Put Some Energy Into It! Use a Calorimeter to Measure the Heat Capacity of Water
AbstractAlong with its many other interesting properties, water has the ability to absorb a lot of heat energy, while only experiencing a relatively small change in temperature. One way this property affects us directly is that our bodies don't change temperature rapidly on hot or cold days, since we are made up of mostly water. In this chemistry-with-an-electronics-flair science fair project, you will determine how the temperature of a small volume of water changes as you add precise amounts of heat energy to it from a heating element.
David B. Whyte, PhD, Science Buddies
Use an electric calorimeter to measure the heat capacity of water. This science fair project allows you to demonstrate and measure the conversion of electrical energy into thermal energy.
Water is a remarkable substance. It is a universal solvent, meaning that it can dissolve many kinds of solid material. It expands when frozen, unlike most other liquids, which is why ice floats. It is the medium in which all of the chemical reactions that are required for life occur. And water is also unusual in its ability to absorb heat. If you add a given amount of heat energy (measured in joules, abbreviated as J) to equal masses of water and lead, the water's temperature will change much less than the lead's. In other words, the water has a higher specific heat capacity than lead does. Specific heat capacity is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by unit degree. For example, in units of grams, degrees Celsius, and joules, the specific heat capacity of water is 4.19 J/°C g. This means that it takes 4.19 joules (again, a joule is a measure of energy) to cause a temperature rise of 1.0 degrees Celsius in 1.0 grams (g) of water.
How much higher is the heat capacity of water than lead? About 33 times as much! So if you add enough energy to raise a given mass of water by 1°C, you will find that the temperature of an equal mass of lead will have increased by 33°C when the same amount of energy is added.
One way to measure the heat capacity of a liquid, such as water, is to add a known amount of heat energy to a known mass of the liquid and measure the change in temperature. The equation relating heat energy to specific heat capacity, where the unit quantity is in terms of mass, is shown in Equation 1.
Q = mc (T2 - T1)
- Q is the heat energy put into the substance, in joules (J).
- m is the mass of the substance, in grams (g).
- c is the specific heat capacity (in J/°C g).
- T2 is the final temperature, in degrees Celsius (°C).
- T1 is the starting temperature, in degrees Celsius (°C).
How can you add a known quantity of energy to the liquid? In this science fair project, you will use a wire that heats up when current is passed through it to add heat energy to a known volume of water. You can calculate the precise amount of energy that is produced by the hot wire if you know the current that is going through the wire, the voltage across the wire, and the amount of time the wire heats the water. The equation relating the energy produced in the hot wire to voltage, current, and time is given in Equation 2, which is a form of Joule's law (see the Bibliography for more information about Joule's law).
E = IVt
- E is the energy generated by the current passing through the wire, in joules (J).
- I is the current, in amps.
- V is the voltage, in volts).
- t is the amount of time the current was active, in seconds (sec).
If you assume that all of the energy produced by the wire is absorbed as heat by the water, you can combine these two equations, as follows:
You can rearrange this equation to find c, the specific heat capacity. What about the other terms? The mass is known because you will use a known mass of water. The temperature change is measured using a thermometer. The current and voltage are measured using a multimeter. And the time is measured using a stopwatch or other timer. The apparatus that you will use, a calorimeter, is shown below. It has two terminals for attaching a 6-V battery, a hole for a thermometer, and a stirrer to mix the water. The two battery terminals are attached to a wire inside the calorimeter that heats up when a current is run through it. The interior of the calorimeter is made of StyrofoamTM to minimize heat loss. To use it, you will fill the calorimeter with water, run a current through the wire to heat it up (a 6-V battery is used to produce the current), and measure the change in water temperature (T2 - T1). As mentioned above, the current and voltage can be measured using a multimeter. Multimeters are easy to use and allow you to obtain numerical data that you can plug into the equations above.
As you work through the procedure, consider the flow of energy: it starts as chemical energy stored in the battery, becomes electrical energy when it a current is created, and then is converted into heat energy by the wire in the calorimeter. Finally, the heat energy is absorbed by the water molecules, causing the temperature of the water to rise. The calorimeter is fairly simple to use, but it allows you to perform some precise and elegant explorations into the nature of energy.
Terms and Concepts
- Joule (J)
- Specific heat capacity
- Joule's law
- Volt (V)
- Ohm's law
- What is it about the water molecule that explains why water has a relatively high specific heat capacity?
- What is the difference between specific heat capacity and heat capacity?
- What is the equation for c, the specific heat capacity, derived by rearranging Equation 2?
- If the heating wire has a resistance of 3 ohms, and it is connected to a 6-V battery, what is the value for the current (in amps) that will pass through the wire? Hint: Use Ohm's law.
- In the example in the previous bullet, how many joules are transferred to the water per second? Hint: Use Equation 2, with I = 2 amps, V = 6 V, and t = 1 sec.
- What is heat?
