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

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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
021Dry, shiny steel
54Hissing; tire filling
10−11Thin white frost appears
15−17Valve closed; thick white frost
60−7White frost
1200Frost turning into water drops
1800Mostly drops, a little frost
2406Wet, no frost left
36015Drops drying up
1.
Use Table 1.

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

2.
From 0 s to 15 s, CO₂ was rushing out of the cartridge into the tire. Which particle explanation fits the cooling best?
Question 2 options
3.
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?
Question 3 options
4.
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?
Question 4 options
5.
Match each sample to the particle model that describes it best.

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
SubstanceMelts/freezes at (°C)Boils/condenses at (°C)
Water0100
Isopropyl alcohol−8982
Carbon dioxide−78 (solid ↔ gas)−78 (solid ↔ gas)
6.
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.
7.
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?
Question 7 options
8.
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?
Question 8 options
9.
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.

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.