Science

Directions: Read the following article. As you read, annotate it by adding comments and answer the embedded questions throughout. Try to use each of the following annotations with the symbols at least once as you read.  You should have at least one annotation per page.      

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→ Finally, answer the analysis questions embedded within, and at the end as you move through the article. 

Liquid Oxygen 

      In your daily life, oxygen is everywhere. It’s in the air all around you, and even inside you, traveling from your lungs to every cell in your body. You probably think of oxygen as a gas, because that’s how you encounter it—but it isn’t always a gas. Oxygen can also exist as a liquid with a beautiful blue color. However, you’ll never see puddles of liquid oxygen on the ground, and you can’t buy a bottle of liquid oxygen at the store. Oxygen is very 5different from water, the most common liquid on Earth. Scientists can produce liquid oxygen, but it’s not easy. Why is oxygen gas all around us, while liquid oxygen is so rare?

Prediction:  

• How do you think scientists produce liquid oxygen?

They make the oxygen very cold and condense it until there is now place to go other than together.

The Role of Attraction

Liquid oxygen is much harder to find than liquid water because oxygen molecules aren’t  strongly attracted to other oxygen molecules. Every substance has a certain level of attraction between its molecules that pulls the molecules toward each other. This molecular attraction is stronger in some substances, like water, than it is in other substances, like oxygen. Molecules in liquids and solids stay close together because of the  attraction between molecules. In the gas phase, molecules are able to move away from each other because they have a lot of kinetic energy, but attraction between the molecules still keeps them in the same general area. Different substances have different levels of attraction, but the level of molecular attraction for any one substance is always the same.

Since attraction never changes for a substance, something else has to change before the substance can change phase. You can think of each substance as always being in a game of tug-of-war at the molecular level. Attraction is always pulling the molecules together, while kinetic energy allows the molecules to move around. Substances change phase when either molecular attraction or kinetic energy wins the tug-of-war—and since each substance’s molecular attraction is always the same, the change must be in the amount of kinetic energy the molecules have. A large increase in kinetic energy can move molecules so fast that they overcome the pull of attraction and move away from each other. The opposite is also true: a large decrease in kinetic energy allows attraction to overcome kinetic energy and pull the molecules toward each other.

Question:  

• Why do they say the phase of a substance depends on the “Tug-of-War” between molecular attraction and kinetic energy?  Which of these cannot change in a substance?

There has to have less kinetic energy than molecular attraction. So it is a Tug of War between kinetic energy and molecular attraction.

For oxygen gas to change to a liquid, the attraction that pulls the oxygen molecules toward each other must overcome the kinetic energy that pulls them away from each other. Because the attraction between oxygen molecules is weak, a great deal of kinetic energy needs to be removed from the molecules (by making the oxygen very cold) before their attraction can pull the molecules toward each other.

Analysis:  

• Explain the diagram to the right:  What is the relationship between the red circles and the spaces between the molecules (colored circles).  

Substance B is an oxygen substance because the big dots mean it is cold and there is no kinetic energy in the molecules.

• How could you change substance A so that the molecules are closer together?

When they are cold they have less kinetic energy and stronger alteration.

Using Attraction to Make Liquid Oxygen

Scientists use their knowledge of attraction to produce liquid oxygen. They know they need to allow the weak attraction between oxygen molecules to pull them together. They do this by transferring energy out of the oxygen, lowering the temperature of the oxygen molecules until their kinetic energy is so low that it can’t overcome the attraction between molecules. The temperature required to turn oxygen gas to liquid is far colder than what’s required to turn water vapor to liquid. That’s why oxygen needs to be extremely cold to change phase into a liquid, while water doesn’t need to be very cold at all.

When scientists need liquid oxygen, they can get it by separating oxygen molecules right out of the air. Air is a mixture of nitrogen and oxygen combined with small amounts of other gases like water vapor and carbon dioxide. Since the gases in air have different levels of molecular attraction, scientists can separate them by sending the air through a machine with different tanks that are at different temperatures.

Question:  

• Based on the reading and the diagram above, order the three molecules from strongest to weakest molecular attraction:   oxygen, water, nitrogen

• Strongest  = Water  

• Medium = oxygen

• Weakest = nitrogen

In the first tank, scientists decrease the temperature enough to condense the water into a liquid—that is, they decrease the kinetic energy of the molecules so the attraction between them can pull the molecules together. The liquid water that forms in the first tank is drained away. Then the remaining gases are put into a second tank and cooled below -183°C (-297.4°F). That’s cold enough that the molecules in those gases have very little kinetic energy to push molecules apart. Because of this, the attraction between the molecules pulls them together and condenses the gases into a liquid, just like the water in the first tank. This liquid contains the molecules from the remaining gases in the air, including oxygen. The liquid mixture is then moved into a third tank that is heated up just a little so the nitrogen evaporates—the kinetic energy of the nitrogen becomes too strong for the attraction between molecules, and they escape into a gas. When the nitrogen evaporates, the liquid left behind is mostly oxygen.

Oxygen isn't the only substance that's hard to condense because of weak attraction. Hydrogen and methane also have very weak attraction, and so are difficult to condense. Like oxygen, these are substances we tend to think of as only being gases on Earth. However, also like oxygen, they can become liquid if they are cooled enough. Cooling these substances just a little won't cause them to condense: these gases reach extremely cold temperatures before condensing into liquids. It’s all about the attraction.

Analysis:

• Theoretical science research involves gathering information to better understand phenomena and concepts, like the scientist in this reading, and it may not have a direct impact on the world.  Applied science research involves trying to solve a particular problem, like when you were a geohazard engineer intern and you designed a tsunami warning system.  They are both important for different reasons.  Which of these two types of research do you find more interesting and explain why?  

I don’t like the geohazard job more than the titan job because I like to think about space and how it can change the world.