Density
Have you ever wondered why oil floats on water? It's because oil particles are less dense or spread out in a larger area than are water particles. Icebergs also float on liquid water because the icy particles are spread further apart than the particles of liquid water.
Density Rule:
| Density = | Mass |
| Volume |
Examples of density:
- Because fats and oils are less dense and float on water, a fat person should float more easily than a thin person.
- Oil spills float on sea water.
- Whales have a high proportion of blubber (fat deposits) to insulate against the cold of the ocean waters and also to assist in flotation.
- Small rounded structures on the stems of deep sea red algae contain oils for the plant to remain upright.
- Liquids of different densities are used to make layered cocktails.
| DENSITIES OF SOME SUBSTANCES (g / ml) | |
| Earth's atmosphere | 0.001 |
| Styrofoam packaging | 0.03 |
| Cork | 0.2 |
| Polypropylene plastic | 0.9 |
| Ice | 0.9 |
| Water | 1.0 |
| Salt water | 1.03 |
| Aluminium | 2.7 |
| Diamond | 3.5 |
| Planet Earth | 5.5 |
| Iron | 7.8 |
| Copper | 8.9 |
| Lead | 11.3 |
| Gold | 19.3 |
| Black hole in space | 400000000000000 |
Example - How Dense is a Human?
The average male human in USA has a mass of 87 kg and a volume of 8.5 cubic metres. How dense is the average human male (in g/ml)?
Answer:
Mass = 87 kilograms = 87 000 grams
Volume = 8.5 m3 = 8.5 × 100 × 100 = 85 000 cm3 or ml
| Density = | Mass | = | 87000 | = 1.02 g/ml |
| Volume | 85000 |
Did You Know That...?
Over 2000 years ago, the brilliant Greek mathematician Archimedes did a calculation just like this. He estimated his body volume by immersing himself in a bath of water.
There are some obvious problems that make human density estimates unreliable. Describe some of the problems.
Question - Is My Boyfriend's Ring Real Gold?
For my birthday, my boyfriend gave me a ring. In chemistry class, I used laboratory equipment to test it. The ring's mass is 850 grams. When I put it in a tiny measuring cylinder of water, it displaced 0.044 millilitres of water. Is it gold?
Answer
Density = 19.3 g/ml; Yes
Did You Know That...?
Population density is used to compare the populations of animals in the wild over period of time. It is used to determine if populations are increasing to pest proportions or decreasing to endangered species numbers. For example, the population density of a bat colony consisting of 1000 bats and inhabiting an area of 5000 square metres is shown below.
| Population Density = | Numbers of animals | = | 1000 | = 0.2 bats per square metre |
| Area | 5000 |
The most populated country in the world is China. Its population is 1 354 000 000 people in an area of 9 707 000 square kilometres. What is China's population density?
| 
| 
|