Definitions
What is density?
The ratio of mass to volume (how much matter is in a given space).
Mass = 20 g, Volume = 10 mL. What is the density?
20g/10mL=2g/mL.
You place a block in water and it sinks. What does that tell you about its density compared to water?
Its density is greater than water (1 g/mL).
Why do hot air balloons rise?
Warm air inside is less dense than surrounding air, making the balloon rise.
A liquid has a density of 1.2 g/mL. Will a block with density 0.8 g/mL float or sink?
It will float (0.8 g/mL < 1.2 g/mL).
What two properties do you need to calculate density?
Mass and volume.
An object has a density of 5 g/mL and a mass of 50 g. What is its volume?
V=m/d=50g/5g/mL=10mL.
A student measures the volume of a rock by water displacement. What tool are they using?
A graduated cylinder.
Why do ships made of steel float, even though steel is denser than water?
Ships are hollow and displace a lot of water, reducing their average density below water’s.
You have two blocks of the same material, one small and one large. Which has greater density?
They have the same density (density is independent of size).
What is a unit for density?
g/mL or g/cm³.
Volume = 25 cm³, Density = 4 g/cm³. Find the mass.
m=d×V=4g/cm3×25cm3=100g.
If a liquid floats on another, what can you say about their relative densities?
The one that floats has lower density than the one below it.
Why does ice float in water?
Ice has a lower density because water expands when it freezes.
A 200 g object has a density of 4 g/mL. What is its volume?
V=200g/4g/mL=50mLV = 200 g / 4 g/mL = 50 mLV=200g/4g/mL=50mL.
True or False: Density changes when you cut an object in half.
False — density is an intrinsic property and does not change when the object is cut.
A cube has sides of 2 cm and a mass of 32 g. What is its density?
Volume of cube = 23=8 cm32^3 = 8 \, cm³23=8cm3. Density = 32g/8cm3=4g/cm332 g / 8 cm³ = 4 g/cm³32g/8cm3=4g/cm3.
You drop a piece of wood in alcohol. It floats halfway. What does that tell you about its density?
Its density is about the same as alcohol (close match).
How does density help explain why oil spills spread on oceans?
Oil is less dense than water, so it spreads on the surface.
You mix liquids with densities 0.8 g/mL, 1.0 g/mL, and 1.3 g/mL. Which one settles at the bottom?
The liquid with density 1.3 g/mL sinks to the bottom.
Explain why oil floats on water in terms of density.
Oil is less dense than water, so it floats.
A rock displaces 15 mL of water in a graduated cylinder. Its mass is 45 g. What is its density?
45g/15mL=3g/mL45 g / 15 mL = 3 g/mL45g/15mL=3g/mL.
A student finds an object’s density once with 2.1 g/mL and another time with 2.3 g/mL. How should they report their final result?
Take the average or range (about 2.2 g/mL).
Why do your ears “pop” when you dive deep into a swimming pool?
Water pressure increases with depth, compressing air in your ears until pressure equalizes.
A submarine adjusts its density to dive or rise. Explain how changing its density allows this.
By adjusting the amount of water in ballast tanks, submarines change their overall density to sink (more dense) or rise (less dense).