Density and Buoyancy Lesson Plan with a Virtual Lab
Updated 2026-10-02
This density and buoyancy lesson plan uses a free virtual lab so students can answer the oldest question in the science room, "Why do some things float?", with measurements instead of guesses. They calculate density from mass and volume, compare blocks with the same mass and the same volume, identify mystery materials, then weigh the water a floating block pushes aside. Everything is ready to use: learning goals, setup, predictions, a step-by-step activity with the readings students should get, a five-question set for the class link, a buoyancy extension, and ideas for differentiation. Every number comes from the simulations' own models.
Lesson at a glance
- Level: grades 6–9 (ages 11–15), physical science. The buoyancy extension suits grades 8–10.
- Time: one 55–60 minute period, plus an optional 20-minute extension.
- Prior knowledge: mass in kilograms, volume in liters, dividing decimals.
- Format: pairs on laptops, tablets or phones, or the whole class with one projector.
- Simulations: Density – sink or float. Extension: Buoyancy – lab and shapes.
Learning goals
By the end of the lesson, students can:
- Calculate density with D = m/V and give it in kg/m³.
- Predict whether a block floats or sinks in water by comparing its density with 1,000 kg/m³.
- Explain why mass alone and volume alone don't decide floating.
- Identify a material from its measured density.
- State that a floating object displaces its own mass of water (Archimedes' principle).
What the simulation does
The density simulation has three screens.
- Intro: one block in a 100 L tank. Choose a material, Foam (150 kg/m³), Wood (800), Ice (920), Brick (2,000), Aluminium (2,700) or Iron (7,800), or Custom. Sliders set the mass (0.1–30 kg) and volume (1–10 L). For a library material, moving one slider moves the other, so the density stays fixed. The readout shows D = m/V, "floats" or "sinks", the volume under water and the tank's water level.
- Compare: four blocks, A to D, with the same mass, the same volume or the same density. One slider sets the shared value. Tap a block to read its m, V and D.
- Mystery: five unlabeled blocks. Drag one onto the digital scale to read its mass, and into the tank to read its volume from the water level. Students then pick the material from a list and press Check. A reference table of densities can be shown or hidden.
A few details worth knowing before class:
- Liters and kilograms make the arithmetic easy. 1 L of water has a mass of 1 kg, so D in kg/m³ is 1,000 × (m in kg) ÷ (V in L).
- Floating blocks fool the tank. On the Mystery screen, a floating block only shows part of its volume unless students hold it fully under (step 4).
- New blocks reshuffles the Mystery screen. The answers in this plan are for the blocks that appear when the simulation opens.
Materials and setup before class
Materials: one device per pair (or a projector), the tables below on paper, and a calculator.
Setup (10 minutes, once):
- Open the simulation on the Intro screen with Wood selected. These are the defaults: a 4.00 kg, 5.00 L block.
- Click Share and create a link for each class, for example "Science · Period 1". Leave Show reference density table on for most classes.
- Optional: on the Questions tab, enter the question set below and attach it to the link. The two predictions lock the simulation until each student commits. See the Predict–Observe–Explain guide for the routine.
- Post the link, or open it in present mode and show the QR code.
Lesson sequence
1. Hook and predictions (7 minutes)
Project the Intro screen. Drag the wooden block under the water and let go: it bobs back up and settles with 80% under water. Then have students commit to two predictions, on paper or on the link:
- P1. "A wooden block floats with 80% of it under water. You cut it in half. How does the half block float?"
- P2. "A 4.0 kg block floats in water. How many liters of water does it push aside?"
Many students expect the small half to float higher, "because it's lighter". Few have a number for P2. Don't correct anyone yet.
2. Density on the Intro screen (12 minutes)
Pairs choose each material, set the volume slider to 2.0 L and copy the readout. The last three columns are for you; leave them blank on the student copy.
| Material | V (L) | m (kg) | D (kg/m³) | Floats or sinks | Under water | Tank level |
|---|---|---|---|---|---|---|
| Foam | 2.00 | 0.30 | 150 | floats | 0.30 L (15%) | 100.3 L |
| Wood | 2.00 | 1.60 | 800 | floats | 1.60 L (80%) | 101.6 L |
| Ice | 2.00 | 1.84 | 920 | floats | 1.84 L (92%) | 101.8 L |
| Brick | 2.00 | 4.00 | 2,000 | sinks | 2.00 L (100%) | 102.0 L |
| Aluminium | 2.00 | 5.40 | 2,700 | sinks | 2.00 L (100%) | 102.0 L |
| Iron | 2.00 | 15.60 | 7,800 | sinks | 2.00 L (100%) | 102.0 L |
Ask pairs for two patterns. Good answers: "Everything under 1,000 kg/m³ floats" and "For a floating block, the liters under water equal its kilograms; for a sinking block, they equal its volume."
