Key Concepts

The way a thermometer works is an example of heating and cooling a liquid. When heated, the molecules of the liquid in the thermometer move faster, causing them to get a little further apart. This results in movement up the thermometer.When cooled, the molecules of the liquid in the thermometer move slower, causing them to get a little closer together. This results in movement down the thermometer.

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Summary

Students will look closely at the parts of a thermometer. After placing a thermometer in hot and cold water, students will look at molecular model animations of the liquid in a thermometer. Students will then draw a model of the molecules of a thermometer after it has been placed in hot and then cold water.

Objective

Based on experimental observations, students will describe, on the molecular level, why the liquid in a thermometer goes up when it is heated and down when it is cooled.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Be sure you and the students wear properly fitting goggles.Students should use care when handling hot tap water.When using isopropyl alcohol, read and follow all warnings on the label. Isopropyl alcohol is flammable. Keep it away from any flames or spark sources.

Materials for Each Group

Student thermometerMagnifierCold waterHot water (about 50 °C)

Notes about the materials

Student thermometers are available from Sargent Welch (WL5679), Flinn Scientific (AP5406) and other suppliers.


Explore

Do an activity to investigate what makes the liquid in a thermometer go up and down.

Question to investigate

What makes the liquid in a thermometer go up and down?

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Materials for each group

Student thermometerMagnifierCold waterHot water (about 50 °C)

Procedure

Look closely at the parts of a thermometer.

Look closely at your thermometer. The liquid inside is probably a type of alcohol that’s been dyed red.

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Practice reading the temperature in °C by having your eye at the same level as the top of the red liquid. What is the temperature?

Use a magnifier to look closely at the thermometer from the front and from the side. Look at the bulb and the thin tube which contain the red liquid.

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Put your thumb or finger on the red bulb and see if the red liquid moves in the thin tube.

Observe the red liquid in the thermometer when it is heated and cooled.

Place the thermometer in hot water and watch the red liquid. Keep it in the hot water until the liquid stops moving. Record the temperature in °C.

Now put the thermometer in cold water. Keep it in the cold water until the liquid stops moving. Record the temperature in °C.

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Expected results

The red liquid goes up in hot water and down in cold water. Students will have an opportunity to relate these observations to an explanation on the molecular level, of why the liquid moves the way it does.

If you have time, you can have students pick a temperature somewhere between the temperature of cold water and hot water and then attempt to combine an amount of hot and cold water to achieve that temperature in one try. They can see how close they can get.

Record and discuss student observations

Give students time after the activity to record their observations by answering the following questions on their activity sheet. Once they have answered the questions, discuss their observations as a whole group.

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Based on what you know about the way molecules move in hot liquids, explain why the liquid in the thermometer goes up when heated. Based on what you know about the way molecules move in cold liquids, explain why the liquid in the thermometer goes down when cooled. Why do you think the tube which contains the red liquid is so thin?What do you think is the purpose of the larger outer tube?

When heated, the molecules of the red liquid inside the thermometer move faster. This movement competes with the attractions the molecules have for each other and causes the molecules to spread a little further apart. They have nowhere to go other than up the tube. When the thermometer is placed in cold water, the molecules slow down and their attractions bring them a little closer together bringing them down the tube. The red liquid is contained in a very thin tube so that a small difference in the volume of the liquid will be noticeable. The large outer tube has two purposes—to protect the fragile inner tube and act as a magnifier to help you better see the red liquid.