WHICH CAN HOLD MORE HEAT?

PART II

Knight Foundation Summer Institute

Terry Newirth, Haverford College

Objectives:

  1. To have the students measure the heat capacity of water a.~d isopropyl alcohol
  2. To have the students understand heat capacity
  3. To have the students learn to calculate the heat capacity

Vocabulary:

Heat loss

Heat gain

Materials:

each group will need:

Procedure:

Some student teams will do this experiment with water and some will do it with pure isopropyl alcohol. They will share their data.

  1. With the graduated cylinder measure 50mL of water or isopropyl alcohol into your beaker
  2. Fill the graduated cylinder with 25 ml water and record the temperature
  3. Bring the beaker with your liquid to the hot plate and heat it to about 70¡C, then with a pot holder pour the hot liquid into a double Styrofoam cup
  4. Place the thermometer in the Styrofoam cup and monitor the temperature
  5. When it cools to exactly 60¡C, pour the 25mL of water into the cup and stir gently with the thermometer. The moment you pour the water into the cup is time = 0. Monitor and record the temperature every 15 seconds for one minute

Results/Conclusion:

After one minute the temperature of the water/water mixture will be higher than the alcohol/water mixture. In both cases the heat given off as the liquid cools is used to heat up the room temperature water that we have just added. Which ever liquid releases more heat as it cools will be able to heat up the added water more. Clearly water is the winner here. [The difference is actually more than we observe in this experiment as heat is actually generated when water and alcohol are mixed. If you have interested students you can have them measure the temperature of 50ml of alcohol and 25ml of water and then record the temperature when you mix them. I noted an increase from 23o to 25o when I did this experiment.]

In the first experiment we compare the amount of heat it takes to raise the temperature of equal volumes of two low liquids. In the second experiment, we compare the heat released when equal volumes (at room temperature) of two liquids are cooled.

Note we cannot compare the heat released by monitoring the temperature as the pure liquids cool in wither the beaker or the Styrofoam cup, as then we are comparing the rate of cooling, not the heat released in cooling. In fact water cools off much faster than isopropyl alcohol, as it has a much higher thermal conductivity, i.e. it transfers heat five times more effectively than the alcohol, while water's heat capacity is only 1.6 times that of isopropyl alcohol.

Heat capacity is actually be definition related to heat absorbed/g. Its easy to see why mass was chosen as the standard. volumes can change dramatically with temperature, while mass remains constant with temperature. One can do both experiments keeping mass constant and get similar, though not as dramatic results. For pure isopropyl alcohol, the density is .7855 g/ml so for 55ml (55g) water you would need 70ml isopropyl alcohol to have 55g. For qualitative demonstration purposes for middle schoolers this is probably not necessary.