Cold Boiling Water
One difference you'd notice on Mars is that water would boil at a much lower temperature. This is because the pressure of the martian atmosphere is much less than that on Earth. In this activity, you'll create a vacuum by covering the end of a syringe containing water and then pulling on the plunger. This lowers the pressure and makes the water bubble like water in a hot pot on the stove.
What You'll Need
- a clear plastic syringe (The type for giving measured doses of medicine will work fine, or you can remove the tip from a medical syringe so that it can't accept a needle.)
- tap water
Fill a syringe 1/4 full of water. Try to fill it so that there's as little air as possible in the syringe. To do this, point the tip of the syringe upward, flick the tip with a fingernail to dislodge bubbles, then push the air out by pushing inward on the plunger (like nurses and doctors do on medical shows on TV).
Cover the tip with a finger.
Slowly pull on the plunger. Notice that as you pull on the plunger, it pulls back in the opposite direction. When you pull, the pressure inside the syringe is reduced below atmospheric pressure (the air outside the syringe). This results in a net force being exerted by the outside air pushing the plunger back into the syringe and the gas inside the syringe pushing outward less strongly. Notice also that a space appears inside the syringe that isn't filled by water.
Allow the plunger to slide slowly back into the syringe. Notice if there are any air bubbles.
Slowly pull the plunger out again.
Release the plunger suddenly. Notice that it snaps back quickly.
Pull on the plunger a third time. Notice that this time bubbles form in the water. The water appears to be boiling.
What's Going On?
When you pull on the plunger, you increase the volume inside the syringe and decrease the pressure on the water. A space appears above the water, and in this space there's a partial vacuum. It's not a perfect vacuum because it has some water vapor in it as well as some air.
Tap water has air dissolved in it. When you reduce the pressure in the syringe, the dissolved air comes out of solution, forming bubbles. When you slowly allow the plunger to slide back into the syringe, the air that has come out of solution stays out of solution. Water vapor changes from a gas to a liquid very quickly. Any gas bubbles that form when you pull out the plunger and then go away when you allow it to return are bubbles made of low-pressure water vapor. When these bubbles form inside the liquid, we say that the liquid boils.
It's difficult for small bubbles to form so that boiling can start in a clean liquid. However, when you pull out the plunger and allow it to snap back, you create small "seed" bubbles throughout the water. The next time the pressure is reduced, boiling happens at these seed bubbles.
The Martian Connection
The water in the syringe is actually boiling at room temperature. If you reduce the atmospheric pressure even further using a vacuum pump, the water can boil at the freezing point. It's therefore possible for liquid water, solid ice, and gas bubbles to coexist indefinitely. This is called the triple point of water, where all three phases exist in equilibrium. The triple point of water is 32°F (0°C) and 6 millibars (a bar is one atmosphere of pressure). The triple point of water exists on the surface of Mars: You could hold a beaker of boiling water on the martian surface that had ice cubes floating in it, and the ice cubes wouldn't melt because the liquid water would be at the freezing point.
Thinking about the activity you just did, imagine you'll be making a spaghetti dinner on Mars. Would the pasta cook more quickly or more slowly than on Earth? Why?
If you were to cook spaghetti on top of a high mountain on Earth, how would the elevation affect your boiling of the pasta water (Hint: Compared to sea level, is the atmospheric pressure higher, lower, or the same on a mountaintop?)
The difficulty of getting bubbles to form explains why it's dangerous to boil water in a microwave oven. If you're heating water in a clean ceramic or glass cup, it's possible that the water can be heated above the boiling point and yet be unable to form bubbles. In this case, the water is superheated. When you remove the cup of water from the microwave, you might jiggle it and shake loose seed bubbles, causing the water to suddenly boil vigorously and splatter hot water around.