Investigate the 'Death' of an Orange: How is Rate of Heat Loss Based on the Surrounding Temperature?
Abstract
Imagine that you are a detective investigating a murder. You have a body that was found in a swimming pool, and someone reported hearing an argument near the pool at 10:00 PM the night before. Your first question might be "Was this person killed around 10:00 PM last night, or at some other time?" One way to answer this question is to measure the internal temperature of the body. The longer ago the killing occurred, the colder the body. But can you assign a specific time of death to a specific temperature? In this science fair project, you will measure the rate of heat loss from an orange and see how it depends on the temperature of the surroundings.Summary
David B. Whyte, PhD, Science Buddies
- StyrofoamTM is a registered trademark of The Dow Chemical Company.
Objective
Measure the rate of heat loss from an orange (representing a body) and see how it depends on the temperature of the surroundings.
Introduction
Say you are investigating a murder and want to know when the victim was killed. If the body is cold, you can guess that the murder did not happen very recently. But can you be more precise in order to catch the killer? First, you will want to measure the internal temperature of the body, its core temperature. Once you have the core temperature, what else do you need to know to determine when the murder took place? Well, you will want to know what the temperature of the surroundings was in the recent past. If the body was found in the morning, you will want to know what the temperature in the area was overnight, for example. You will also want to determine if the body was insulated, say, by heavy clothing. More clothing will obviously slow the rate of heat loss. Suppose the body was found in a swimming pool or in a lake. How would this affect the rate of heat loss? These are all questions that you can model, without having to work with actual victims. In this case, you will use an orange as a model for a human body. One concept that is particularly important for this science fair project is rate of change. See Equation 1.
Equation 1:
Rate of change = Change in temperature (°C)/time (hours)
If the temperature of an object decreases by 10°Celsius (C) in 1 hour (h), the rate of change of the temperature is -10°C/h—the negative sign indicates that the temperature is going down. If the temperature of an object decreases by 10°C in 10 h, the rate of change is -1°C/h (10 divided by 10 is 1, and the answer is -1 because again, the temperature is going down). The change in temperature is the same (10°C) in both cases, but one rate is 10 times greater than the other. The time can be calculated in seconds, minutes, days, or in any other time unit.
In this forensics science fair project, you will investigate the "time of death" of an orange. The orange will be warmed to near human core body temperature (100°F) and allowed to cool for various amounts of time in surroundings of various temperatures. You will make a graph that shows how the temperature changes over time. You can also investigate how factors such as clothing and size affect heat loss.
Terms and Concepts
- Core temperature
- Heat loss
- Model
- Rate of change
- Forensics
Questions
- What is the "normal" core temperature of the human body?
- How would the size of an object affect the rate of change in temperature?
- How would the shape of an object affect the rate of change in temperature? (Interesting fact: A sphere has the smallest surface area possible for a given volume, so it will theoretically have the slowest rate of change.)
- How would other factors affect rate of heat loss? For example, consider humidity levels, air movement, and fat levels in the body.
Bibliography
- Simonson, L.J.D. (2010). Heat Transfer. Retrieved January 26, 2010.
- Wikipedia Contributors. (2010, January 27). Heat transfer. Wikipedia: The Free Encyclopedia. Retrieved January 26, 2010.
Materials and Equipment
- Cooler, StyrofoamTM
- Thermometer, with stainless steel stem to insert into the orange. Obtain 2 thermometers if you want to measure temperature in two oranges at the same time. Use a thermometer that is meant for measuring ambient temperatures (meaning, temperatures around the house or outside). Thermometers for body temperature or for oven use may be suitable. Should have clear readings between 50 and 100°F (10–38°C ). For example, this pocket thermometer from www.amazon.com
- Tap water
- Lab notebook
- Pot, 1-gallon
- Oranges (6), should be the same size
- Stove
- Tongs
- Timer
- Ice (1 bag)
- Graph paper
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Experimental Procedure
Cooling the Orange in Room-Temperature Water
- Add room-temperature water to the cooler, so that there are about 3 inches of water on the bottom.
- Record the water temperature in your lab notebook.
- Fill the pot three-fourths full with water.
- Add one orange to the water in the pot.
- Heat the water slowly until it is 100°F (38°C).
- Keep the temperature at 100°F for 30 minutes.
- Remove the orange from the water with a pair of tongs. Be careful, but quick, so as to avoid dropping the orange and splashing hot water onto yourself.
- Immediately insert a thermometer into the orange. Be sure to place the tip of the thermometer into the center of the orange.
- Record the temperature of the orange and the time. Call this Time zero in your lab notebook.
- Place the orange in the room-temperature water in the Styrofoam cooler and start the timer.
- Record the temperature every 2 minutes, until the temperature is 80°F (~27°C).
- Repeat steps 1–11 two times, with fresh oranges, water, and make sure your pot has cooled to room temperature.
Cooling the Orange in Ice-cold Water
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Make an ice bath, as follows:
- Empty the room-temperature water from the previous section out of the cooler.
- Add ice to the cooler. Make a layer of ice about 1 inch thick.
- Add 1 quart of water to the ice.
- Add ice, as needed, to the ice water to keep it "icy."
- Fill the pot three-fourths full with water.
- Add one orange to the water in the pot.
- Heat the water slowly until it is 100°F (38°C).
- Keep the temperature at 100°F for 30 minutes.
- Remove the orange from the water with a pair of tongs. Be careful, but quick, so as to avoid dropping the orange and splashing hot water onto yourself.
- Immediately insert a thermometer into the orange. Be sure to place the tip of the thermometer into the center of the orange.
- Record the temperature of the orange and the time. Call this Time zero in your lab notebook.
- Place the orange in the Styrofoam cooler with ice water and start the timer.
- Record the temperature every 2 minutes, until the temperature is 80°F (~27°C).
- Repeat steps 1–10 two times, with new oranges, ice, and water. And again, make sure your pot has cooled to room temperature.
Analyzing the Results
- Average the temperatures of the oranges that were cooled at room temperature for each time: 0 minutes, 2 minutes, 4 minutes, etc.
- Average the temperatures of the oranges that were cooled in the ice bath for each time: 0 minutes, 2 minutes, 4 minutes, etc.
- Graph the time, in minutes, on the x-axis and the temperature, in °F or °C, on the y-axis.
- Looking at the graphs, when do you see the greatest rate of change: when you first add the oranges to the water (when there was the greatest temperature difference), or later?
- If you found a "dead" orange in a pool of room-temperature water, and the orange had a core temperature of 85°F, how long had it been "deceased"?
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Calculate the rate of change in the temperature of the oranges as they cooled from 90°F to 80° F.
- Look at the graphs of temperature vs. time.
- Determine how long it took for the oranges in room-temperature water to cool from 90°F to 80°F.
- Determine how long it took for the oranges in ice water to cool from 90°F to 80°F.
- Divide 10°F (the temperature change) by the times (in minutes) to get the rates.
Ask an Expert
Variations
- Try larger or smaller oranges. How does the rate of change in the temperature vary with the diameter of the orange?
- Determine the rate of cooling (in air, not in water) when the orange is placed on a piece of Styrofoam vs. sitting on a metal surface.
- "Dress" the orange; for example, with duct tape. How does this affect heat loss?
- Cool the oranges at other temperatures, such as 40°F, 50°F, and 60°F. Add ice or cold water to keep the temperature of the water bath constant. How does the rate of change of the temperature vary with the temperature difference?
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