CBL
Lab 6

Heating Curve

A heating curve is helpful in illustrating what occurs as a substance changes from a solid to a liquid, then a gas. When a solid sample is heated, its temperature increases until its melting point is reached. At this point, the solid begins to melt. The heat being absorbed goes into breaking the cohesive forces holding the substance as a solid, not into increasing the temperature. Once all the solid has melted into liquid, if you continue to heat, this additional heat will be absorbed to increase the temperature. The temperature will continue to increase until the liquid reaches its boiling point. When the liquid begins to boil the temperature will remain constant as the energy goes into moving the molecules further apart and the liquid changes to a gas.

Based on the paragraph above, draw a graph of Temperature Vs. Time that you expect to produce when you heat a beaker of ice water to boiling.

Objectives: Students should

• predict the results of the experiment (graphically and in words)
• use the CBL and temperature probe to create a heating curve
• explain the process of phase change

CBL TI-82 or TI-83 calculator 250 mL beaker

hotplate/stirrer ice water

stir bar CBL temperature probe

1. Pour 30 mL of water in the beaker and add an equal amount of ice.
2. Put a magnetic stir bar in the beaker and set the beaker on a stirrer/hotplate.
3. Position the temperature probe into the water-ice mixture so that the probe is not touching the side or bottom of the beaker.
4. Set up the CBL by placing the temperature probe in channel 1 and linking the CBL to the calculator.

a) Turn on the CBL and the calculator.

b) Press PRGM on the calculator.

c) Using the arrow keys, highlight the program called "HEAT". Press ENTER.

(Display should read "PRGM HEAT")

d) Press ENTER.

(Display should read "How much time between points in seconds?")

Since this program will take 99 temperature samples, you should enter the sampling rate (in seconds) accordingly. Press ENTER. (The calculator display should read "Press Enter to start" and the CBL should have three dashes across the display.)

5. Turn the the hotplate on highest temperature setting and the stirrer on a medium speed. Press ENTER.
6. Observe the melting of the ice and note the temperature when no more ice is present in the beaker. Note also the temperature when the water begins to boil.
7. When the CBL says DONE, sketch the graph in the chart below. You can find the data in L₃ and L₄ by pressing STAT then ENTER. You can also look at the plotted points on the graph by using the [TRACE] and arrow keys.

1. How well did your predicted graph match your experimental graph? _____________________________________________________________________
2. One the graph above, put an "X" where you last saw ice.
3. On the graph above, put an "Y" where you first observed boiling.
4. When water boils, gas bubbles form at the bottom of the beaker. What is the gas inside the bubbles.______________
5. Predict the temperature of water if your continued to heat for 10 more minutes._____
6. Describe what you observed just above the surface of the boiling liquid. _____________________________________________________________________

***optional ---determine how to monitor the temperature again

7. What happens to the boiling temperature of the water after you dissolve some table salt in the water?

8. What happens to the boiling temperature of the water after you dissolve some table sugar in the water? (careful! don’t add salt and sugar to the same water unless you want to complicate your results.)

9. What conclusion can you draw from your observations in numbers 7 and 8?

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