Preparing cold Horlicks drinks [back]

Key Learning Points

1. Specific latent heat
2. Specific heat capacity
3. Mixing

Teachers' notes: (zip 61k)

Typical results, suggested answers, students' worksheet.

Introduction

This is how people in restaurants prepare cold Horlicks drink: First dissolve some Horlicks powder with hot water, and then add a lot of ice to cool down the drink. People use hot water to dissolve the Horlicks powder first because the powder is more soluble at a higher temperature. They usually put in an excessive amount of ice to make the drink as cold as possible. As physics students, however, we may want to ask an interesting question, namely, how much ice should be added to the drink to cool it down to a given temperature?

 Dissolve the Horlicks powder with hot water. Put in an excessive amount of ice to make the drink as cold as possible.

The problem is not as straightforward as it seems. The specific heat capacity of Horlicks drink is not known and so we cannot find the answer by a direct calculation. We have to do an experiment to find out the specific heat capacity of Horlicks solution first. This activity consists of a series of experiments that will guide you step by step to solve these problems.

Apparatus

Horlicks powder, melting ice, polystyrene cups, towel, temperature sensor, datalogger, electronic balance, adhesive tape, heater / Bunsen burner.

Experiment 1: Finding the specific heat capacity of a Horlicks solution

We will mix the Horlicks solution with some ice to find out its specific heat capacity.

Procedure

1. Heat up some water. Pour the hot water into a beaker and measure the mass of the water using an electronic balance.
2. Weight some Horlicks powder with the balance and dissolve it in the hot water. The mass ratio of Horlicks powder to water can be 1:10 or 2:10.
3. Use the adhesive tape to fix the temperature sensor to the polystyrene cup and make sure that the sensor part (bottom part) is not in contact with the cup.
4. Re-heat the Horlicks solution to above 50 . Pour 100 ml of the solution into the polystyrene cup. Measure the initial mass of the solution by the electronic balance.
5. Record the initial temperature of the solution by the datalogger.
6. Use a towel to dry some melting crushed ice. Add the ice gradually to the solution. Keep stirring until the ice is completely dissolved.
7. Stop adding ice when the temperature of the solution becomes, say, 15 .
8. Measure the final mass of the solution by the balance.
9. Repeat the step 1-8 with Horlicks solution of same concentration. At least three trials for each concentration should be done.

Discussion

1. Suppose the initial and final masses of the solution are 100 g and 140 g respectively, and the initial and final temperatures are 60 and 15 respectively. Find the specific heat capacity of the Horlicks solution. Show your steps clearly.
2. Will the heat exchange with to the surroundings cause errors in this experiment? How can we make correction to this error?
3. The initial temperature of the trials should be kept more or less the same. Why?
4. Why do we have to use crushed ice in the experiment?
5. Why do we use melting ice? Why should the ice be dried by the towel before adding to the solution.
6. Do you expect the specific heat capacity of the Horlicks solution be larger or smaller than that of water? Why?
7. How does the specific heat capacity of the Horlicks solution depends on the mass ratio of Horlicks powder to water?

Experiment 2: Predict the mass of ice needed to cool down a Horlicks solution

Now we will use the value of the specific heat capacity of Horlicks solution found to estimate the mass of ice required to cool down a Horlicks solution to a given temperature.

Procedure

1. Choose a target temperature to which the Horlicks solution will be cooled.
2. Follow steps 1-6 in Experiment 1.
3. Calculate the amount of ice needed to cool down the solution.
4. Add dry crushed ice to the solution over the electronic balance. Stop adding ice when the calculated mass is reached.
5. Record the final temperature of the mixture as soon as all the ice is melted. See if the final temperature matches with your target. Find the percentage difference between the calculated and the actual masses of ice used.

Discussion

1. Suppose the initial mass of the Horlicks solution is 100 g, and the initial and final temperatures are 60 and 10 respectively. Find the mass of ice required to cool down the solution. Take the specific heat capacity of the Horlicks solution to be 3700 . Show your steps clearly.