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Matter Lab 6-8 / Week 04 / Wednesday
3/6
Week 04 · Density: Why Things Float

Wednesday

Bench Lab: Salt water and a density column
// Mass packed into a space
⏱ about 20 min

Wednesday: Bench Lab: Salt Water and a Density Column

Rocket has three cups lined up on the Bench and a spoon in each hand. "Corn syrup, water, oil. Pour them all in and see!"

Raven holds up a hand. "Predict first, then pour. Which one lands at the bottom?"

"Syrup," Rocket says at once. "It was 1.36 on the table."

Nova hovers over a fourth cup, where Raven has stirred salt into water until it vanished. "This one is not on your table," Nova says.

"Salt water," Raven says. "What do you notice? It looks exactly like plain water."

Rocket tilts his head. "Looks the same. But it has more stuff in it. So maybe it is denser."

"Then let us find out," Raven says, and reaches for the food coloring.

Your mission

Today you run two tests. First, you find out whether fresh water floats on salt water. Second, you build a density column from three kitchen liquids.

Predict before each pour. Then record what you see in the data table and your Bench Log.

  • Four clear cups, and one tall clear cup or jar for the column
  • Water, salt, corn syrup and cooking oil
  • Food coloring, two colors
  • Measuring cups, a spoon, and a kitchen scale if your family has one
  • Paper towels for spills, and your Bench Log
Safety first
Use only cool or warm tap water. Nothing is heated.
Nothing goes in your mouth, and nothing is tasted. Keep hands away from your face while you work.
Wear safety glasses or sunglasses while pouring, in case a liquid splashes.
Wash your hands when you finish. Pour leftovers down the sink with running water.
  1. Pour one cup of water into cup A. Stir in two spoonfuls of salt until you cannot see any grains.
  2. Pour one cup of plain water into cup B and add two drops of one food coloring.
  3. Tilt cup A and pour the colored fresh water in very slowly, down the inside of the cup.
  4. Watch for one minute. Record whether the colored water stays on top, sinks or mixes.
  5. For the column, pour corn syrup into the tall cup until it is about one third full.
  6. Slowly pour water with a drop of the second color down the inside of the cup until it is two thirds full.
  7. Slowly pour cooking oil on top. Wait one minute and record the order of the layers.
  8. If a kitchen scale is handy, measure the mass of 100 milliliters of each liquid and record it.
PREDICT BEFORE YOU POUR
  • Read the question.
  • Tap your answer.
Salt water has dissolved salt in it. Compared with fresh water, it is:
Which liquid should be at the bottom of your column?
Which liquid should be on top of your column?
TestMy predictionWhat I sawMass of 100 mL (if measured)
Colored fresh water on salt water
Corn syrup layer
Water layer
Cooking oil layer

Reading the result

Ocean water, which contains dissolved salt, is denser than pure water. So fresh water can rest on top of salt water, at least for a while.

In the column, each liquid floats on any liquid denser than itself. The order of the layers is the order of the densities.

If your layers mixed, pour more slowly next time. Pouring down the inside of the cup keeps the layers calm.

If you measured mass, divide each mass by 100 to get grams per milliliter. Compare your numbers with Tuesday's table.

Your numbers will not match exactly. Kitchen scales round, and cooking oil is not exactly corn oil. Close is good science.

What the lab showsTrue or false?
Dissolving salt in water makes the water more dense.?
Oil floats on water because oil is less dense.?
The densest liquid ends up on top of a density column.?
Salt water and fresh water look different, so you can tell them apart by eye.?
WHY THIS EXERCISEThe column is a density table you can see. The bottom layer has the biggest number.
How many layers should your finished column have? Type a number.
WHY THIS EXERCISEEach liquid stays its own layer because each has a different density.
Sketch your density column. Label each layer with its liquid and its density from Tuesday's table.

Careful pouring. Tomorrow you will read the U.S. Geological Survey's own water density table, temperature by temperature.

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