1. Mass
&
Density

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 the measure 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 same whenever 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 kg 1 g = 1  10-3 kg = 1/1000 kg 1 mg = 1  10-6 kg = 1/ kg

6. Measuring mass
A beam balance is used to give very accurate measurements.
However, the electronic balance is easy to handle. It gives highly accurate measurements and allows easy digital reading.

8. Weight
Weight is the gravitational force acting on a body.
The weight of a body is greater on Earth than on the moon because the earth exerts a greater gravitational force on it than 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. SI unit for density is kg/m3. Another common unit for density is g/cm3
Definition of Density: The density of a substance is its mass per unit volume.
Formula:
Mass
Volume
Density =
SI unit for density is kg/m3. Another common unit for density is g/cm3

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. Finding the density of a substance
Step 1: Measure the mass and the volume of the substance
Step 2: Divide the mass of the substance by its volume.
Mass
Volume
Density =

15. Finding the density of a liquid
Step 1: Measure the mass and the volume of the substance
Mass
Volume
Density =
Step 2:

16. Finding the density of a regularly shaped object
Step 1: Measure the mass and calculate from dimensions, the volume of the object l h b r
Mass
Volume
Density =
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
Volume
Density =

20. Solid Substance Density in g/cm3 Platinum 21.5 Gold 19.3 Lead 11.3
Silver 10.5
Copper 8.9
Iron 7.9
Zinc 7.1
Aluminium 2.7
Glass 2.5
Ice 21.5
Pinewood 19.3
Cork 0.24
Solid

21. Liquid Gas Substance Density in g/cm3 Mercury 13.6 Glycerine 1.3
Sea water 1.03
Pure water (at 4OC)
Olive oil 0.92
Turpentine 0.87
Petrol 0.80
Alcohol 0.79
Air
Hydrogen
Liquid
Gas

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 normally has the highest density - solid, liquid or gas?
Solid
(d) Which solid with a mass of 1 kg has the smallest volume?
Platinum
(e) Which liquid with a volume of 1 cm3 has the largest mass?

23. Normally, substances become less dense when they are hotter
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!!!

25. Think!!!

26. Some substances float and some others sink.
Floating and sinking
Some substances float and some others sink.
Substances such as pebbles or ball bearings sink in water because they have a higher density than water.
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. cork 0.24 g cm-3
corn oil 0.9 g cm-3
ice 0.92 g cm-3
water 1.0 g cm-3
glycerine 1.3 g cm-3
glass 2.5 g cm-3

28. The solid will float in the liquid.
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.