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Hot or Cold? Boiling Water
 
 

With the Thanksgiving holiday, many kitchens will be alive with cooking and festivities with family and friends. One of the traditional side dishes, mashed potatoes, is usually cooked with boiling water. We decided to honor chefs everywhere who are busy making one of the most memorable meals of the year, and do an experiment with something they all use. So, let’s do an experiment about boiling water.


Question:

Does cold water really boil faster than hot water? The legend says that cold water will boil faster than hot water because the water molecules are closer together and will therefore heat up faster. What do you think?


Let’s Try It.

Purpose: This experiment will investigate the effects of gravity on objects of different masses.

Hypothesis: Hot water will boil faster than cold water.

Equipment/Materials: 1 medium sauce pan, measuring cup, thermometer and timer.

Procedure:

Step #1. Measure 1 cup of the cold water, and record the temperature using your thermometer.
Step #2. Place the 1 cup of water into the pan with the thermometer. Let the temperature stabilize and record the starting temperature.
Step #3. Place the pan on the burner and start the timer.
Step #4. When the water starts to boil (you will be able to tell because the temperature will not increase anymore), stop the timer and record the boiling water temperature and the elapsed time.
Step #5. Repeat using warm (approx. 80◦ F) and hot water (approx. 120◦-140◦ F)

What Happened:

Over the course of running the experiments, did you discover that if we minimize air resistance, all objects fall to the ground at the same rate?


Conclusion:

My results are listed in the table. As expected, the hot water took the least amount of time to boil, followed by the warm water and in last place, the cold water. This was, in fact, our hypothesis. So why did the hot water boil faster? This is because it takes a certain amount of energy to make water boil (in this case we are talking only about thermal, or heat, energy).The initial hot, warm and cold water all have energy. We need to increase the energy of the water to make it boil. Since the hot water already has more energy than the warm or cold water we need to add less. This is why the hot water boils faster than the warm and cold water.

 

Starting Temperature (◦ F) Boiling Temperature (◦ F) Elapsed Time (min.)
Cold Water 43 203 3:09
Warm Water 79 203 2:09
Hot Water 140 203 1:04

Discussion:

Did you notice something else interesting? The boiling temperature of my water was only about 203◦ F. I thought potable water boiled at 212 ◦ F. Why did my water boil at a lower temperature? That is because water boils at 212°F at sea level and we are at approximately 5,000 feet in elevation above sea level where there is not as much atmospheric pressure. Because there is less pressure it does not take as much energy to boil water (or any other liquid) because the molecules are not pressed together and move more freely. As can be seen by the phase diagram below, as the pressure goes down the temperature at which water will turn to gas also goes down (follow the red line). On top of Mt. Everest (approx. 29,000 ft in elevation above sea level) water will boil at 156 ◦ F. The boiling point of water will decrease by 1.8 ◦ F for every 935 ft of elevation. So, if the boiling point of water at sea level is 212°F, then at 5,000ft we can expect the boiling point of water to be 9.6◦ F less. The final expected boiling water temperature is then 202.4◦ F. That is pretty close to our thermometer!


Resources:

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