Materials and Equipment
- Electric calorimeter, 6-V; available online from suppliers such as Amazon.com
- Thermometer; available online from suppliers such as Amazon.com
- Distilled water (1 gallon); available at grocery stores
- Graduated cylinder, 250-mL; available from online suppliers such as Amazon.com
- Multimeter, such as the Equus 3320 Auto-Ranging Digital Multimeter; available online from Amazon.com
- Battery, 6-V
- Test wires with alligator clips, 14-inch; available from online suppliers such as Amazon.com
- Lab notebook
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Setting Up the Calorimeter
Remove the aluminum cup from inside the calorimeter.
- Some of the heat energy will be used to heat the aluminum cup, rather than the water, if it is left in the calorimeter.
Place the thermometer in the rubber stopper on top of the calorimeter.
- Use a drop of liquid soap to lubricate the thermometer, if needed.
- Add 175 mL of distilled water to the calorimeter.
- Place the top on the calorimeter so that the thermometer and the heating wire are immersed in the water.
Attaching the Multimeter to the Battery
Set the multimeter to read DC current.
- The current will be in the range of 1 to 3 amps.
- Make sure the probes are inserted into the correct sockets (holes) on the multimeter to read DC current.
- For more information about how to use and read a multimeter, visit the Science Buddies resource How to Use a Multimeter.
- Attach the red lead (wire) from the multimeter to the positive terminal on the 6-V battery (see Figure 1, wire A). Use a test wire with alligator clips, if needed.
A closed circuit is formed between a calorimeter, a multimeter and a battery. The red colored lead of a multimeter is connected to the positive terminal of a 6 volt battery. The black colored lead of a multimeter is connected to the lead of the heating wire in the calorimeter. The wire between the negative terminal of the battery and the remaining lead of the calorimeter should remain disconnected until the water in the calorimeter is ready to be heated.
Figure 1. Wiring diagram for calorimeter. Note that the multimeter is part of the circuit. The wire will start to heat up when wire C is attached to the calorimeter.
- Attach the black lead from the multimeter to one of the metal stems attached to the heating wire of the calorimeter (See Figure 1, wire B). Don't worry about the colors of the wires. The multimeter will be able to read the current either way.
- Attach a 14-inch test wire with alligator clips to the black (negative) terminal on the battery (See Figure 1, wire C). Don't attach the other end to the calorimeter yet.
A closed circuit is formed between a calorimeter, a multimeter and a battery. The red colored lead of a multimeter is connected to the positive terminal of a 6 volt battery. The black colored lead of a multimeter is connected to the lead of the heating wire in the calorimeter. The wire between the negative terminal of the battery and the remaining lead of the calorimeter are connected and the current (DC) across the multimeter is displayed as 1.80 amps.
Figure 2. Calorimeter setup. The calorimeter, 6-V battery, and multimeter are shown. The multimeter is set to read DC current in the range of 1 to 10 amps.
Warming the Water
- Read the temperature of the water. It should be near room temperature. Note the temperature and time in your lab notebook.
- Turn on the multimeter.
- Start the timer.
- Attach the other end of the wire from the black terminal of the 6-V battery to the unused stem on the calorimeter. Current should start to flow.
- Record the current and time in your lab notebook. The resistance of the wire is approximately 2–3 ohms, so according to Ohm's law, the current will be in the range of 2–3 amps.
Recording the Temperature Change Over Time
- Watch the thermometer to note change in temperature.
- Mix the water with the stirrer every minute or so. If the stirrer hits the metal wire, adjust its position.
Record the time, the current, and the temperature in your lab notebook every 2 minutes.
- Feel free to take more readings, such as every minute.
- When the temperature has increased by 10 degrees Celsius, disconnect the battery from the calorimeter.
Repeating the Procedure
- Put fresh room-temperature water in the calorimeter.
- Perform the entire procedure above two more times to demonstrate that your results are repeatable.
Analyzing Your Results
Calculate the specific heat capacity of water, using Equation 2 from the Introduction.
- T2 is the final temperature, and T1 is the starting temperature.
- If the current varied between readings, use the average value.
- m is the mass of water in the calorimeter, which will be 175 grams (g) if you used 175 mL.
- Average the results of your three trials to get your final value.
- Look up the specific heat capacity of water. How does your value compare to the published value?
Ask an Expert
- Devise ways to make your results more accurate. For example, use a high-quality thermometer, measure the exact voltage of the battery with a multimeter, weigh the amount of water with a scale that is accurate to 0.1 g (such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com), correct for the heat that is used to warm the stirrer, etc.
- Graph the temperature on the y-axis and the time on the x-axis for each trial. Do you expect a straight line?
- Use the calorimeter to determine the specific heat capacity of aluminum. Place the aluminum cup (30 g) inside the calorimeter. Repeat the procedure above with a known mass of water, but add a term for the mass of the cup and the specific heat capacity of aluminum to the left side of Equation 3. Try other metals. Compare your results to published values.
- Use the calorimeter to determine the specific heat capacity of other liquids, such as salt water, vegetable oil, etc.
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