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Freezer Science Fair: Watching Water Freeze

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Freezer Science Fair: Watching Water Freeze

Use a cooling data log and a particle model to explain freezing.

Two cups in the freezer

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

Watching Water Freeze

1For the Oakridge Middle School science fair, seventh graders Ines Carvalho and Kwame Asante asked a question: what happens to the temperature of water while a freezer takes thermal energy away from it? Their teacher, Ms. Abara, lent them a digital temperature probe that records a reading every hour.

2Ines poured 100 mL of tap water at 20 °C into a plastic cup, which they called Cup A. Kwame poured 200 mL of the same water into an identical cup, Cup B. Each cup got a probe in the middle of the water. They placed both cups side by side on the same shelf of a home freezer set to −18 °C and left them for 8 hours. Each hour, Ines opened the door for a few seconds to look at Cup A and write down what she saw. Their data are in Table 1.

3To explain their results, the students used a particle model. All matter is made of tiny particles that are always moving. Temperature is a measure of the average kinetic energy, or energy of motion, of the particles. In liquid water, the particles are close together and slide past one another. In ice, the particles are held in fixed positions, where they vibrate. Thermal energy always moves from a warmer object to a colder one, and it moves faster when the temperature difference between them is larger.

4The students noticed two more things. First, the inside walls of the freezer were covered with a thin layer of frost. The frost formed from water vapor, a gas in the air, that touched the cold walls. Second, when Kwame took the frozen Cup A out and set it on the kitchen counter, where the air was 22 °C, tiny drops of water appeared on the outside of the cup within a minute.

Table 1. Freezer Data Log
Time (h)	Cup A (°C)	What Cup A looked like	Cup B (°C)
0	20	All liquid	20
1	0	Thin ice on top	5
2	0	About half ice	0
3	0	Nearly all ice	0
4	−10	Solid ice	0
5	−16	Solid ice	0
6	−17	Solid ice	0
7	−18	Solid ice	−8
8	−18	Solid ice	−15

1.

How many hourly readings of 0 °C did Cup B have? readings

2.

From hour 1 to hour 3, Cup A stayed at 0 °C as it changed from a thin layer of ice to nearly all ice. Which description fits the particles during those hours?

A. They lost energy and settled into fixed positions as the liquid froze.B. They broke apart into smaller pieces that packed in more tightly.C. They escaped from the cup as a gas and left only ice behind.D. They stopped moving, because 0 °C is the coldest that water can get.

3.

Why did Cup B stay at 0 °C for more hours than Cup A?

A. A larger amount of water freezes at a lower temperature, so Cup B froze below 0 °C.B. Cup B had more water particles, so more thermal energy had to be removed to freeze all of it.C. Cup B started at a higher temperature, so it needed more time to cool down to 0 °C.D. The freezer sent its cold into the smaller cup first and reached Cup B later.

4.

From hour 4 to hour 7, Cup A cooled from −10 °C to −18 °C. What happened to its particles?

A. They vibrated more slowly in fixed positions.B. They moved farther apart from one another.C. They became smaller as they got colder.D. They slid past one another more slowly.

5.

Cup A's temperature stopped dropping at −18 °C. Which explanation fits the data?

A. Ines opened the door each hour, which let warm air stop the cooling.B. Ice has no thermal energy left once it reaches −18 °C.C. Cup A reached the freezer's temperature, so thermal energy stopped flowing out of it.D. The water had finished freezing, so the ice could not get any colder.

Frost, droplets and predictions

Use paragraphs 3 and 4, Table 1 and the particle model to answer.

6.

Choose the two events in which water vapor from the air lost thermal energy and changed state.

A. Frost forming on the inside walls of the freezerB. Cup A warming from −18 °C on the kitchen counterC. Drops of water appearing on the outside of the cold cupD. The water in Cup B cooling from 20 °C to 5 °C

7.

Where did the drops on the outside of Cup A come from?

A. Water from the melting ice leaked out through the plastic cup.B. Water vapor in the room lost energy to the cold cup and became liquid.C. The ice gave off water vapor that landed on the cup's outside.D. Cold from the cup changed some of the room's air into water.

8.

Sort each change by what happens to the average speed of the particles named.

9.

Suppose the students repeat the test with 100 mL of water in a freezer set to −10 °C instead of −18 °C.

At what temperature would the water stay while it freezes? °C

10.

In that warmer −10 °C freezer, how would the time Cup A spends at 0 °C most likely compare with the first test?

A. Shorter, because a warmer freezer makes water freeze fasterB. The same, because the water still freezes at 0 °CC. It would stay liquid, because −10 °C is too warm for water to freezeD. Longer, because thermal energy flows out more slowly when the temperature difference is smaller

11.

Use evidence from Table 1 and the particle model to explain why Cup B took longer to freeze than Cup A. Include what was happening to thermal energy and to the particles while each cup stayed at 0 °C. Write a claim, evidence and reasoning.