# Liquid Cooling

## Objective

To determine whether some liquids cool faster than others and why.

## Base Difficulty

Medium, because it requires precise timing and concentration on the task at hand.

## Concept

Suppose you had a bunch of different liquids - sodas, juices, syrup, etc. - and they all were heated up to a hot temperature. Which one would cool down to room temperature first? Is there some explanation for why some liquids would cool faster than others? What do you think will happen?

## Hypothesis

Which liquids do you think will cool the fastest? Why do you think that is? Can you successfully test your guess?

## Materials

You will need a large tub, either a bath tub, a cooler, or just a large pot, and you will need to fill it with hot water. Be careful not to boil the water, as it will evaporate too fast to do the experiment with. Also, be sure to keep the water temperature as constant as possible in order to keep the experiment as consistent as possible. One way to accomplish this is to have a large pot of water on the stove heated to a constant temperature.

You will then need to select a diverse group of liquids to test. Good suggestions include water as a control liquid, sodas, juices, syrup, cooking oil, and molasses. As long as you choose several different liquids with differing properties (thickness, color, etc.) your experiment will be interesting, informative, and effective. You will need several of the same type of containers that will not melt to keep the liquids in. We recommend coffee mugs, especially if they have lids that can completely close them off.

You will also need cooking thermometers, since they are built to measure the hot temperatures you will be observing in the heated liquids. Measuring cups will be necessary to ensure that the same amount of each liquid is added to the containers.

## Procedure

First, note the temperature of the room that you are in. This will be the temperature your liquids will cool to, and thus is a very important number to have on hand.

Using a stove - remember, BE SAFE! - prepare a hot water bath that will remain at a fairly constant temperature. Remember, don't boil it, or you will lose water too quickly, and boiling is dangerous, too. Measure an equal amount of your test liquids into the containers and place them in the bath. After a few minutes, measure the temperature of the bath and the temperature of the liquids. If all are the same temperature, you are ready to begin the experiment.

Record the temperature of the bath and the liquids. Remove each of the containers from the bath (use an oven mitt or a strong set of tongs) and set them on potholders or a ceramic/tile countertop. If you have a limited number of thermometers, you may want to stagger your removal of the containers to fit with your measurement interval. For example, if you only have two thermometers, you should remove two containers, then wait about 1 minute before removing the next two. This way you can measure each mug at equal times after being removed from the bath. Clean the thermometer between containers!

Measure the temperatures of each mug at fixed intervals until you reach a fairly room temperature. We recommend checking each container at two minute intervals at the most. Your results will be even better if you check each container at one minute intervals. Alternatively, set a time limit and see which liquid is the hottest at the end and which is coolest. If your liquids cool very quickly (less than 15 minutes) you may need to shorten your intervals and measure fewer liquids at once.

## Research

You will need to understand how convection works, and how different liquids have different heat constants, so check out this website.

## Analysis

Since all liquids should start at a uniform temperature, a line graph is a good way to display your data points. Be sure to check our Guides section for a detailed explanation on making line graphs in Microsoft Excel. To estimate the rate of cooling, you can assume (incorrectly) that the liquids will lose heat linearly and divide the temperature difference by the duration. For an added challenge and a better fit with your data, learn about Newton's Law of Cooling here and try to find the heat constant for each of your liquids. You can then use these values to show why your liquids cooled at their respective cooling rates.

## Conclusion

This is the section where you discuss why your hypothesis was correct or incorrect, using the data you gathered to support your arguments.

## Extensions

Instead of testing different liquids, you could test the same liquid in different containers, like plastic containers, metal containers, Styrofoam containers, etc., and see which containers are better for cooling and which are better for keeping things hot.