Mass & Density

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

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

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.

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 = 1/1000000 kg

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.

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.

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

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.

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

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?

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.

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 =

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

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:

Finding the density of an irregularly shaped object

Finding the density of an irregularly shaped object

Finding the density of an irregularly shaped object Mass Volume Density =

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

Liquid Gas Substance Density in g/cm3 Mercury 13.6 Glycerine 1.3 Sea water 1.03 Pure water 1.00 (at 4OC) Olive oil 0.92 Turpentine 0.87 Petrol 0.80 Alcohol 0.79 Air 0.00129 Hydrogen 0.00009 Liquid Gas

(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?

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.

Think!!!

Think!!!

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.

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

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.

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.