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Lab Experiment: Phase Changes and Energy Transfer of Water

Objective

Water is one of the most common substances found on Earth. It exists as a solid, a liquid, and as a gas. Transforming water from one of these phases to another involves the transfer of energy. In this lab you will record the temperature of water as it changes from ice to water and then into steam. You will use your data to compare the relative amounts of heat energy transferred in each phase change.

1. Hypothesize: What will require the most energy, changing 1 gram of ice at 0° C into to liquid water at 1° C, or changing 1 gram of liquid water at 100° C to steam at 100° C? What is your reasoning?

Mass the empty beaker and record the mass on the data table. Tare the scale and then add 200 g of ice. Add 200 g of cold water to make the total mass in the beaker 400 g. Put a thermometer into the beaker and use it as a stirring rod. Stir the water and ice mixture until the temperature stops going down. Be sure to hold the thermometer so that it does not touch the sides or the bottom of the beaker. When the thermometer reaches its lowest reading, record this temperature at time 0 on the data table below.

Turn on the hot plate to its highest setting Wipe any moisture from the bottom and sides of the beaker. Place the beaker on the hot pad. Read and record the temperature every 30 seconds, continuing for at least 5 minutes after the water reaches a full boil. Take turns continuously, gently, stirring throughout the experiment.

A) When the ice begins to melt
B.) When the ice is entirely melted
C.) When the water begins to boil.
Graph your data on the graph paper below. Be sure to add to the graph: a title, axes labels, symbols representing the special times you noted and a legend for these symbols.

1. During the entire experiment the heater was kept at the same setting. That means that the heater was adding energy to the system at a constant rate. In other words, energy was always going into the water from the heater.

2. Figure 1 shows an ideal graph for this experiment. Is your graph the same as the ideal graph? How is it different?

3. Even though energy was constantly being added at the same rate by the heater what happened to the temperature as the ice melted and as the water boiled?

## Procedure

4. How many minutes did it take to melt the 200 g of ice? This is the difference in time between the point it started to melt, and the point the ice was all melted.

5. Divide the number of minutes it took to melt the ice by 200 g. This tells us the number of minutes to melt each gram of ice. Show your work.

6. After the ice melted you had 400 g of water to heat from 0 °C to 100 °C. How many minutes did it take to do this?

7. Divide this number of minutes by 400 g. This tells you the minutes to heat 400 g from water at 0 °C to water at 100 °C. Show your work.

8. Divide your answer to #7 by 100 °C to find the minutes to heat 1 g of water 1°C. Show your work.

9. After the ice melted you had 400 g of water that started to boil. Find the mass of the water that boiled away during the experiment.

10. How many minutes passed between the time the water started to boil until the end of the experiment?

11. Divide this number of minutes the water boiled by the mass of water that boiled away to give you the minutes to boil away each gram of water. Show your work.

12. Define:
a. Phase change –
b. Latent heat -

13. Even though energy was constantly added to the beaker, what happens the temperature of water during a phase change? What should have happened? Look at both your data and the ideal graph in figure 1.

14. What did you calculate for the following?

Minutes to melt 1g of ice = min./g (your answer to #5)
Minutes to heat 1 g of water 1°C = min./g °C. (your answer to #8)
Minute to boil away 1 g of water = min./g (your answer to #11)

15. If you assume that equal amounts of energy were added to the beaker each minute, which phase change requires more energy, melting or boiling. How does this compare with your hypothesis?

Textbooks tell us the amount of energy measured in Joules (J) required to do each of the following:
Energy to melt 1g of ice = 330 J/g
Energy to heat 1 g of water 1°C = 4.18 J/g °C
Energy to boil away 1 g of water = 2260 J/g

16. If you assume that equal amounts of energy were added to the beaker each minute, how do your measurements in #14 compare? They aren’t the same units but are they proportionally about the same?

17. You must add energy to liquid water to change it to water vapor. That energy can come from the surroundings. If you take energy out of the surroundings what does that do to the temperature of the surroundings?

18. When water vapor turns back into a liquid to become rain or freezes to become a solid to make snow, what must happen to the energy or latent heat stored in the water vapor? What would that do to the temperature of the surroundings?