Monday
Monday: Why Does Ice Float?
Rocket drops an ice cube into a clear cup of water at the Bench. It bobs, then settles with its top above the surface.
"Ice is water," he says. "Why does it float on itself?"
Raven leans in close. "What do you notice? Part of the cube is above the water line."
Nova hovers over the cup, her light making the ice glow. "Think about how much water fits in a cube that size," she says.
Rocket drops in a spoonful of corn syrup. It slides straight to the bottom and sits there in a golden lump.
"Same spoon, different result," Raven says. "So floating is not about size."
"It is about how much stuff is packed into the space," Rocket says. Nova hums. "Scientists have a word for that."
Density is mass packed into a space
Every object has mass, the amount of matter in it. Every object also has volume, the space it takes up.
Density is the mass of a substance for a specific volume. Scientists say it as mass per unit volume.
To find density, divide the mass by the volume. A lump with a mass of 20 grams and a volume of 10 milliliters has a density of 2 grams per milliliter.
The units matter. Mass is in grams. Volume is in milliliters, and one milliliter is the same as one cubic centimeter.
The density of water is roughly 1 gram per milliliter. One milliliter of water has a mass of about one gram.
| Quantity | What it measures | Kitchen unit |
|---|---|---|
| Mass | How much matter | grams, on a kitchen scale |
| Volume | How much space | milliliters, in a measuring cup |
| Density | Mass for a specific volume | grams per milliliter |
- Read the question.
- Tap your answer.
Less dense floats, more dense sinks
Ice is less dense than liquid water, which is why your ice cubes float in your glass.
Corn syrup is more dense than water, so it sinks when placed in water. That is what Rocket saw.
The rule is simple. A substance less dense than the liquid around it floats. A substance more dense than that liquid sinks.
Size does not decide it. A huge log floats and a tiny pebble sinks, because the log is less dense than water.
| Statement | True or false? |
|---|---|
| Density is the mass of a substance for a specific volume. | ? |
| Ice is more dense than liquid water. | ? |
| Corn syrup sinks in water because it is more dense than water. | ? |
| Bigger objects always sink. | ? |
- Tap a card.
- Then tap its spot.
Mark the water line with a piece of tape. Notice how much of the cube sits above it.
Write "ice floats" in your Bench Log, and your first guess about why.
Only cool or warm tap water is used. A kitchen scale and measuring cups make the numbers real, but estimates are fine.
The density facts this week come from the U.S. Geological Survey Water Science School, which you can read at usgs.gov.
Good start. Tomorrow you will read a real density table and find out why ice spreads out when it freezes.