We did still finish up the gas lesson from last week. I started out with a quick introduction to the gas laws: Charles' Law and Boyle's Law.
Boyle's Law. Robert Boyle was a British scientist who studied how gas pressure and volume are related. He found that as the volume of a container decreases, the pressure of the gas in the container increases. I gave the example of "pouring" a gas out of a 2-cup measuring jug into a baby food jar. The baby food jar is much smaller so, if we moved the gas from the jug to the jar, the pressure of the gas would increase. This is due to the tiny particles in the gas being pushed or compressed. They are forced to move closer together giving them less space to move around.
Boyle's law also tells us that if we were to move the gas from the baby food jar to the measuring jug, the pressure would decrease because those tiny particles have much more space to move.
Charles' Law. Jacques Charles was a French scientist who studied how the volume of a gas relates to its temperature. Charles' law states that as the temperature of a gas increases, the volume also increases. If the temperature decreases, then, the volume decreases. I used an example of a hot air balloon. As the temperature of the gas in the hot air balloon increases, the volume increases filling the balloon with air. To allow the balloon to land, the temperature and volume of the gas are both decreased.
Fading Color lab - This was one of the labs we didn't get to last week. It shows the effect of dry bleach on color.
For this lab, each student was given a baby food jar filled with water. They added two drops of food coloring to the water. The students then added one teaspoon of powdered bleach and stirred carefully. We let the jars sit and observed them periodically. The colors started to fade almost immediately; by the end of class they were much lighter!
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| 1st photo taken immediately after adding the bleach; probably around 10:20. |
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| 2nd photo taken around 10:25. |
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| 3rd photo taken around 10:35 |
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| Final photo taken after class (11:40). We really noticed a difference in the colors! |
I gave them a quick quiz to see what they remembered about the phases of matter. The students also discussed what they already know about how the phases can change (melting, solidifying, vaporizing, condensing). One student also knew about sublimation that we covered today. We spent some time discussing examples of each phase change.
Colder Water lab - In this experiment we saw the temperature of icy water become even colder.
We filled an empty metal can with crushed ice and then added water to cover the ice. The students added a thermometer to the can and we then waited 30 seconds before measuring the temperature. At this point, the temperature of the water was about 40 degrees F.
I then added 1 tablespoon of table salt to the icy water and stirred. We replaced the thermometer in the can and waited another 30 seconds. The students then recorded the new temperature and found it had lowered to just over 30 degrees F.
This happens because the salt requires energy to dissolve in the water. The salt gets that energy by removing heat from the water causing the temperature of the water to lower.
Frosty Can lab - This was another lab to show the effect of salt on the temperature of water.
We used the same can of salty, icy water from the Colder Water experiment. We added two more tablespoons of salt and stirred well. The students let the can sit and we watched as water condensed on the outside of the cold can. The amount of condensation increased as the can rested. Again, this is due to the salt taking heat from the water to provide energy for it to dissolve. If we left the can long enough, we would probably see a layer of frost form on the outside of the can.
Chilling Effect lab - Each student was given a thermometer and asked to blow their breath across the bulb about 15 times. They noticed the temperature of the thermometer had risen due to the heat of their breath.
We then moistened cotton balls with rubbing alcohol and rubbed the alcohol-soaked ball on the bulb of the thermometer. The students then removed the cotton ball and blew across the bulb another 15 times. We noticed that, despite the hot breath, the temperature of the thermometer was lower.
As the students blew on the thermometer, their hot breath caused the liquid in the bulb of the thermometer to expand (we spent a minute discussing what expand means). The liquid molecules move farther apart, causing the liquid to rise in the thermometer. The cool rubbing alcohol evaporated from the thermometer and takes energy away from the liquid in the bulb. As the liquid cools it contracts (again, we took some time to discuss the meaning of contract). The molecules of liquid take up less space and the liquid moves down the thermometer.
Sublime Sublimation
Sublimation is the process of a solid changing directly to a gas without going through the liquid phase. It can be seen easily with dry ice.
Dry ice is carbon dioxide in its solid form so, as it sublimates, it changes back into carbon dioxide gas. We reviewed what we already know about carbon dioxide - it's used to make drinks fizzy (carbonated beverages). Carbon dioxide is also the gas that is created in the vinegar-baking soda reaction.
Before starting these labs we went over some safety considerations for the dry ice. Carbon dioxide is heavier than oxygen so it actually removes/takes over the oxygen in the air. This is how CO2 fire extinguishers work. I told the students we'd go outside to complete most of the dry ice observations so we'd have plenty of ventilation. We also went over reasons why we shouldn't touch the dry ice with bare skin or taste it.
Observing Sublimation
1) I put a few pellets of dry ice into a styrofoam cup and then used a hair dryer to heat them up. The students were able to see the gas vapors coming off the pieces of ice. We found this worked a little better when I held the hair dryer at the side of the cup as opposed to at the top/opening of the cup.
