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

Friday

Density in everyday life
// Mass packed into a space
⏱ about 20 min

Friday: Density in Everyday Life

Rocket arrives at the Bench with a question. "If salt water is denser, do people float better in the ocean?"

Raven opens the Bench Log. "The ocean has about 35,000 parts per million of salt. That is a lot of dissolved stuff."

"And some lakes have even more," Nova says. "In the saltiest water, people float without even trying."

Rocket imagines bobbing like a cork. "So density is not just a number. It changes what happens to you."

Raven flips back a page. "What do you notice about everything we tested? The rule never changed."

"Less dense floats, more dense sinks," Rocket says. "Ice, oil, syrup, salt water. All of it."

Nova brightens. "Then you are ready to review the week."

Salt in the water

The U.S. Geological Survey measures dissolved salt in parts per million. Fresh water has less than 1,000 parts per million.

Ocean water contains about 35,000 parts per million of salt, which is about 3.5 percent.

Because ocean water contains dissolved salt, it is denser than pure water. Your Wednesday test showed the same thing in a cup.

In some extremely salty lakes, you truly do not sink the way you do in normal fresh or ocean water.

Kind of waterDissolved salt (parts per million)
Fresh waterless than 1,000
Slightly saline water1,000 to 3,000
Moderately saline water3,000 to 10,000
Highly saline water10,000 to 35,000
Ocean waterabout 35,000
DENSITY ALL AROUND YOU
  • Read the question.
  • Tap your answer.
Why does a person float more easily in the ocean than in a pool of fresh water?
Oil spilled on a pond spreads across the top. What does that tell you about oil?
A pond freezes at the top first. Which fact explains it?

Density tells you what something is

Density is a property of a substance. Every sample of pure water at 4 degrees Celsius has the same density, 1.000 grams per milliliter.

That makes density useful for telling substances apart. Gold is 19.3, copper is 8.92, and aluminum is 2.70 grams per milliliter.

If a shiny metal block has a density of 8.92, it is not gold. The numbers do not lie.

This is the idea behind this week's standard. Chemists compare properties like density before and after a change to see what a substance is.

Next week you will mix substances together and learn which mixtures can be separated again.

Week reviewTrue or false?
Density is mass divided by volume.?
Water is roughly 1 gram per milliliter.?
Ice sinks in liquid water.?
Dissolved salt makes water less dense.?
Warm water is less dense than cool water above 4 degrees Celsius.?
WHY THIS EXERCISEThese five ideas explain every float and sink you saw at the Bench.
OUR WEEK, IN ORDER
  • ?We defined density as mass per unit volume.
  • ?We read a density table and learned why ice spreads out.
  • ?We tested salt water and built a density column.
  • ?We read the U.S. Geological Survey water density table.
  • ?We found density in oceans, ponds and metals.
WHY THIS EXERCISEThe week moved from one floating ice cube to a property that identifies substances.
A metal block has a density of 2.70 grams per milliliter. Using Tuesday's table, which metal is it? Type one word.
WHY THIS EXERCISEMatching a measured density to a table is how chemists identify an unknown substance.
Ocean water contains about ____ parts per million of salt.
Fresh water has less than ____ parts per million of dissolved salt.
Draw a cup of ocean water and a cup of fresh water. Show the same object floating higher in the salt water.
Try it
Look around your kitchen for liquids that separate into layers when left alone, such as oil and vinegar dressing.
Without tasting, note which layer is on top. Which one is less dense?

Strong week, chemist. Tomorrow is Bench Day: a family float challenge in fresh and salt water.

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