Now check P1. Choose Wood and move the volume slider from 4.0 L to 1.0 L. The mass follows (3.20 kg, then 0.80 kg), the density stays at 800 kg/m³, and the block floats with 80% under water every time. Cutting a block doesn't change what it's made of.
3. Same mass, same volume, same density (10 minutes)
Switch to the Compare screen. Keep Same mass at 1.5 kg and tap each block.
| Block | m (kg) | V (L) | D (kg/m³) | Result |
|---|---|---|---|---|
| A | 1.50 | 5.00 | 300 | floats |
| B | 1.50 | 1.88 | 800 | floats |
| C | 1.50 | 0.56 | 2,700 | sinks |
| D | 1.50 | 0.19 | 7,800 | sinks |
Same mass, yet two float and two sink. Press Same volume (4.0 L): the masses become 1.20, 3.20, 10.80 and 31.20 kg, and A and B still float. Finally press Same density and set 800 kg/m³: all four float, big or small. Set 1,500 kg/m³: all four sink.
The conclusion writes itself: neither mass nor volume decides, density does. The pillar guide uses this screen as a whole-class demo if you want a shorter version.
4. Identify the mystery blocks (12 minutes)
Switch to the Mystery screen. Pairs weigh each block on the scale, measure its volume in the tank, calculate D and choose the material. The answer key for the blocks that appear when the simulation opens:
| Block | m (kg) | V (L) | D (kg/m³) | Material |
|---|---|---|---|---|
| A | 7.80 | 1.00 | 7,800 | Iron |
| B | 23.40 | 3.00 | 7,800 | Iron |
| C | 10.80 | 4.00 | 2,700 | Aluminium |
| D | 4.00 | 5.00 | 800 | Wood |
| E | 17.80 | 2.00 | 8,900 | Copper |
Watch block D. Dropped into the tank, it floats and reads "4.00 L (partial)", which gives 1,000 kg/m³ and no match in the table. Pairs who push it fully under while holding it read "5.00 L (full)" and get 800 kg/m³. That's a real lab skill: a floating object needs a sinker or a push to measure its volume. Notice also that A and B are both iron, with very different masses.
For a second round, or a different answer key per class, press New blocks. Two of the five new blocks are always the same material.
5. Exit check (5 minutes)
Use questions 3–5 of the set below. Then open View answers and show the Prediction and After columns for P1 and P2 side by side.
Question set for this lesson
Enter these on the simulation's Questions tab. Suggested Instructions for students: "Use the Intro screen first. Read m, V and D from the readout under the sliders. Write densities in kg/m³."
1. Multiple choice · Before, as a prediction · Ask again after the simulation
- Question: "A wooden block floats with 80% of it under water. You cut it in half. How does the half block float?"
- Options: Higher, with less than 80% under / The same, with 80% under (correct) / Lower, with more than 80% under / It sinks
- Explanation: "Cutting the block halves its mass and its volume, so the density stays at 800 kg/m³. The fraction under water is the block's density divided by the water's: 800 ÷ 1,000 = 80%. On the Intro screen, a 4 L and a 1 L wooden block both float 80% under."
2. Number · Before, as a prediction · Ask again after the simulation
- Question: "A 4.0 kg block floats in water. How many liters of water does it push aside?"
- Answer: 4.0, tolerance ± 0.1, unit L
- Explanation: "A floating block sinks until the water it pushes aside weighs as much as the block. 4.0 kg of water has a volume of 4.0 L. It doesn't matter how big the block is, as long as it floats: the 5 L wooden block of the Intro screen has 4.00 L under water."
3. Number · After the simulation
- Question: "A block has a mass of 17.80 kg and a volume of 2.00 L. What is its density?"
- Answer: 8900, tolerance ± 100, unit kg/m³
- Explanation: "D = m/V = 17.80 kg ÷ 2.00 L = 8.9 kg per liter. One liter is 0.001 m³, so D = 8,900 kg/m³. The reference table on the Mystery screen matches that to copper, and it sinks because 8,900 is more than 1,000."
4. Multiple choice · After the simulation
- Question: "On the Compare screen with Same mass, blocks A and B float and blocks C and D sink. Why?"