2) We covered the cup/dry ice with some plastic wrap. We know the dry ice sublimates into a gas so we should have seen the plastic wrap puff up. It did a little but we were hoping for something more dramatic! :)
3) I lit a candle and "poured" the vapors from the cup with dry ice over the candle. Since carbon dioxide overrides the oxygen, the candle was extinguished (this again shows how the CO2 fire extinguishers work).
4) I placed a piece of dry ice on the ground and gave each student a towel and a penny. The students held the penny with the towel and then held the edge of the penny on the dry ice. This makes a great squealing sound!
We know carbon dioxide gas is released as the dry ice sublimates. When the students held the pennies on the chunk of dry ice, it caused that gas to become trapped. The gas pushed against the penny and caused the penny to vibrate/squeal.
5) We added some water to a cup with dry ice. This really showed the vapors/the dry ice sublimating.
One of the students asked what dry ice is used for. It's mostly used for refrigeration. Dry ice can be used if the power goes out so food doesn't spoil. It's also used in refrigerated trucks. Dry ice is also used in special effects and fog machines. We saw this with the water and dry ice.
We finished up class by making our own ice cream. It's very easy to make at home and uses minimal ingredients. Here's the recipe if you'd like to try this again. This makes enough for one person.
1/2 cup milk
1/2 tsp. vanilla extract
1 tbsp. sugar
4 cups ice
4 tbsp. salt
2 quart size zipper freezer bags
1 gallon size zipper freezer bags
Hand towels/gloves to keep your fingers warm
Mix the milk, vanilla, and sugar together in one of the quart size bags. Carefully squeeze out as much air as you can (to prevent the bag from bursting open) and seal it tightly. Place this inside the other quart size bag (we double-bagged to prevent salt and/or ice from mixing in with our milk mixture). Put the two small bags inside the gallon bag. Add the ice to the gallon bag and sprinkle salt on top. Squeeze out the excess air and seal the bag tightly. Now, shake the bag until the milk mixture starts to solidify. It can take anywhere between 5 to 10 minutes so patience is key.
*You can also add a little chocolate syrup to the milk and sugar mixture to make chocolate ice cream.
Why salt? That relates to the Colder Water lab above. The salt removes energy from the ice as it melts the ice. This causes the temperature of the liquid milk to lower and become more solid.
We ran out of time and didn't get a chance to discuss freezing, boiling, and melting points. Here it is so you can go over it at home if you'd like.
Phase changes occur due to a change in energy which changes due to a change in temperature. We know that for a liquid to change to a gas, we increase the temperature. The boiling point of water is 212 degrees F. Remember, though, that liquids don't need to boil to become a gas/vapor. Changing a solid to a liquid also requires an increase in temperature/energy. The temperature required to change a solid to a liquid is the melting point. The melting point of water is just over 32 degrees F. As we learned earlier, the freezing point of water is 32 degrees F. Salt and sugar have higher melting points than water which explains why it's easier to dissolve salt or sugar in hot liquids than cool liquids. For a gas to condense or change into a liquid, it needs to lose some of its energy or cool down. The dew point of water (the temperature at which the water vapor in the air changes to liquid) is just over 32 degrees F.
To look forward to next week:
Polymers! We'll discuss natural and man-made polymers. The students will make their own slime and will learn how to insert a toothpick into a balloon without popping it.
References:
The Colder Water, Frosty Can, Chilling Effect, and Fading Color labs are all from Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
I found the ice cream recipe on this website:
http://teachnet.com/lessonplans/science/plastic-bag-ice-cream-recipe/
The dry ice/sublimation ideas came from this website:
http://tlc.ousd.k12.ca.us/~acody/Dry_Ice.html
The students told me about the squealing pennies. Here's some more information on that topic:
http://www.seriouslyfunnyscience.com/node/73
Additional information on matter and states of matter:
Bose-Einstein Condensates.
http://www.chem4kids.com/files/matter_becondensate.html
Well, it really is true that we learn something new every day! I had never heard of this type of matter. This state of matter was created in 1995 and is based on work (from the 1920s) by Satyendra Bose and Albert Einstein.
The students know plasma is very hot; gases have to be really excited (their temperature has to be increased considerably) for them to become a plasma. The Bose-Einstein Condensates, however, are very cold. The particles in this state of matter barely move at all.
More basics on matter and the states of matter.
http://www.chem4kids.com/files/matter_intro.html
http://www.chem4kids.com/files/matter_states.html
Chem4Kids is a really great website and includes all areas of science (and some math!) It's written in kid-friendly language, includes colorful graphics and charts, and makes a great complement to the wonderful (home school-friendly) activities and experiments in the Janice VanCleave books.
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