To start the class, I asked the students to describe a votive candle. We then lit the candle and the students discussed changes they observed and any new states of matter they noticed. I also took this opportunity to provide a brief introduction to chemical and physical changes. The students understood that the candle, when lit, was undergoing both physical and chemical changes.
We completed two labs at the beginning of class. These both demonstrated that atoms have positive and negative parts.
Paper hop lab - For this, each student was given a balloon as well as several small circles from a hole punch. The students rubbed the inflated balloons on their shirts and then held the balloon close to (but not touching) the paper circles. We watched the circles "jump" up and stick to the balloon.
The students knew that all matter is made up of atoms. I built on this by telling them about the neutral, positive, and negative components: neutrons, protons, and electrons. When the students rubbed the balloons on their shirts, the balloons picked up electrons from the shirts. This gave the balloons an excess of negative charge. The positive part of the paper circles were attracted to those extra negatively-charged electrons on the balloon. This attractive force is so great that it overcame gravity allowing the paper circles to jump up and attach themselves to the balloon.
Do Not Touch lab - This was another lab that included charging a balloon and using it to move another object. This is a fun and easy one to try again at home.
Each student was given a nickel, a balloon, a toothpick, and a plastic cup. The students had to stand up the nickel on its edge and then balance the toothpick on the top edge of the coin. We had a little trouble getting the nickels to stand up so the students used a little modeling clay to help. Good thinking! We managed to do this with round toothpicks but I'm sure it's much easier if you have flat ones.
Once the toothpicks and nickels were balanced, the students carefully placed a cup over the coin. They then rubbed the inflated balloons on their shirts and held the balloon near the cup. As they moved the balloon, the toothpick also moved.
Again, rubbing the balloon against a shirt (or your hair) will cause it to gain extra electrons. The negative charge of these electrons attracts the positive charge of the toothpick causing the toothpick to move with the balloon.
We moved on to discuss some other basic characteristics of matter. We talked about how all matter takes up space and has mass but it can exist in different states of forms. These states depend on temperature.
Solids - Every solid has a definite shape and a definite volume. The tiny particles inside an object are in constant motion. In solids, these particles are very close together so they don't move much; they mostly just vibrate next to each other. Since they can't move over or around each other, they are able to hold their shape.
Before moving on to liquids, I asked the students if every metal is a solid. They knew there was one, mercury, that is a liquid at room temperature. We discussed some uses of mercury (old thermometers, amalgam fillings, fluorescent light bulbs). Mercury will not become a solid until it reaches -40 degrees celsius. Brrrr!
Liquids - Just like solids, liquids have a definite volume. If you poured 1 cup of water into a different container, you would still have 1 cup (provided, of course, you didn't spill any). However, liquids do not have a definite shape. Liquids will take on the shape of their container.
Liquids are able to move a little more than solids but the particles in liquids are still relatively close together. They do have enough kinetic energy to move around and over each other, allowing them to take on the shape of whatever container you pour them into.
Unseen Movement lab - This lab allowed the students to observe molecular motion.
For this lab we had a jar full of water and some red food colouring. I added three drops of food colouring to the water and the students watched as they flowed down through the water and started to spread out. We left the red water in the jar in the classroom so we'll see what has happened during the week. The water should be completely red since the particles of food colouring will evenly spread through the water. This is called diffusion.
Where Did It Go? - For this lab, we used another jar of water along with some rubbing alcohol and food colouring. I poured one cup of water into the measuring jar and then added six drops of blue food colouring. I then measured one cup of rubbing alcohol and added that to the water. The students then each checked the level of the liquid.
The liquid level was below the two cup mark. There are small pockets of space between the water molecules. The rubbing alcohol filled in these pockets so we had a combined volume of less than two cups.
Gases - We have a whole class dedicated to gases next week so we only discussed them briefly during this class session.
We started out this section with a lab:
An Empty Sack? - I filled an empty plastic bread sack with air and then held the top to close it. The students tried squeezing the bag but were unable to completely deflate it. This showed that air, while we can't see it, does take up space.
Gases do not have a definite volume. Gases do not have a definite shape. They can contract or expand to fill the space available to them. Gases can also be squeezed (or compressed) into a smaller space. I showed the students a picture of a CNG logo and asked if they had ever seen that logo on a vehicle such as a city bus. I also asked them if they knew what CNG stands for. This stands for Compressed Natural Gas (they got the Natural Gas part). Compressed means squeezed so the natural gas that is powering those buses has been squeezed to fit into a smaller container. Nitrogen, one of the gases we breathe, can be compressed to form liquid nitrogen, which is very cold. I talked a little about liquid nitrogen and how it immediately turns into its gaseous form at room temperature. If you pour liquid nitrogen it looks like you are just pouring out a gas; all you see is the vapour.
The particles of a gas have enough kinetic energy to separate completely from one another. They are free to move in all directions.
Plasma - I started this section by asking the students which of the states of matter they thought was the most common. While we encounter solids, liquids, and gases throughout the day, the most common is actually the one we don't really talk about. Plasma makes up stars and nebulae. 99% of the mass of the solar system is contained in the sun making plasma the most common form of matter in the universe. We looked at some Google images of nebulae.
Plasma can also be seen in fluorescent lights. When you turn on a fluorescent light bulb, the electricity causes particles of mercury inside the bulb to form plasma.
Rising Ball lab - I filled a glass jar with rice and buried a small ball in the rice. After adding the lid to the jar, the students were each given the jar and instructed to shake it from side to side (not up and down). As they shook the jar, the ball rose to the surface of the rice.
As we saw earlier in the Where Did It Go? lab, there are small spaces between the particles of any sample of matter. When the students shook the jar, the grains of rice moved closer together (settled) causing the ball to be pushed up to the surface. This shows, then, that two pieces of matter cannot occupy the same space at the same time.
Not at the Same Time lab - This lab further demonstrated that two pieces of matter cannot occupy the same space at the same time.
For this, each student was given a plastic cup, 6 marbles, some water, and some masking tape. The students placed the masking tape on the cup to mark the top of the water level. They then added the marbles to the cup and noticed that the water level rose. The marbles push the water out of the way causing the water level to rise. As we know, though, the volume of the water did not change. The change in the water level is equal to the volume of the marbles.
Next week: We will continue our study of matter by learning more about gases. Pop a cork from a soda bottle using yeast and make foaming soda.
Reference
The labs from this class session are all from Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
