Question from Course Book
Question from Course Book
9.1 The Idea of the Density
Question
1. (a) The mass of an object tells us how much matter it is made of.
What is the unit of mass?
(b) Which has more mass, 1 kg of water or 1 kg of air?
Answer
1. (a)
(b)
kilogram (kg)
They have the same mass (1 kg)
Question
2. Look at the picture of different materials on the previous page.
(a) What is the density of gold?
(b) Which is denser, water or ice?
Answer
2. (a)
(b)
19.3
1 cm
g/cm3
1 cm
water is denser than ice
1 cm
Question
3. When water boils, it becomes steam. Its particles spread out and
occupy a greater volume. Has its density increased or decreased? Explain
your answer.
Answer
•
•
The density of water decreases when it becomes steam.
The same mass of particles now occupies a bigger volume so the
density is less.
9.2 Measuring Density
Question
1. What is the mass of tomato shown in the photograph?
Answer
150 g
Question
2. Here is a description of an experiment to measure the density of salty water.
Step 1: An empty measuring cylinder was placed on an electronic balance. The
reading on the balance was recorded (54.0 g)
Step 2: Salty water was poured into the measuring cylinder. The new reading on the
balance was recorded (115.2 g).
Step 3: The volume of the salty water was read from the measuring cylinder (60 cm3).
(a) Draw diagram to show the three steps described above.
(b) Calculate the mass of salty water.
(c) Calculate the density of the salty water.
Answer
(a)
(b) mass of salty water
= 115.2 – 54
=61.2 g
(c) Density of the salty water =
61.2g/60cm3 = 1.02 g/cm3
9.3 Density Calculation
1. A rectangular block of steel measures 4 cm x 2 cm x 1.5 cm.
Its mass is 93.6 g.
(a) Calculate the density of steel.
(b) Steel is made almost entirely of iron atoms, but it is denser
than iron. Suggest a reason for this.
Answer
(a) density = mass/volume
= 93.6 g/ (4 cm x 2 cm x 1.5 cm)
= 7.8 g/cm3
(b) The atoms are packed more closely together in steel than in
iron. (There is less empty space in steel)
2. A half liter bottle has a mass of 80 g. It contains 500 cm3 of liquid
when full. The bottle is filled with olive oil of density = 0.90 g/cm3.
(a) Calculate the mass of oil in the bottle.
(b) Calculate the mass of the full bottle.
Answer
(a) density = mass/volume
mass = density x volume
= 0.90 g/cm3 x 500cm3
= 450 g
(b) Mass of the full bottle = 450 g + 80 g = 530 g
3. A coin has a mass of 7.0 g. It is made of a metal alloy of density
5.6 g/cm3. Calculate the volume of the coin.
Answer
density = mass/volume
volume = mass / density
= 7.0 g/ 5.6 g/cm3
= 1.25 cm3
9.4 Pressure
Question
1. Who will create a greater pressure, a heavy person standing on
a bed or a lighter person lying on the bed? Explain your answer.
Answer
The heavy person standing on the bed creates greater
2. Explain the following; in your answers, use the words
‘pressure’, ‘force’, and ‘area’.
(a) A truck used in the desert has wide tyres.
(b) A cricket stump has a sharp point at one end.
(c) A drawing pin has a sharp point at one end and a large, flat head
at the other.
Answer
(a) The pressure is lower because the large force is spread over a big ar
so that the truck will not sink in the sand.
(b) It will be easier to press the sharp end into the ground because the
pushing force acts on a smaller area, creating a greater pressure.
(c) The point is sharp so there is a greater pressure (force acting on sma
area); the head has a large area so that the force acts on a bigger area
creating less pressure on your thumb when you push.
9.5 Pressure Calculation
1. A force of 6 N acts on an area of 2 m2. What force acts on each
square meter? What is the pressure on the surface?
Answer
Force in 1 m2 = 6N/2 = 3 N
Pressure = force/area
= 3 N/ 2 m2
= 3 N/m2
2. A rectangular block weights 240 N. The area of the block in
contact with the floor is 20 cm2. Calculate the pressure on the
floor. (Give your answer in N/cm2).
Pressure = force/area
Answer
= 240 N/ 20 cm2
= 12 N/cm2
When the wind blows, it creates pressure on any surface it blows
against. A large window has an area of 3.5 m2. If the wind causes a
pressure of 2000 Pa on the window, what force acts on it?
Answer
Pressure = force/area
Force
= Pressure x Area
= 2000 Pa x 3.5 m2
= 7000 N (or) 7000 Pa m2
Workbook Exercises
9.1 Understanding density
1.The table shows the densities of seven solid materials.
Study the table. In the space below, list the materials in order from least
dense to most dense.
Material
nylon
copper
cork
concrete
marble
gold
silver
Density in g/cm3
1.2
8.9
0.24
2.4
2.6
19.3
10.5
Answer
Least dense
Most dense
cork
nylon
concrete
marble
copper
silver
gold
2. The picture shows cubes of four materials. Each cube has a mass
of l.0 kg.
In the space below, list the materials in order, from least dense to
most dense.
Answer
Least dense
Most dense
ice
brick
iron
lead
3. Use the information from the table to answer this question:
Which has a greater volume, 1 kg of gold or l kg of silver? Explain your
answer.
