The Item:
Question #1:  Why are controls used at each temperature?
Question #2:  Compare the times it took the milk in each of the cups to curdle? What relationship, if any, exists between the temperature of the milk and the time it takes to curdle? Compare this result with your prediction. Explain any agreement/differences.

Item Description:

This exercise presents a scientific test of a simple multivariate problem examining the effects of the enzyme rennin and temperature on curdling in milk.

Students are asked to explain the use of multiple controls in the experiment. To answer this question correctly, students should recognize the fundamental comparative use of a control. The students must also note that multiple controls are used in this experiment to test for the effect of rennin on milk curdling rate as well as for the effect of temperature on the rennin/curdling relationship.

Students are also asked to explain the results of the experiment and to discuss the observed relationship between temperature and the rate of the rennin induced curdling reaction. Here students must recognize the different variables involved in the experiment (i.e. rennin and temperature). Only the best students will recognize that the experiment deals not just with the effect of temperature on milk but also with the effect of temperature on milk containing rennin. The average student will simply recognize that warmer milk curdles faster.

Rubric

Criterion 1:		Some fundamental understanding of the use of a control in an experiment should be presented. This should include a statement that the controls are used to test the effects of rennin on milk curdling. NOTE: Simply stating, "To see how long it would take to curdle", is not sufficient. Examples of acceptable statements: To see whether milk without rennin would curdle. Because it shows the difference between what the rennin does to the milk. They are used to see what happens between the two. Like one normal and one not.
Criterion 2:		Students should explain the use of controls at each temperature to test for the effects of temperature on milk curdling rate. This may or may not be included with a statement satisfying Criterion 1. Examples: Controls are used at each temperature so you can see which temperature will curdle faster. You need to use the same temperature in both cups of milk to see the reactions of the rennin. The controls are used to see if the controls or the rennin curdle faster at each temperature. (This satisfies Criteria 1 and 2.)
Criterion 3:		Explain results of experiment in terms of the rate of curdling at different temperatures. If the experiment was successful, it should be noted that curdling rate increased with temperature. If the experimental results were inconclusive, credit should be given if mention is made of this. Examples: The hot milk curdles quicker than any other. It was just like I predicted. Warmer milk curdles faster. It was like I predicted, only the cold milk curdled faster than room temperature. I was wrong. I thought hot milk would curdle, but it didn't.
Criterion 4:		Students should understand the multivariate nature of the experiment and should briefly described the effects of both rennin and temperature on curdling rate. This can be stated either in the prediction or in questions 1 or 2 of the event. Correct answers will state that milk with rennin curdles more rapidly at higher temperatures or that higher temperatures make the enzyme "work" faster. Examples: My prediction was right. I chose hot milk, because the hot water...made the enzyme work faster. (This statement satisfies Criteria 3 and 4.) Warmer milk with rennin or not can curdle a lot faster, but controlled milk at 35 degrees did not curdle. There's a difference there.