Materials & Prep
Gather the teacher’s block of wood, steel bolt, clear container of water, balance, ruler, graduated cylinder, calculator, and exit tickets. Prepare or display this class data: wood block, mass 16 g, dimensions 2 cm × 2 cm × 5 cm; steel bolt, mass 39 g, water displacement 5 mL. Confirm the measurements with the available objects before class and adjust the recorded values if needed. Remind students to handle water and the metal bolt carefully and confirm cleanup and spill procedures against school rules.
Opening
Ask“The steel bolt is much heavier than the block of wood. Which one will float, and which one will sink?” Have every student commit by writing W or S for each object, then briefly explain the prediction to a partner. Place both objects in water. Students observe that the wood floats and the bolt sinks.
Confront“Heavy means sinks” is not enough to explain what happened. The bolt is heavier, but the wood floats. Today we will use a calculation to determine whether an object’s mass is packed into a small or large amount of space.
ConnectAsk students to recall that mass tells how much matter an object has and volume tells how much space it takes up. Record the observations separately from inferences: “The wood stays partly above the water” is an observation; “The wood is less dense than water” is an inference.
Direct instruction and model
DefineDensity is mass divided by volume. Water has a density of about 1 g/mL. An object with a density less than 1 g/mL floats in water. An object with a density greater than 1 g/mL sinks.
Display
Density = mass ÷ volume
Think aloud“I cannot decide from mass alone. I need both mass and volume. I will calculate density, then compare that number with 1 g/mL.”
Worked exampleFor the wood block, first calculate volume from its dimensions: 2 cm × 2 cm × 5 cm = 20 cm³. Since 1 cm³ equals 1 mL, the volume is 20 mL. Then calculate density: 16 g ÷ 20 mL = 0.8 g/mL. Because 0.8 g/mL is less than water’s 1 g/mL, I predict the wood floats.
Worked exampleFor the steel bolt, use water displacement to find volume. The bolt displaces 5 mL, so its volume is 5 mL. Its density is 39 g ÷ 5 mL = 7.8 g/mL. Because 7.8 g/mL is greater than 1 g/mL, I predict the bolt sinks. The common mistake is stopping at “39 g is heavy.” The decision requires the mass-to-volume calculation.
Guided practice
Display
Object A: mass 24 g, volume 30 mL Object B: mass 45 g, volume 40 mL Object C: mass 18 g, volume 12 mL
Have students calculate each density with a partner and predict float or sink. Require partners to say, “The density is g/mL. Since this is [less than/greater than] 1 g/mL, the object will [float/sink].”
Check for understandingAsk students to hold up F or S for Object A before discussing. Then ask, “What number did you compare with 1 g/mL?” Expected: Object A has a density of 24 ÷ 30 = 0.8 g/mL, so it floats. Listen for students who choose sink because 24 g sounds heavy, and return them to the division by volume.
ExpectedObject B has density 45 ÷ 40 = 1.125 g/mL and sinks. Object C has density 18 ÷ 12 = 1.5 g/mL and sinks.
ScaffoldProvide this three-line organizer for students who need it: “Mass = g. Volume = mL. Density = mass ÷ volume = g/mL. Since is [less than/greater than] 1, I predict .” Students may use a calculator after writing the division.
Independent application
Students complete the following problems independently, then compare answers with a partner.
- A plastic cube has a mass of 12 g and a volume of 20 mL. Calculate its density and predict whether it floats or sinks.
- A glass marble has a mass of 30 g and a volume of 10 mL. Calculate its density and predict whether it floats or sinks.
- An object has a mass of 16 g and a volume of 16 mL. Calculate its density. Predict what will happen in water and explain what makes this case different from the others.
Students must show the calculation and write a claim with evidence: “I predict because the density is g/mL, which is than water’s density of 1 g/mL.”
CirculateCheck that students divide mass by volume, include units, and compare density rather than mass alone. Ask, “What evidence from your calculation supports your prediction?”
ExtensionStudents create a fourth object with a mass greater than 30 g that would still float. They must choose a volume, calculate its density, and explain why the large mass does not determine the result by itself.
Closing check
Exit ticket“An object has a mass of 42 g and a volume of 60 mL. Calculate its density. Predict whether it will float or sink in water. Use one sentence to justify your prediction with the calculated density and the density of water.”
Expected42 g ÷ 60 mL = 0.7 g/mL, so the object floats because 0.7 g/mL is less than water’s 1 g/mL. Collect the ticket to check whether each student can calculate density, compare it with water, and justify the prediction.