Rocket holds an empty one liter bottle upside down over the sink. "Full of air," he says. "How much does the air weigh?"
Raven checks the Bench Log from last week. "The balloon test showed air has mass. It did not tell us how much."
Nova hovers over a reference page glowing on the wall. "Engineers measured the air at sea level long ago," she says. "Compare it with water."
Rocket reads the numbers and whistles. "A liter of water is about a thousand grams. A liter of air is barely one gram."
"What do you notice?" Raven asks. "Same volume, very different mass."
"Because the particles are so far apart," Rocket says. "Mostly empty space. The table from Tuesday was right."
Raven writes it down under a new heading: States in Real Life.
NASA engineers use a standard value for air at sea level: about 1.2 kilograms for every cubic meter.
A cubic meter holds 1,000 liters. So one liter of air has a mass of about 1.2 grams.
Water is about 1 gram per milliliter, according to the United States Geological Survey. A liter is 1,000 milliliters, so a liter of water is about 1,000 grams.
The same volume of liquid has hundreds of times more mass than the gas. Liquid particles are packed in contact; gas particles are far apart.
Gases are very compressible. A large amount of oxygen can be stored in a small tank, which is how people who need breathing help carry it.
When the tank is opened, the gas expands and the pressure drops, so it can be breathed at normal pressure.
A liquid in a gravitational field takes the shape of its container with a free surface. In microgravity, a liquid forms a ball instead.
Oxygen and carbon dioxide are gases at room temperature. Water and mercury are liquids. Most metals are solids.
At the very high temperatures on the Sun, atoms break apart into a mixture of charged particles called a plasma. Scientists often call plasma a fourth phase of matter.
This week you sorted matter into solids, liquids and gases by shape and volume. You explained each state with the particle model.
You tested the rules at the Bench with water, ice and a bag of air. You read the states and melting points of real elements.
Next week you will add thermal energy to the model and watch particles speed up, melt and boil.
| Week review | True or false? |
|---|---|
| A solid keeps its shape and volume. | ? |
| A liquid keeps its shape but changes volume. | ? |
| Gas particles are far apart and mostly empty space. | ? |
| A liter of air has more mass than a liter of water. | ? |
| Mercury is the only metal that is liquid at room temperature. | ? |
Strong week, chemist. Tomorrow is Bench Day: a family state hunt and a bag of air on the scale.