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

Tuesday

Reading a density table
// Mass packed into a space
⏱ about 20 min

Tuesday: Reading a Density Table

Raven lays a printed table on the Bench. Each row names a substance and gives a number.

"Corn oil, 0.922. Water, 0.998. Corn syrup, 1.36," she reads. "What do you notice about the order?"

Rocket runs his finger down the column. "Smallest at the top, biggest at the bottom. Like a cup of layers."

Nova dims her light to make the numbers easier to read. "Which of these would float on water?" she asks.

"Anything smaller than 0.998," Rocket says. "So oil floats, and syrup sinks."

Raven taps the ice row. "Ice is 0.917. That is why your cube floated yesterday."

"So a table can predict the experiment before we do it," Rocket says. Nova hums. "Now you are thinking like a chemist."

A table of densities

Chemists measure the density of many substances and list them in tables. Here are some from a chemistry text hosted by the University of California, Davis.

Liquids and solids are given in grams per milliliter at 20 degrees Celsius. Gases are much less dense, so they are given in grams per liter.

Read the water row first. Then compare every other row to it.

SubstanceDensity (grams per milliliter)
Ice (at 0 degrees Celsius)0.917
Corn oil0.922
Water0.998
Water (at 4 degrees Celsius)1.000
Corn syrup1.36
Aluminum2.70
Copper8.92
Gold19.3
USE THE TABLE
  • Read the question.
  • Tap your answer.
Corn oil is 0.922 and water is 0.998. Where does corn oil go when poured on water?
Which substance in the table is the most dense?
A block of aluminum and a block of copper are the same size. Which has more mass?
LEAST DENSE TO MOST DENSE
  • ?Corn oil
  • ?Water
  • ?Ice
  • ?Aluminum
  • ?Copper
  • ?Corn syrup
WHY THIS EXERCISEPutting substances in density order predicts how they will layer in a cup.

Why ice spreads out

When water freezes, its molecules lock into a lattice, a repeating pattern. The lattice arrangement lets water molecules be more spread out than in the liquid.

The same mass of water now fills more space. Upon freezing, the density of ice decreases by about 9 percent.

Temperature changes density in liquids too. Water at about 4 degrees Celsius is denser than water at 0 degrees Celsius.

Most materials expand as temperature increases, so their density usually decreases as they warm up.

That is why a density table always says what temperature it was measured at. The table above says 20 degrees Celsius.

Same substance, same temperature, same density. That makes density a property you can use to identify a substance.

Liquid water cools toward freezing
Molecules lock into a spread-out lattice
The same mass fills more space
Ice is less dense than liquid water
Ice floats
Upon freezing, the density of ice decreases by about ____ percent.
Water is densest near ____ degrees Celsius.
Most materials become less dense when they get warmer or colder? Type one word.
WHY THIS EXERCISEDensity depends on temperature, so a density table always states the temperature it was measured at.
StatementTrue or false?
In ice, water molecules are more spread out than in liquid water.?
Water at 0 degrees Celsius is denser than water at 4 degrees Celsius.?
Gases are much less dense than solids and liquids.?
Corn syrup is less dense than water.?
WHY THIS EXERCISEReading density data correctly is the skill tomorrow's Bench Lab depends on.
Try it
Put a spoonful of cooking oil in a clear cup of water. Do not stir. Watch where the oil goes.
Check the table: corn oil is 0.922 and water is 0.998. Did the cup agree with the numbers?
Record the result in your Bench Log with both densities written beside it.
Draw two boxes the same size. Fill one with water molecules close together and one with molecules in a spread-out lattice. Label liquid and ice.

Strong data reading. Tomorrow is Bench Lab: you will test salt water against fresh water and build a density column.

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