8th grade · Science · Unit 2 of 16
Density & Buoyancy
Mass, volume, floating and sinking
What your child practices
Calculating density from mass and volume, explaining why solids, liquids and gases differ in density using the particle theory, and predicting whether objects float or sink in different fluids.
Alabama Course of Study content standard: 8.1 · density as a property that identifies a pure substance
How we explain it
Children can open this short lesson before they practice.
- Density = mass ÷ volume.
- Objects less dense than a fluid float; denser ones sink.
- Gases are less dense than liquids because their particles are farther apart.
Example: A block has a mass of 15 g and a volume of 5 cm³. What is its density? Density = mass ÷ volume. 15 ÷ 5 = 3. Answer: 3 g/cm³
Density & Buoyancy in Alabama
In the Alabama Course of Study, this is unit 2 of 16 in 8th grade Science, after Mixtures & Solutions and before Particles & Matter.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
A hot-air balloon rises because the heated air inside it is…
- more dense than the cooler air around it
- made of a different gas
- not made of particles
- less dense than the cooler air around it
less dense than the cooler air around it
Hint: Warm air expands, so the same mass takes up more volume and is less dense.
Question 2
Water has a density of 1.00 g/cm³. Does each material float or sink in water?
Sort into: 🛟 Floats · ⚓ Sinks
- 🪵pine wood (0.5 g/cm³)
- 🫒vegetable oil (0.92 g/cm³)
- 🍾cork (0.24 g/cm³)
- 🪙copper (8.9 g/cm³)
- 🎣lead (11.3 g/cm³)
- 🧊ice (0.92 g/cm³)
- 🔩iron (7.9 g/cm³)
- 🥫aluminum (2.7 g/cm³)
🛟 Floats: pine wood (0.5 g/cm³), vegetable oil (0.92 g/cm³), cork (0.24 g/cm³), ice (0.92 g/cm³) · ⚓ Sinks: copper (8.9 g/cm³), lead (11.3 g/cm³), iron (7.9 g/cm³), aluminum (2.7 g/cm³)
Hint: A material floats if its density is less than 1.00 g/cm³ and sinks if it is greater.
Question 3
A metal has a density of 4 g/cm³. What is the mass of 7 cm³ of it, in g?
answer boxg
28 g
Hint: Mass = density × volume.
Question 4
A lake freezes from the top down. Which fact about density helps explain this?
- Ice is denser than water, so it sinks.
- Cold water has no particles.
- Ice is less dense than liquid water, so it stays at the top.
- Ice has no volume.
Ice is less dense than liquid water, so it stays at the top.
Hint: The ice layer floats and protects the water below it.
Question 5
A sample of a liquid has a density of 5 g/mL and a mass of 50 g. What is its volume in mL?
answer boxmL
10 mL
Hint: Volume = mass ÷ density.
Question 6
Which state of matter usually has the lowest density?
- gas
- They are all the same.
- liquid
- solid
gas
Hint: Gas particles are very far apart.