- Options: A and B are bigger, so they are lighter / A and B have a density below 1,000 kg/m³ (correct) / C and D are heavier / The water is deeper under C and D
- Explanation: "All four blocks have the same mass, 1.5 kg, so mass can't be the reason. A (300 kg/m³) and B (800 kg/m³) are less dense than water and float. C (2,700) and D (7,800) are denser than water and sink."
5. Short answer · After the simulation
- Question: "On the Mystery screen, a floating block gave a density of 1,000 kg/m³. What went wrong, and how do you fix it?"
- Accepted answers (optional): *under*
- Model answer: "The block floated, so only part of it was under water and the tank showed a volume that was too small. That made the density look bigger than it is. Push the block fully under while holding it and read the full volume: 5.00 L instead of 4.00 L gives 800 kg/m³, wood."
Questions 1 and 2 are the predictions, asked again after the simulation so students can compare. The formative assessment guide explains how to read the results across classes.
Extension: Archimedes' principle in the buoyancy lab
The buoyancy lab adds forces. On its Lab screen, sliders set the liquid density (300–3,000 kg/m³), gravity (with Moon, Earth and Jupiter buttons), and the block's mass and volume. A beaker beside the tank collects exactly the volume of liquid the block displaces, and the readout shows the upthrust F_A and the weight P in newtons.
The default block is 5 kg and 5 L, exactly as dense as water, so the readout says "suspended". Have pairs work through this sequence, letting the block settle each time:
| Setting | State | Beaker | F_A (N) | P (N) |
|---|---|---|---|---|
| m = 4.0 kg, V = 5.0 L, water, Earth | floats | 4.00 L | 39.20 | 39.20 |
| same block, Moon | floats | 4.00 L | 6.40 | 6.40 |
| same block, Jupiter | floats | 4.00 L | 99.20 | 99.20 |
| same block, liquid 1,600 kg/m³, Earth | floats | 2.50 L | 39.20 | 39.20 |
| m = 10.0 kg, V = 5.0 L, water, Earth | sinks | 5.00 L | 49.00 | 98.00 |
Questions to ask:
- "Why are F_A and P equal for every floating block?" Floating means the forces balance, so the upthrust equals the weight.
- "Why doesn't the block float higher on the Moon?" Gravity shrinks the weight and the upthrust by the same factor, so the same 4.00 L of water still balances the block.
- "The sinking block rests on a scale on the tank floor. What does it read?" 49.0 N: the weight minus the upthrust, 98.00 − 49.00. Ask what a diver feels when lifting a stone under water.
For a full lab write-up, see how to create a virtual lab activity.
Differentiation
Support:
- Use only Foam, Wood, Brick and Iron in step 2, and give the table with the materials filled in.
- On the Mystery screen, keep the reference table on and do only blocks A, C and E.
- Give sentence starters: "The block floats because its density is…" and "Mass alone doesn't decide because…"
- Pair students so one drags the blocks and the other records, then swap.
Stretch:
- Use Custom on the Intro screen to make a block that floats exactly half under water (for example 2.5 kg and 5.0 L, 500 kg/m³), then explain the rule.
- Predict the step 3 "Same volume" masses before pressing the button.
- In the buoyancy lab, find the lightest liquid on the slider in which the 4.0 kg, 5.0 L block still floats (810 kg/m³; at 800 kg/m³ it is suspended), and explain why it sinks at 700 kg/m³.
English learners: the simulations are available in six languages. Create a second link in the student's language so the labels are familiar while the discussion stays in English.
Standards alignment
This lesson fits the density unit of middle school physical science and the density and upthrust topics of GCSE Physics. NGSS has no performance expectation dedicated to density or buoyancy, so we don't claim one. The lesson does exercise three NGSS practices: Planning and Carrying Out Investigations, Analyzing and Interpreting Data, and Using Mathematics and Computational Thinking.
For more physics ideas, see interactive physics lesson ideas. Chemistry teachers who use density to identify substances will find related activities in interactive chemistry lesson ideas.
FAQ
Why does the simulation use liters and not cubic centimeters?
Liters keep the numbers simple and link straight to kilograms of water. Convert for students who need cm³: 1 L = 1,000 cm³, and 1,000 kg/m³ = 1 g/cm³.
Why does a block at exactly 1,000 kg/m³ behave oddly?
A block exactly as dense as water has no reason to rise or fall, so the result depends on where you let go of it. Avoid that case in the main lesson. The buoyancy lab labels it "suspended".
Can students do this lesson on phones?
Yes. Students open the link without an account. The sliders work well on touch screens; the Mystery screen's dragging is easier on a tablet or laptop.