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Matter Lab 6-8 / Week 04 / Bench Day
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Week 04 · Density: Why Things Float

Bench Day

Bench Day: Float challenge
// Mass packed into a space
⏱ about 20 min

Bench Day: Float Challenge

Saturday morning, two big bowls sit on the Bench. One holds fresh water, and one holds water with plenty of salt stirred in.

"Same water, different bowls," Rocket says. "Let the float challenge begin!"

Raven lines up a crayon, a coin, a grape, a cork and a paper clip. "Predict each one before you drop it."

Rocket drops the grape into the fresh water. It sinks. He drops another into the salt water. It bobs near the top.

"What do you notice?" Raven asks. "The grape did not change. The water did."

Nova hovers between the bowls. "Which bowl has the denser liquid?" she asks. Rocket grins. "The salty one. We proved it."

  1. Density is the mass of a substance for a specific volume, mass per unit volume, in grams per milliliter.
  2. Water is roughly 1 gram per milliliter and is most dense at about 4 degrees Celsius.
  3. Ice is less dense than liquid water because freezing spreads the molecules into a lattice.
  4. A substance less dense than the liquid around it floats. A denser one sinks.
  5. Dissolved salt makes water denser. Ocean water has about 35,000 parts per million of salt.
◇ FAMILY FLOAT CHALLENGE
Fill two bowls with cool tap water. Stir salt into one until no more grains disappear.
Gather five small waterproof objects from around the house. Each person predicts float or sink in each bowl.
Test each object in the fresh water first, then in the salt water. Dry it between tests.
Talk it over. Did any object float in one bowl and sink in the other? What does that say about the two liquids?
For a grown-up
Choose objects that are safe to get wet and too large to swallow. Food items used as test objects are not eaten afterward.
Only tap water is used. Everything is rinsed and the water goes down the sink when you finish.
The density and salinity facts this week come from the U.S. Geological Survey Water Science School, at usgs.gov.
Safety first
Nothing goes in your mouth. Nothing is tasted.
Wipe up spills right away so the floor stays dry.
Wash your hands when the challenge is over.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
An object sinks in fresh water but floats in salt water. Which statement is true?
Which bowl holds the denser liquid?
Bench Day checkTrue or false?
An object that floats is less dense than the liquid it floats in.?
Adding salt to water makes it denser.?
A heavier object always sinks.?
WHY THIS EXERCISEThe float rule compares densities. Weight alone does not decide it.
Draw your two bowls side by side. Show where each object ended up in each one.
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