MiStar 7.3 L4 CYP

​MiStar 7.3
Unit 7.3 Lesson 4 Embedded Assessment - Teacher Guide

Camping Rocks

By Amber Furst

​When camping, we want to make soup and hot cocoa, but don’t like to carry all the heavy cooking pots needed to make hot water. Our new Wilderness Survival kit claims we can make hot water without a stove or pot! The kit provides a plastic bag to hold water, and matches to make a fire. The instructions say, “Build a fire. Find rocks. Put rocks in the fire until they get hot. Add hot rocks to the bag of water until the water is hot.”
To prepare for our trip, we created a laboratory model of the hot water system. Our goal was to test it out in the lab to see how it could work best in the wilderness. Figure 1 shows the laboratory model, consisting of a plastic bag, partially filled with water, and the added hot rock.
Initial rock temperature = 200oC
Initial water temperature = 20oC

Multiple Choice

Which model's arrow represents the flow of thermal energy in the system from the story on the previous slide.

Multiple Choice

Describe the reasoning for the placement of the arrow to show energy flow in the system.

• Thermal energy transfers from colder to hotter regions. The cold rock is at a lower temperature than the water, thermal energy will transfer from the water to the rock.

• Thermal energy transfers from hot to hotter regions. The hot rock is at a higher temperature than the water, thermal energy will transfer from the rock to the water.

• Thermal energy transfers from colder to colder regions. The cold rock is at a colder temperature than the water, thermal energy will transfer from the rock to the water.

• Thermal energy transfers from hotter to colder regions. The hot rock is at a higher temperature than the water, thermal energy will transfer from the rock to the water.

​We conducted an experiment to see if changing the amount (mass) of rock in the system would affect the heating of the water (Figure 2). 

We used two rocks of different masses and heated them both in an oven until they reached 200oC. We used equal amounts of 20oC water  in each container. 

Figure 2 shows the change in water temperature Before and After a given amount of time.

Multiple Choice

Describe why the change in water temperature is different for Rock A and Rock B in Figure 2.

• Thermal energy transfers from the water to the rocks, depending on the temperature of the water.

• Thermal energy transfer from the rocks to the water. If a rock is hotter than the water it does not matter how big the rock is, it will transfer more thermal energy.

• Thermal energy transfer from the rocks to the water, depends on the amount of rocks. Rock B has more mass so it has more thermal energy than Rock A for the same temperature.

• Thermal energy transfer from the rocks to the water, depends on the temperature of the water and of the rocks. Both Rock A and B will move the same thermal energy.

When camping, we like to have hot cocoa made with milk. We conducted a second experiment to compare using milk in the plastic bag, instead of water. We used two rocks of the same mass at a temperature of 200oC, and added the hot rocks to two separate plastic bags, each containing equal amounts of milk or water (Figure 3).

We found that the final temperature of the milk was different from the final temperature of the water, even though all the other experiment conditions were the same.

Multiple Choice

Use your knowledge of thermal energy transfer to describe why the final temperature of the milk was different from the final temperature of the water.

• The amount of thermal energy transfer and the relationship of thermal energy to temperature depends on the materials.

• The amount of thermal energy transfer and the relationship of thermal energy to temperature depends on the freshness of the milk.

• The amount of thermal energy transfer and the relationship of thermal energy to temperature depends on the hottest temperature

• The amount of thermal energy transfer and the relationship of thermal energy to temperature depends on the brand of plastic bag.

Multiple Choice

The lab tests are useful to get general information about how to heat the water using rocks, but when camping, there may be factors that can’t be easily tested in the lab.

Identify one way the “Hot Water System Model” would be different in the lab than in the actual camping setting (limitation of the model).

• The type of rock

• Environmental factors, such as air temperature or wind

• The amount of water and the mass of the rock

• All of these

Open Ended

Describe how one of the differences identified in the previous question could affect thermal energy transfer in the actual setting as compared to the lab experiments.