Fargo has been FROZEN the last few weeks! In the spirit of winter, we’re going to learn about a few frozen phenomena.
These are the questions we'll be exploring today!
- What is a polar vortex?
- What is wind chill, and how does it work?
- What is the Mpemba effect?
- What freezes faster, hot water or cold water? Why do you think that?
Make some predictions. Write down what you know about each question.
First of all, let's talk about the Polar Vortex.
Maybe you’ve never heard of a polar vortex before, but they exist at the poles all the time! A polar vortex is a large, low-pressure area of cold air located near the North and South Poles. In the winter, the Northern polar vortex sometimes weakens and expands. Below, the picture on the left shows a strong polar vortex that is well defined. On the right is an example of a weak polar vortex that has expanded.
Check out Scijinks (https://scijinks.gov/polar-vortex/) for a great explanation of how a Polar Vortex works!
Because of this polar vortex, you might have heard people talking about the intense wind chill that we've been having. A -10 degree (Fahrenheit) day with a little extra wind suddenly feels much colder! So, does windchill affect everything? Will objects freeze faster because of higher windchill? This is a tricky question. Only living things are affected by windchill. That's why you see the term "real feel" when there is a significant windchill. The air stays the same temperature, but living things feel colder because of the wind.
Check out the windchill chart below. What do you notice? When the air is warmer, high wind speed doesn’t make as much of a difference in wind chill. When it’s cold, however, a little wind goes a long way!
Now, let’s go back to one of our first questions: which freezes first, hot water or cold water? You might have predicted that cold water would freeze before hot water, because that seems like the most logical answer! The hot water should take longer to cool off and get to freezing temperature than cold water, right? Well, sometimes that's not the case. In certain circumstances, hot water can freeze more quickly than cold water. This is called the Mpemba Effect.
The history behind the Mpemba Effect started with ice cream. Erasto Batholomeo Mpemba noticed in 1963 that when he put hot ice cream mixture and cold ice cream mixture in the freezer, the hot mixture sometimes froze more quickly. This sparked more interest in studying which water temperatures freeze faster.
There are many explanations suggested for the Mpemba effect. One is that some of the heated water evaporates, so less liquid ends up needing to be frozen than the same amount of cold water. Another is that cold water tends to freeze from the top down, while warm water freezes from the bottom up. In the first case, the top layer of ice insulates the rest of the water, and thus the water takes longer to freeze. Density is another factor that has been suggested; hot water is less dense which could allow for quicker cooling.
Unfortunately, the Mpemba Effect is yet to be fully explained! Results are often inconsistent, making it hard to understand exactly why hot liquid sometimes freezes faster. Can you explain the Mpemba Effect? Test it out for yourself!
YOU WILL NEED:
* 4 identical containers with caps
* Tape and writing utensil
* Paper to write down your observations
* Freezer or temperatures colder than 32 degrees Fahrenheit.
Here's what to do!
1. Gather your four identical containers. Label two of them "hot" and two of them "cold."
3. Measure 1/2 cup of water and pour it carefully into your first container labeled “cold”. Seal with the lid. (If you don't have lids for your containers, use plastic wrap).
4. Measure 1/2 cup of cold water and pour into your second “cold” container. Leave this container open.
5. Turn your faucet to the hot setting, and measure 1/2 cup of hot water into the container labeled “hot”. Seal with the lid.
6. Repeat with the second “hot” container, but leave the lid off.
8. Write down your prediction. In which order will the containers freeze? Why?
9. Set your timer and check on your containers every 15 minutes. Write down your observations.
10. After 90 minutes (when you can see significant freezing taking place) bring your containers inside. Which container contains the most ice? The least ice? Which one started to freeze first? Last? Do your results match your prediction?
Observations for round 1:
After 15 minutes, the hot containers had melted into the snow, but no freezing had taken place. Notice the condensation in the containers, especially in the ones with hot water.
We broke up the ice in order to tell which had frozen the most.
In our testing, we never observed the Mpemba effect. We repeated the test four times under the conditions below.
1. 1 cup of water/container outside in -20 degrees F, 3 mph wind, on snow.
2. 1/4 cup of water/container in -20 degrees F, 7 mph wind, on baking sheet.
3. 1/2 cup of water/container in -20 degrees F, 10 mph wind, on baking sheet.
4. 1/2 cup of water/container in a normal freezer (no wind).
However, we DID determine that the open containers froze faster than their sealed counterparts. Each time the order of freezing was cold/open, cold/closed, hot/open, hot/closed. Why do you think that might be?
Another note: The cold containers froze from the surface first, while the Hot containers seemed to freeze more uniformly from the sides of the bottles.
Did you observe the Mpemba effect? If not, what factors might be worth testing? You could try the same experiment with different containers (example: glass instead of plastic or wide mouth instead of narrow).
Polar Vortex and Windchill Chart are Public Domain images found in Wikimedia Commons.
Photographs of experiment taken by Maddie Van Beek.