Answer
1 kg of silver has a greater volume than 1 kg of gold because silver is less
dense than gold.
4. Gold is denser than silver. A 1 cm cube of gold has more mass than a 1
cm cube of silver.
However, if we could see the atoms of gold and silver, we would see that they
are exactly the same size. A l cm cube of gold has the same number of atoms as
a 1 cm cube of silver. What does this tell you about the atoms of gold and
silver?
Answer
An atom of gold has more
mass than an atom of
silver.
9.2 Measuring density
1. To find the density of a piece of material, you need to measure its
volume.
The picture shows a piece of wood. It is 0.5 cm thick.
Measure the picture and find the volume of the piece of wood. Give
your answer in cm3.
Answer
a length of wood = 7.4 cm
b width of wood = 1.7 cm
c volume of wood = 6.27 cm3
2 Bijan wanted to find the density of a piece of metal. He weighed it.
Then he measured its volume by placing it in a measuring cylinder which wa
half-full of water.
Here are his measurements:
mass of metal = 196.0 g
reading on measuring cylinder (water only) = 45 cm3
reading on measuring cylinder (water + metal) = 68 cm3
(a) Calculate the volume of the metal, in the space below.
68 – 45 = 23 cm3
Answer ---------------------------(b) Calculate the density of the metal, in the space below.
196.0 g/ 23 cm3 = 8.5 g/cm3
Answer
-----------------------------
Bijan used a table of densities to try to identify the metal. The table is shown
below.
(c) Which metal might Bijan have been investigating?
Answer The density of brass is closest to the measured value.
Can Bijan be sure of this answer? Explain why/why not.
Answer He cannot be sure, however, because there might be other metals
(or alloys) not included in the table with a similar density.
3. Polystyrene foam is less dense than water. Molly measured the volume of
a small block of foam by submerging it in water in a measuring cylinder.
The pictures show her results.
(a) Explain why Molly placed a block of
steel on top of the foam.
(b) Calculate the volume of the foam.
(c) Molly weighed the foam to find its mass
mass of foam = 6.6 g
Calculate the density of the foam.
Answer
(a) To stop the foam floating; it must be
completely submerged.
(b) 55cm3.
(c) Density = 6.6 g/ 55 cm3 = 0.12 g/cm3.
3. Pat says that people are less dense than water. Explain how you
could test this idea. Do you agree with Pat.
Answer
Yes, People are less dense than water. People float in water – this is
good evidence. It isn’t essential to measure the density of a person.
9.3 Calculation involving density
1. A classroom measures 3 m x 4 m x 6 m.
(a) Calculate the volume of air in the room. Give your answer in cubic metr
(m3).
(b) The density of air is l.3kg/m3. Calculate the mass of air in the room.
Answer
(a) volume of air 3 m x 4 m x 6 m = 72 m 3
(b) density = mass/volume
mass of air = density x volume = l.3kg/m3 x 72 m 3 = 93.6 g
2. A steel cube has sides of length 20 cm. Calculate its mass.
(Density of steel = 7.7 g/cm3.) Give your answer in g, and in kg.
Answer
volume= 8000 cm3
mass = density x volume = 7.7 g/cm3 x 8000 cm3 = 61600 g = 61.6 kg
3. A stone is made of material with a density of 2. 7 g /cm3.
94. 5 g.
Calculate the volume of the stone.
Answer
density = mass/volume
volume = mass/ density = 94.5 g/ 2.7 g/cm3 = 35 cm3
Its mass is
e density of water is 1.0 g/ cm 3. This is useful as it means we can
easure the mass of water and easily work out its volume.
)
)
)
)
What is the mass of 1 cm3 of water?
Answer
1 g
.
What is the mass of 20 cm3 of water?
Answer
20 g
What is the volume of 10 g of water?
Answer
10 cm3
What is the volume of 22.5 g of water?
Answer
22.5 cm3
e) Milan is measuring the density of ethanol. He weighs a bottle three
imes: first, empty; then filled with water; then filled with ethanol. Here are
his results:
mass of empty bottle = 30.0 g
mass of bottle filled with water
= 90.0 g
mass of bottle filled with ethanol = 78.0 g
i) Calculate the volume of the bottle.
Answer
(i) mass of water = (mass of bottle filled with water – mass of empty bottl
= 90.0 g – 30.0 g = 60 g
Density of water = 1.0 g/cm3
Volume of the water = mass of water/ density of water = 60 g / 1.0 g/cm3
= 60 cm3
 the volume of the bottle = 60 cm3
ii) Calculate the mass of the ethanol.
i) Calculate the density of ethanol.
Answer
mass of ethanol
= mass of bottle filled with ethanol - mass of empty bott
= 78.0 g - 30.0 g = 48.0 g
density of ethanol = mass of the ethanol / volume
= 48.0 g/ 60 cm3 = 0.80 g/cm3
5. Mercury is a liquid metal. Its density is 13.6 g/cm3. Mercury is
supplied in a plastic bottle whose volume is 50cm3. The empty
bottle has a mass of 25.0 g. Calculate the mass of the bottle when it is
filled with mercury
Answer
mass of mercury= 50 x 13.6 = 680g
total mass of bottle plus mercury is 680 g + 25 g = 705 g