Science of the Physical Universe 27

PY 107 Physics of Food/Cooking Lab 1

Physics of Food and Cooking:

Lab 1– Phase Transitions: Salt /Ice mixture Melting Point and Ice Cream

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Part 1 Dependence of melting temperature in ice + salt mixtures on salt concentration

NOTE: 1 tsp salt ~ 5 g (check this for accuracy once) 1 g of water = 1 mL 100g of ice ~ 2/3 of the ice cubes a standard ice maker tray in a fridge.

Alternate procedure for mixing salt and ice water steps 1 and 2

Ingredients: Tap Water Ice (~150g or one tray’s worth of ice cubes) 7 tsp salt Procedure:

2 PY 107 Physics of Food/Cooking Lab 1

1. Pour 0.5 cups (100 mL) tap water (room temperature water) into each of the four containers. Dissolve 0, 1 tsp, 2 tsp, 4 tsp of salt in each container. 2. Start with the first container of water. Add one fourth of the ice (~37.5 g) and stir quickly. 3. Do step 3 as above. 4. Repeat step 3 with each of the salt water containers. 5. Calculate the concentration of salt from the worksheet. 6. Plot your data on the chart.

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Part II. Make ice cream using the salt water mixture to lower the freezing point of water

*Note that weight-to-volume calculators for these ingredients can be found online (e.g. http://www.onlineconversion.com/weight_volume_cooking.htm (http://www.onlineconversion.com/weight_volume_cooking.htm)) ¼ tsp vanilla ~ 2 g of vanilla extract **This procedure also works with other milks - soy, almond, rice, etc for a non-- dairy version. Just replace the cream and milk.

Instructions/ Questions for Lab Report

The report should include:

 Objective of the lab.  Brief description of how you made the measurements. If you made any changes to the experimental set up please describe that.  Include the data on the worksheets and note the observations that you made.  Briefly explain your observations in terms of the underlying science.  Conclusions

Specific Questions to address in the report: Salt + ice Freezing lab (1 part A)

Assuming that the specific heat of salt is 0.22 cal/ (g °C) and that of water

1 cal/ (g °C), and ice is 0.5 cal/ (g °C), calculate the specific heat of the water salt mixtures you made (use mass weighted averages). Also calculate the specific heat of the ice salt mixture given for the ice cream recipe in this same lab write up.

Ice Cream Lab (1d)— Give the actual weight to volume conversions you used. Was the salt concentration given in the recipe enough or if you changed it what did it finally become. How long did it take for the ice cream to freeze? Describe the shaking method which worked the best.

At the salt concentration you used what would be the melting point of ice? (Use the graph in the lecture notes).

Did you measure the temperature of the ice + salt and of the ice cream mixture before and after it was done?

Assuming that the milk cream sugar mixture was at 20°C initially how much heat must be removed from it to cool it down to the temperature of the ice -salt mixture. Use the total mass of the mixture (adding all component masses) and take its specific heat as 0.8 cal/(g °C) (The specific heat of cream is about 0.6 cal/(g °C) and that of milk is

6 PY 107 Physics of Food/Cooking Lab 1 close to that of water, 1 cal/(g °C), so the average of approximately equal wt. mixture of the two gives 0.8 cal/(g °C) ).

Assuming that the latent heat of ice in a salt water mixture is the same as that of pure ice (80 cal/g), how much heat does it take to melt 600 g of ice.

Is this greater or less than the heat removed from the milk cream mixture. Depending on the above comparison, how much of the ice in the ice salt mixture should have melted? Is this reasonably consistent with your observations.

If not, what are some of the reasons for the differences between the estimate and your observation.

For more on the science of ice cream see the book (available in Mugar Library) The science of ice cream

Author: Chris Clarke

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