To explore what happens to the amount of different substances as they change from a solid to a liquid or a liquid to solid.
This lesson is the third in a three-part series that addresses a concept that is central to the understanding of the water cycle—that water is able to take many forms but is still water. This series of lessons is designed to prepare students to understand that most substances may exist as solids, liquids, or gases depending on the temperature, pressure, and nature of that substance. This knowledge is critical to understanding that water in our world is constantly cycling as a solid, liquid, or gas.
In these lessons, students will observe, measure, and describe water as it changes state. It is important to note that students at this level "...should become familiar with the freezing of water and melting of ice (with no change in weight), the disappearance of wetness into the air, and the appearance of water on cold surfaces. Evaporation and condensation will mean nothing different from disappearance and appearance, perhaps for several years, until students begin to understand that the evaporated water is still present in the form of invisibly small molecules." (Benchmarks for Science Literacy, pp. 66-67.)
In Water 1: Water and Ice, students explored how water can change from solid to liquid and back again.
In Water 2: Disappearing Water, students focused on the concept that water can go back and forth from one form to another and the amount of water will remain the same.
In this lesson, students will investigate how melting and freezing impact three everyday items: water, chocolate, and margarine. They will be introduced to the concept of conservation of matter. They will experience that when a substance changes from a solid to a liquid, the amount (weight) does not change.
Please bear in mind that while water is a single substance, margarine and chocolate are not. Therefore, it is important that discussion be focused on the amount of the substances, not on their characteristics. For example, water has a specific freezing and melting point, whereas margarine and chocolate do not. And while you can melt and freeze water indefinitely without any noticeable changes, the same is not true of margarine and chocolate.
You could search the following sites for useful background information as you prepare to do this lesson:
Ask students the following questions:
- Has anyone ever eaten ice cream on a hot summer day? What happens to it if it is not eaten?
- Have you ever left a glass of ice water out on the table? What happened to the ice?
- Have you ever noticed what happens to frozen objects as they heat up?
- What happens to the amount of a substance if it is changed from a solid to a liquid?
Next, show students a baggie filled with ice. Ask students to predict what will happen to the substance (including to the amount of it) over time. This should be done in an open, large group format, and you should record a list of the students' predictions.
Break students into small groups and give them a Ziploc baggie, ice, paper towels, and a scale or balance.
Have students put the ice into the baggie and seal it. Have them record their observations of the ice at this time, and then record the weight of the bag of ice. Have students take turns holding the bag in their hands, wiping the outside of the bag as necessary to get rid of any moisture.
- What do you see?
- What is happening to the ice?
- Why is this happening to the ice?
- What do you think is happening to the amount of ice?
Have students weigh the bags of ice again.
- What happened to the amount?
- Does the ice look the same as it did in the beginning? How is it different?
Allow the solid to completely change to a liquid, and have students wipe the bag and weigh it again. If time allows (and there is a freezer available), put the baggies into the freezer to solidify the liquid. Then wipe the bags and weigh them a final time.
- Does the amount of water change when it changes from a solid (ice) to a liquid?
- Does the amount of water change when it changes from a liquid to a solid?
- Imagine that a younger friend has just told you that the amount of ice is gone because she saw it melt. How would you explain her mistake to her?
- How would you explain what you observed to someone who did not perform this experiment?
At a later time perform this same experiment with chocolate chips. Have students hold the baggie in their hands to melt the chocolate chips.
- What happened to the amount of chocolate chips as they melted?
- What happened to the amount of chocolate chips as they changed to a solid?
- Did you expect this? Why or why not?
- If you took a cake and cut it into pieces, would the amount of the cut cake be more, less, or the same as the uncut cake? Draw a picture of both cakes that explains your answer. Be prepared to explain your drawing.
At a later time perform this experiment with margarine. Have students hold the baggie in their hands to melt the margarine.
- What happened to the amount of margarine as it changed from a solid to a liquid?
- What happened to the amount of margarine as it changed from a liquid to a solid?
- At dinner you see a bowl of vegetables on the table with a pat of margarine on the top. As the margarine melts into the vegetables, what happens to the amount?
Revisit the predictions that students made in the Motivation of this lesson. Discuss how their predictions related to actual experiences with the substances.
Next, have each of the students respond to the following prompt:
You leave a bowl outside overnight. It snows and fills the bowl. You notice the bowl in the morning and leave it. The snow melts as the temperature increases. Diagram what happens to the snow as it melts. Be prepared to explain if you were to put the bowl on a scale what would happen to the amount of water.
Read the story, White Snow Bright Snow by Alvin Tresselt to the class.
- What is snow?
- What happened to the snow as the temperature increased?
- Did the amount change as the snow melted and turned to a liquid?
- Why did the snowman appear to get smaller? What was really happening to him? Did the amount of the snowman really change? Why or why not?