Freezing and Melting

Adapted from: A Year of Hands-on Science

S1E2 Students will observe and record changes in water as it relates to weather.

  1. Recognize changes in water when it freezes (ice) and when it melts (water).

c. Determine that the weight of water before freezing, after freezing, and after melting stays the same.

Students investigate how a change in temperature can change the state of water. After freezing cups of water, students may observe that there is more ice than water. Recognizing that the level of ice is above the water line (marked on the cup) or that the water level is lower than where the top of the ice was, can be an interesting discovery for young students. Water is an interesting substance in that it expands as it freezes. Most other substances contract, or shrink, when they freeze.

As they conduct this experiment, children will see evidence that the water expanded by noting that the level of ice is above the black line. The child's stage of development will determine whether he or she perceives this as being more water or recognizes that the amount of water has remained unchanged.

Materials

  • sheet of chart paper and marker (set up chart paper similar to reproducible for this activity)
  • tray
  • Freezing and Melting Science Journal page
  • A thermometer, a clear plastic cup – 1 for each group of two or three students

Note:This is a two-part activity requiring two days to complete. To prepare, use a permanent black marker to make a line a half-inch below the top of each cup (on the outside). Number the cups 1 through how ever many groups of students you have. Fill a pitcher with water about an hour before starting, giving the water a chance to come to room temperature. This will ensure that all groups begin the experiment with water that is the same temperature, thus allowing for fair comparisons of results. Finally, divide the class into small groups.

PART 1 (Day 1)

1 / Begin this activity by asking children: What is ice? Let their descriptions, definitions, and experiences with ice guide other discussions during the course of the experiment.
2 / Next, ask students what they think will happen if you place a cup of water in the freezer. (If you live someplace where the temperature is cold enough, place the cups of water outside.) Many children will respond that it will turn into ice. How long do they think it will take for the water to become ice? This is a good time to let students set up an operational definition for the experiment. How will they define ice? Will it be when there is some ice starting to form in the water or when the entire cup of water has turned to a solid block of ice? The important thing is that everyone uses the same definition.
3 / Hand out reproducibles and have each group predict and record how long it will take a cup of water to turn to ice.
4 / Give each group a cup of water filled to the black line and a thermometer. Have students record the number of the cup on their worksheets. Next, they should record the number of the cup on their worksheets. Next, they should record the staring time and place the thermometer in the cup. While they wait two minutes to record the temperature, they can write their initial observations of the cup of water on their worksheet. Let them know when it is time to read the thermometers and remind them to record the temperature on the worksheet.
5 / Place the cups in the freezer for 30 minutes.
Be sure to weigh the cup of water before placing it in the freezer. Make a chart to show the weight of the water before freezing.
6 / After 30 minutes, let each group make a quick check of its cup, noting and recording time, temperature, and additional observations. Have students check their cups at regular intervals, noting the point at which they can no longer insert the thermometer in the cup due to the thickness of ice.
7 / When students' definition of ice has been reached, let them complete the worksheet.

PART 2 (Day 2)

Materials

  • Chart. Cups of ice, thermometers, and reproducibles from Part 1
  • marker

1 / Discuss students' experiences from Part 1. What did they learn? (The water froze and turned to ice. Ice forms when the water reaches 32° F, etc.) What would happen if the cup of ice were left out on their desks? Would it take as long for the ice to melt as it did for it to freeze? How can they find out?
2 / Discuss ideas for designing an experiment to test how long it takes for ice to melt. Remember the idea of operational definitions. How would students like to define the end result of this part of the investigation? (What will determine when the ice is melted? Is it when they can insert the thermometer in the water or when there is no ice left in the water?)
When you remove ice cups from the freezer be sure to weigh the cups and record the weight on the chart you started yesterday.
3 / After recording predictions for how long it will take the ice to melt, have students mark the top of the ice on the outside of the cup. Then, as in Part 1, students should record starting time and observations on their worksheets, then make observations every 30 minutes.
4 / Bring students together to compare and discuss the results and what they have learned.

Note:What's the science behind ice expanding when it freezes? While most substances contract (shrink) when they cool down, water is unique in that it expands (the water molecules move farther apart). Because of this, students will observe that the ice line on their cups is above the water line.

5 Compare the weight of the water before and after freezing. Discuss why the weight of the water stayed the same before and after freezing.