wood = $$\frac {m}{v}$$ = $$\frac{20 \mathrm{~g}}{192 \mathrm{~cm}^{3}}$$
= 0.104 g/cm3
As density of the piece of wood (0.104 g/cm3) < density of water (1 g/cm3), therefore, the piece of wood floats in water.

Question 3.
The relative density of oak wood is 0.64. What is its density in
a. the SI system?
b. the CGS system?
Relative density of oak wood = 0.64
a. In SI system, density of oak wood = relative density of oak wood x 1000 kg/m3 = 0.64 x 1000 kg/m3
= 640 kg/m3

b. In CGS system, density of oak wood = relative density of oak wood x lg/cm3
= 0.64 x 1 g/cm3
= 0.64 g/cm3

Question 4.
The capacity of an RD bottle is 100 cc and its mass is 45 g. Find the mass of the bottle when it is filled with a liquid of density 600 kg/m3.
Mass of empty RD bottle, ml = 45 g
Capacity of RD bottle, V= 100 cm3 Density of the liquid filled in the bottle = 600 kg/m3 = 0.6 g/cm3
Mass of the liquid filled in the bottle,
m2 – Density of liquid filled in it x V
= 0.6 g/cm3 x 100 cm3 = 60 g

Question 5.
An empty RD bottle weighs 60 g but weighs 120 g when completely filled with water. When the bottle is completely filled with an unknown liquid, it weighs 90 g. Calculate the
a. volume of the RD bottle.
b. density of the unknown liquid.
Mass of empty RD bottle, m1 = 60 g
Mass of bottle + water, m2 = 120 g
Mass of bottle + unknown liquid, m3= 90 g
Mass of water = m2 – m1 = 120 g – 60 g = 60 g
Mass of unknown liquid = m3 – m1 = 90 g – 60 g = 30g

a. As density of water is 1 g/cm3, therefore,
volume of water = = $$\frac{60 \mathrm{~g}}{1 \mathrm{~g} / \mathrm{cm}^{3}}$$ = 60 cm3

b. Volume of an unknown liquid = volume of water = 60 cm3
Density of an unknown liquid = 0.5 g/cm3

I. Determine the density of the liquids shown below.

Question 1.
A liquid of mass 1196 g is poured into a measuring cylinder (Fig. 1)
Mass of liquid, m = 1196 g
Volume of the liquid, V= 75 mL
Density of the liquid = $$\frac {m}{V}$$ = $$\frac{1196 \mathrm{~g}}{75 \mathrm{~mL}}$$
= 15.95 g/mL

Question 2.
A liquid with mass 75 g is poured into a measuring cylinder (Fig. 2)
Mass of liquid, m = 75 g
Volume of the liquid, V = 92 mL
Density of the liquid = $$\frac {m}{V}$$
= $$\frac{75 \mathrm{~g}}{92 \mathrm{~mL}}$$
= 0.82 g/mL

Question 1.
Which of the following bodies will sink or float in water and why?
a. A body having a relative density of 1.2
A body having a relative density of 1.2 will sink in water as its density is more than that of water.

b. A body having a relative density of 0.8