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.

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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

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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

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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.

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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

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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.

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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.

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Measuring weight

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

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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.

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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:

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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?

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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.

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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.

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Finding the density of a liquid

Step 1: Measure the mass and thevolume of the substance

Mass

VolumeDensity =

Step 2:

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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:

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Finding the density of an irregularly shaped object

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Finding the density of an irregularly shaped object

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Finding the density of an irregularly shaped object

Mass

VolumeDensity =

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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

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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

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(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

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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.

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Think!!!Think!!!

Think!!!Think!!!

Think!!!Think!!!

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Think!!!Think!!!

Think!!!Think!!!

Think!!!Think!!!

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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.

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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

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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.

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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.