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7th grade · Science · Unit 8 of 14

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.

New York State Learning Standards learning standard: MS-PS1-7 · density as a property that identifies a sample of matter, and why things float or sink

How we explain it

Children can open this short lesson before they practice.

  1. Density = mass ÷ volume.
  2. Objects less dense than a fluid float; denser ones sink.
  3. 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 New York

In the New York State Learning Standards, this is unit 8 of 14 in 7th grade Science, after Particles & Mixtures and before Chemical Reactions.

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.

Source: New York State Education Department (NYSED)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    A block has a mass of 42 g and a volume of 7 cm³. What is its density in g/cm³?

    answer boxg/cm³

  2. Question 2

    A metal has a density of 4 g/cm³. What is the mass of 10 cm³ of it, in g?

    answer boxg

  3. Question 3

    A rock sinks in water. What does this tell you about the rock?

    • It is more dense than water.
    • It has no mass.
    • It is less dense than water.
    • It is a gas.

  4. Question 4

    A hot-air balloon rises because the heated air inside it is…

    • less dense than the cooler air around it
    • more dense than the cooler air around it
    • not made of particles
    • made of a different gas

  5. Question 5

    A block of aluminum is cut in half. What is the density of each half?

    • the same as the original block
    • zero
    • half the original
    • double the original

  6. Question 6

    Pour these liquids into a tall jar. Order them from the bottom layer to the top layer.

    • 🫒Vegetable oil (0.92 g/cm³)
    • 💧Water (1.00 g/cm³)
    • 🍯Honey (1.42 g/cm³)
    • ⚪Mercury (13.6 g/cm³)

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