1

2

Objectives• To explain what mass is• To state the SI unit of mass as the kilogram• To identify g and mg as smaller units for mass• To measure the mass of an object using an

electronic balance• To define density of a substance as its mass

per unit volume and state the SI unit for it• To investigate and describe how the density

of objects (floating and sinking) can be determined

3

Objectives• To demonstrate the ability to calculate

density based on the formula(density = mass/volume) and use the approximate units (kg/m3 or g/cm3)

• To convert one form of units to another (g/cm3 to kg/m3)

• To solve problems using the density formula

4

Mass

The mass of a substance is themeasure of the amount of matter in it.

It depends on the number and size of particles that make up that substance.

The mass of a body is the samewhenever it is measured.

5

The SI unit for mass is the kilogram (kg).

Large masses are measured in tonnes while small masses are measured in grams (g) or milligrams (mg).

1 tonne = 1000 kg1 g = 1 10-3 kg = 1/1000 kg1 mg = 1 10-6 kg = 1/1000000 kg

6

Measuring mass

A beam balance is used to givevery accurate measurements.

However, the electronicbalance is easy to handle.It gives highly accuratemeasurements and allows easy digital reading.

7

8

Weight

Weight is the gravitational force acting on a body.

The weight of a body is greater on Earth than on the moon becausethe earth exerts a greatergravitational force on itthan the moon.

9

Measuring weight

A compression balance and a spring balance can be used to give very accurate measurements.

10

Density

A piece of iron has a greater mass than a piece of wood of the same volume. We say that iron has a higher density than wood.

11

Definition of Density:The density of a substance is its mass per unit volume.

Mass

VolumeDensity =

SI unit for density is kg/m3.Another common unit for density is g/cm3

Formula:

12

Example 1

Sharifah found a ring.

The ring has a mass of 38.6 g and a volume of 2 cm3. Calculate the density of the ring.

Is the ring made of pure gold?

13

Example 2

Osmium, the densest metal found, has a density of 22.6 g/cm3. The mass of a block of osmium was found to be 113 g. Find its volume.

14

Mass

VolumeDensity =

Finding the density of a substance

Step 1: Measure the mass and the volume ofthe substance

Step 2: Divide the mass of the substance by its volume.

15

Finding the density of a liquid

Step 1: Measure the mass and thevolume of the substance

Mass

VolumeDensity =

Step 2:

16

Finding the density of a regularly shaped object

Step 1: Measure the mass and calculate fromdimensions, the volume of the object

l

h

bl

l

l

r

r

h

Mass

VolumeDensity =

Step 2:

17

Finding the density of an irregularly shaped object

18

Finding the density of an irregularly shaped object

19

Finding the density of an irregularly shaped object

Mass

VolumeDensity =

20

Substance Density in g/cm3

Platinum 21.5Gold 19.3Lead 11.3Silver 10.5Copper 8.9Iron 7.9Zinc 7.1Aluminium 2.7Glass 2.5Ice 21.5Pinewood 19.3Cork 0.24

21

Substance Density in g/cm3

Mercury 13.6Glycerine 1.3Sea water 1.03Pure water 1.00(at 4OC)Olive oil 0.92Turpentine 0.87Petrol 0.80Alcohol 0.79

Air 0.00129Hydrogen 0.00009

22

(a) Which state of matter has the lowest density?Gas

(b) Which liquid has the highest density?Mercury

(c) Which on of the following states normallyhas the highest density - solid, liquid or gas?Solid

(d) Which solid with a mass of 1 kg has thesmallest volume?Platinum

(e) Which liquid with a volume of 1 cm3 has thelargest mass?Mercury

23

Normally, substances become less dense when they are hotter. Hence, hot air is less dense than cold air. Do you know why?

For equal masses of air, the hot air has a bigger volume than cold air.

24

Think!!!Think!!!

Think!!!Think!!!

Think!!!Think!!!

25

Think!!!Think!!!

Think!!!Think!!!

Think!!!Think!!!

26

Substances such as pebbles or ball bearings sink in water because they have a higher density than water.

Floating and sinking

Some substances float and some others sink.

A piece of cork or wood which is less dense than water will float.

In general, substances with lower densities will float in fluids (liquids and gases) of higher

densities.

27

corn oil0.9 g cm-3

water1.0 g cm-3

glycerine1.3 g cm-3

cork0.24 g cm-3

ice0.92 g cm-3

glass2.5 g cm-3

28

What happens to a solid placed in a liquid if the densities of the solid and the liquid are the same?

The solid will float in the liquid.

29

References

Tho Lai Hong, Ho Peck Leng, Goh Ngoh Khang, (2001), Interactive Science 1, Pan Pacific Publications.

Chan Kim Fatt, Eric Y K Lam, Lam Peng Kwan, Loo Poh Lim, (2000), Science Adventure, Federal Publications.

Chuen Wee Hong, Lee Khee Boon, Hilda Tan, Ruth Chellappah, Koh Thiam Seng, Yap Kueh Chin, (2000), EPB.