February 24th - Gases

We continued last week's study of the phases of matter by spending more time discussing gases.

To start out, I asked the students what they remembered about gases from last week's class. Do they have a definite volume? What about a definite shape? Are the particles packed close together? Do gases take up space? We discussed the reasoning for the answers to each question.

Dry Paper lab - This lab proved that even though we can't see gases, they do take up space. Each student was given a plastic cup and a piece of notebook paper. They wadded up the paper into a ball so it fit into the bottom of the cup (if you try this at home, make sure the paper has a snug fit and cannot fall out). The students then turned the cups upside down and submerged them in a bucket of water. They were very careful to push the cups straight down into the bucket and to lift them straight out without tilting the cups.
The paper was dry when the students removed it from the cups. Air fills up the cup on top of the paper and prevents water from entering the cup and getting the paper wet.

We spent a little time talking about oxidation. I asked the students what would happen if I left some newspaper on the back window ledge of my car. They knew it would turn yellow. This is actually the opposite of most reactions involving oxygen. Oxygen can be used as a bleaching agent so you would think adding oxygen would turn something white, not yellow. Newspaper is a little different. The materials used to make newspaper are actually yellow. Chemicals are then added that remove oxygen from the newspaper and turn it white. Sunlight heats up the air and acts as a catalyst for the reaction between oxygen in the air and those chemicals in the paper. This reaction then causes the paper to turn back to its original yellow.
We also did a quick demonstration of oxidation of fruit. I cut an apple in half and set one half off to the side. We squeezed a little lemon juice on the other half. When fruits such as apples, bananas, and pears are peeled, cut, or bruised, their cells are broken. Chemicals released by the broken cells react with oxygen and result in the brown color. Vitamin C or the citric acid in lemon reacts with the oxygen before it can react with the chemicals in the fruit, preventing the discoloration.

I asked the students to name any gases they could think of. We also talked about the gases that make up earth’s atmosphere (oxygen, nitrogen, argon, carbon dioxide, and water vapor). We looked at the periodic table of the elements and I showed the students how to identify the gases.

Last week, we learned that all matter is made up of smaller particles. These can then be split up into three groups: positively-charged protons, neutral neutrons, and negatively-charged electrons. I showed the students how to use the periodic table to find out how many of each particle an atom of different elements has.

Protons. To find the number of protons, just look for the element’s atomic number. This is usually located above the symbol. Oxygen, for example is 8, argon (Ar) is 18, and helium (He) is 2.

Neutrons. To find the number of neutrons, subtract the atomic number from the element’s atomic weight. The atomic weight is the number (usually a decimal) underneath the element’s name. Oxygen’s atomic weight is 15.9994. The students knew we could round that up to 16. This means oxygen has 8 neutrons. Argon has an atomic weight of 39.948 so it has 22 neutrons. Helium’s atomic weight is 4 giving it 2 neutrons.

Electrons. The number of electrons is related to the group that each element is found in. There are 8 main groups on the periodic table (look at the Roman numerals with A after them at the top of each column). Elements in group I such as hydrogen, sodium (Na), and potassium (K) all have one outer electron. Oxygen is in group VI so it has six outer electrons.

We moved on to talk about some special groups on the periodic table. Two of the gases, fluorine (F) and chlorine (Cl), are part of the halogens. These are elements in group VII and they like to join with elements from group I. We talked about how sodium from group I likes to join up with chlorine from group VII to form table salt, NaCl. Another special group are the noble gases. These are group VIII. Compounds like to have eight outer electrons so the noble gases do not like to join up with other elements. They already have 8 electrons in their outer shell since they are in group 8. I spent a little time showing them some other combinations of elements and how they like to join up to have 8 outer electrons. I drew dot diagrams for salt and water on the board to show the 8 total electrons.

Escaping Bubbles lab – The students were given a baby food jar filled halfway with soda. We placed the jars on the table and watched the bubbles rise to the top of the jar. We discussed how soda (and other fizzy drinks) are made with carbon dioxide gas. That’s where the word carbonated, as in carbonated water, comes from. Large amounts of carbon dioxide are dissolved in water. This high volume of gas is compressed into a relatively small space (a soda or Perrier bottle) and is immediately sealed so the gas is under high pressure. When the lid is removed, the gas starts to escape right away (that “pssssh” sound is the pressure being released). The bubbles the students observed were bubbles of CO₂ gas that was escaping the open baby food jar.

Foamy Soda lab – This lab showed effervescence, a fancy word for replacing a gas with another substance.

The students kept the baby food jars and soda and added a teaspoon of salt to the liquid. They watched as foam erupted from the jars. Since salt and carbon dioxide are both examples of matter, we know they both take up space and that they can’t be in the same space at the same time. One has to move! The salt pushes the lighter gas out of the way causing lots of carbon dioxide bubbles to move to the top of the jar at the same time. The moving gas creates the great foam that the students observed.

Escape lab – This was another lab to show the removal of carbon dioxide from soda. For this, we placed a balloon around the open mouth of a bottle of soda. The students gently shook the bottle and then set it on the table. We watched as the balloon inflated.

We know the particles in gases are spaced far apart and have plenty of room to move around. Shaking the bottle of soda excited the gases causing their particles to move around even more. The open bottle, as we saw in the last two labs, allowed the excited gas to escape and the pressure from the gas caused the balloon to inflate.

How Long? lab - For this, we set up a pipe using a piece of clear plastic tubing, some clay, a jar, and a soda bottle. We then poured some water into the soda bottle and added a broken up Alka-Seltzer tablet. We placed a little of the tube into the soda bottle and sealed it up with the clay. Water was added to the jar and the free end of the tube was placed into the water in the jar. The students then watched as bubbles traveled through the tube into the water in the jar. OK, so we couldn't really see the bubbles travel along the tube but they did see the water in the jar bubbling. Very cool one to try at home.
The combination of Alka-Seltzer and water creates carbon dioxide which is the gas the students were able to see bubbling in the jar.

We tried one more lab but didn’t have much success with it (most likely due to teacher error!) We’ll try this one again next week since I’m sure it’s really cool when done correctly. J

For this, we’re going to attempt to shoot a cork from a soda bottle. We will add some dry yeast and sugar to warm water inside a soda bottle. We’ll then place a cork in the mouth of the bottle and let the yeast do its thing. Hopefully, the carbon dioxide that is produced by the yeast will build up enough pressure to pop the cork from the bottle.

 Our un-popped corks! I hope we have better luck next time.

To look forward to next week:

*The Pop Cork lab detailed above.

*Fading Color lab – we ran out of time for this. It will show the oxygen gas in bleach.

*We will discuss phase changes and make ice cream!

References:

The labs "Dry Paper," "Escaping Bubbles," "Foamy Soda," "Pop Cork," and "How Long?" are all from Chemistry for Every Kid.

VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
The lab "Escape" is from 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments. 
VanCleave, J. (1993). 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments. New York: John Wiley and Sons, Inc.

Some photos from Mrs. Kole's origami class.