Frost on a CO₂ Inflator: Particles in a Bike Flat Kit

Use temperature data and a particle model to explain a cartridge that frosts over.

The inflator test

Read the passage and study Table 1. Then answer the questions.

A Flat Kit for Winter Riders

1Every February, the bike safety club at Ridgeline Middle School runs a Winter Commute Day. Club members check riders' helmets and lights and show them how to fix a flat tire on a cold morning. This year, seventh graders Leilani Kahananui and Arjun Bhatt studied one tool in the club's flat kit: a CO₂ inflator. It is a small steel cartridge, about as long as a thumb, that a factory fills with 16 g of carbon dioxide and seals. A rider screws the cartridge into a valve head on the tire, opens it, and gas rushes into the tire.

2Carbon dioxide (CO₂) is a pure substance made of molecules. At normal air pressure, solid CO₂, often called dry ice, changes straight into a gas at −78 °C, and CO₂ does not form a liquid at all. Inside the sealed cartridge, though, the CO₂ is squeezed to about 60 times normal air pressure. The package says that the cartridge holds liquid CO₂.

3The club's adviser, Mr. Ruben Castañeda, is a bike mechanic. He ran the test wearing thick gloves and safety glasses, because a cartridge can get cold enough to hurt bare skin. The classroom air was 21 °C. He emptied one cartridge into a tire in about 15 seconds and closed the valve head. Then he set the empty cartridge, still in the valve head, on a tray. Arjun pointed an infrared thermometer at the side of the cartridge and called out readings, and Leilani wrote down what the surface looked like (Table 1).

4Leilani also made Table 2 for three pure substances that riders carry or meet in winter: water in their bottles, isopropyl alcohol that mechanics use to clean disc brakes, and CO₂. On Winter Commute Day, the air outside was −12 °C. Some riders had left a capped water bottle and a sealed bottle of isopropyl alcohol in a bag on the outdoor bike rack overnight. In the morning, the water was frozen solid, but the alcohol still sloshed when the bottle was shaken.

Table 1. Cartridge Surface During and After the Fill (room air 21 °C)
Time (s)	Surface temp. (°C)	What Leilani saw
0	21	Dry, shiny steel
5	4	Hissing; tire filling
10	−11	Thin white frost appears
15	−17	Valve closed; thick white frost
60	−7	White frost
120	0	Frost turning into water drops
180	0	Mostly drops, a little frost
240	6	Wet, no frost left
360	15	Drops drying up

Use Table 1.

From 0 s to 15 s, the surface of the cartridge cooled by °C.

From 0 s to 15 s, CO₂ was rushing out of the cartridge into the tire. Which particle explanation fits the cooling best?

A. Liquid CO₂ molecules broke free as a gas, taking thermal energy from the steel and the liquid.
B. Cold CO₂ particles spread into the steel and gave their cold to the steel's particles.
C. The steel's particles stopped vibrating once the high pressure inside the cartridge was gone.
D. The liquid CO₂ froze into a solid, and freezing pulled thermal energy into the steel walls.

From 15 s to 60 s, the surface warmed from −17 °C to −7 °C while it was still covered in frost. Which way was thermal energy moving?

A. From the frosty steel out into the 21 °C room air
B. From the frost into the steel, which made the frost thicker
C. From the 21 °C room air into the frost and the steel
D. As cold, flowing out of the steel into the room air

The surface read 0 °C at both 120 s and 180 s, even though the room air was 21 °C. Which explanation fits Table 1 and the particle model best?

A. The water molecules in the frost stopped moving at 0 °C until all of the frost had melted.
B. The frost held cold inside it and gave that cold to the steel until the frost was used up.
C. The air right next to the cartridge had cooled to 0 °C, the same as the surface of the steel.
D. Energy from the room was freeing water molecules in the frost from fixed places, not speeding them up.

Match each sample to the particle model that describes it best.

Frost on the cartridge at 15 s (−17 °C)
Frost on the cartridge at 60 s (−7 °C)
CO₂ in the full, sealed cartridge before the test
CO₂ in the filled tire

A. Close together, vibrating a little faster in fixed positions
B. Close together and sliding past one another
C. Far apart, moving freely and hitting the tire's walls
D. Close together, vibrating slowly in fixed positions

Three pure substances

Use paragraphs 2 and 4 and Table 2 to answer the questions.

Table 2. Change-of-State Temperatures at Normal Air Pressure
Substance	Melts/freezes at (°C)	Boils/condenses at (°C)
Water	0	100
Isopropyl alcohol	−89	82
Carbon dioxide	−78 (solid ↔ gas)	−78 (solid ↔ gas)

Use Table 2. Sort each sample by its state. Assume normal air pressure and that each sample has been at the temperature named for several hours.

Water in a capped bottle on the bike rack at −12 °CIsopropyl alcohol in a sealed bottle on the bike rack at −12 °CCarbon dioxide that leaks out of a tire into the −12 °C airWater in a bottle inside the 21 °C classroomSuppose: isopropyl alcohol kept in a lab freezer at −95 °C

Solid	Liquid	Gas

At normal air pressure, CO₂ does not form a liquid, yet the sealed cartridge holds liquid CO₂ at 21 °C. Which explanation fits paragraph 2 best?

A. The steel walls keep the CO₂ inside colder than −78 °C until it is opened.
B. The very high pressure crowds the molecules close together as a liquid.
C. The CO₂ in the cartridge is a different substance from the CO₂ in air.
D. The CO₂ molecules inside are smaller, so they fit together as a liquid.

Use paragraph 4 and Table 2. The water bottle on the rack froze overnight, but the alcohol bottle beside it did not. Which particle explanation fits best?

A. The alcohol bottle was sealed, so it lost less thermal energy overnight than the capped water bottle did.
B. The alcohol molecules stopped moving at −12 °C, so they could not line up and lock into a solid.
C. Water molecules are smaller than alcohol molecules, so they pack into a solid at a higher temperature.
D. At −12 °C, water molecules settle into fixed positions, but alcohol molecules can still slide past one another.

Use Tables 1 and 2.

To reach the temperature at which CO₂ turns solid at normal air pressure, the cartridge surface would have had to cool °C below its coldest reading in Table 1.

What was the frost?

Use the whole passage, both tables and the particle model.

10.

Underline each change in which the matter named lost thermal energy.

The steel surface cooling from 4 °C to −11 °CFrost turning into water drops at 120 sWater in the bottle on the rack freezing overnightThe surface warming from 0 °C to 15 °CWater vapor in the room forming the frost at 10 sThe drops on the cartridge drying up into the air

11.

Arjun says, “The white frost was frozen carbon dioxide that leaked out of the cartridge.” Do you agree? Write a claim, use evidence from Table 1 and Table 2, and use the particle model to explain where the frost came from and what happened to it.