I wanted to show the video below but the school network wouldn't let me access the site. Here it is so you can watch it at home. I've viewed it several times and it's completely appropriate for children. :)
http://www.dailymotion.com/
Chemical Reactions
A chemical reaction occurs when two or more molecules interact and something happens. This also causes a chemical change.
We spent a little time talking about oxidation. The students knew this is a fancy name for "rusting." The oxygen in the air reacts with the iron in steel creating the chemical change that is rust.
Catalysts
The students already know that mixing hydrogen and oxygen in certain amounts causes a reaction that produces water. The atoms in this reaction usually bond very slowly. However, if a chemist adds a spark, the reaction speeds up. The spark is an example of a catalyst, something that lowers the amount of energy needed so a reaction can happen easier.
Inhibitors
Inhibitors slow down reactions. I asked the students why they thought scientists would want to use an inhibitor. These can make reactions easier to control which can be helpful!
Types of Reactions
We reviewed endothermic (reactions that take on heat) and exothermic reactions (reactions that give off heat). We also spent a few minutes talking about some other types of chemical reactions: combustion, decay or decomposition, synthesis (a reaction that results in the production of one or more products), digestion, and oxidation.
Breakdown Lab - This lab showed decomposition. We were able to watch hydrogen peroxide (H2O2) decompose or break down into water and oxygen gas.
I cut a slice from a raw potato and then poured some hydrogen peroxide in a cup. We placed the potato slice into the hydrogen peroxide and then watched as oxygen gas bubbles were produced.
Potatoes contain an enzyme (chemicals found in cells) called catalase (what does that sound like? Catalyst, perhaps?) The breakdown of hydrogen peroxide is actually quite a slow process but the catalase really does act like a catalyst to help the H2O2 break down quickly.
Curds and Whey Lab - This is another one that showed decomposition. For this, we broke milk down into its basic parts: curds (solids) and whey (liquid). We poured a little milk into a baby food jar and then added two tablespoons of vinegar and stirred. As the two liquids mixed, they separated into the two parts and we were able to see some lumpy solids in the milk (curds). After the jar sat for a while, we saw a clear layer (whey) on the top of the milk.
We then completed a few labs on starches. The students had a good idea of things that include starch. We also spent some time talking about how starches are polymers as well as the structure of a starch. Starches are long twisted sets of molecules that have branches coming off them.
Here are some pictures of starches so students can see what that description means (cellulose [found in the cell walls of plants] and amylose [one of the two components that make up starches]):
http://wiki.chemprime.chemeddl.org/images/4/4c/Structures_of_Cellulose_and_Amylose_.jpg
Starch I.D. Lab - This lab showed the students that iodine is a starch indicator. Iodine is a great starch indicator and turns a purple-blue color when it comes in contact with a starch. It remains brown on anything that does not contain a starch.
For this, each student was given a little bit of flour on a paper plate. We mixed a few tablespoons of water in with the flour and stirred well. I then added three drops of iodine to the flour and water mixtures. The students noticed the water turned the purple-blue color to indicate the presence of starch in flour.
Magic Writing Lab - We made an iodine-water solution by mixing a few drops of iodine into a shallow bowl of water. I then squeezed a lemon and gave each student a piece of paper and a paintbrush. The students "painted" a message on the paper with the lemon juice and then let it dry. Once the lemon juice was mostly dry, we dunked the paper into the iodine mixture and observed. The students noticed that the paper started to turn a faint purple while their lemon juice messages stayed white.
Paper contains starch so the iodine reacts with the starch in the paper. Vitamin C and iodine combine to form a colorless molecule which explains why the part with the juice message did not turn purple.
Testing for Starch - We used this lab to test for starches in a variety of foods. The students were each given a piece of cheese, an apple slice, a slice of raw potato, a piece of bread, a saltine cracker, and some sugar. They made predictions about which contain starch then added one drop of iodine to each food item and watched. We noticed that the drops of iodine on the bread, potato, and cracker each turned blue.
Solutions
Simply put, a solution is a special type of mixture. Solutions are classified as homogenous mixtures because the components are evenly mixed (there is an even concentration of the substances in the mixture). Heterogeneous mixtures, on the other hand, are not evenly mixed. There may be a higher concentration of one substance than another. I gave the example of sugar and water and sand and water. If we were to mix sugar into a glass of water, it would eventually dissolve and the sugar would spread evenly throughout the water. Sand would not dissolve; it would just sink to the bottom of the glass. The sugar and water is a homogenous mixture or a solution while the sand and water is a heterogeneous mixture.
Solutes and Solvents
Solutions have different parts. One is the solute which is the substance to be dissolved. In the example above, sugar is the solute.
There is also the solvent. This is the substance the solute is dissolved into. That would be the water in the sugar-water solution from above.
Another example is a soda which is a solution of carbon dioxide gas (the solute) mixed into water (the solvent). There is usually more solvent than solute.
Solubility
Solubility is used to describe how well a solvent can dissolve a solute. Solid solutes dissolve more easily (become more soluble) when the temperature of the solvent is increased. For example, it's easier to dissolve a spoonful of sugar in a mug of hot tea than it is in a glass of iced tea.
Gaseous solutes dissolve more easily when the pressure of the solvent is increased. Adding heat actually makes a gas less soluble. The soda example can be used here. A brand-new, unopened soda contains CO2 gas that is under quite a bit of pressure. When you open the cap, you release some of that pressure (this is what creates that "pssssh" sound). Soda or carbonated water stays fizzy while the bottle is closed because the cap keeps the gas under pressure and makes it easier for the CO2 to dissolve in the water.
Alloys
Alloys are made by melting two or more elements (one must be a metal) and then cooling them so they become solid. Examples of alloys include bronze (mostly copper and tin), brass (copper and zinc), sterling silver (mostly silver and copper), and 14K gold (pure gold is 24K so 14K gold is 14 parts gold and 10 parts of some other metal).
Bicycle frames are often made of aluminum alloys. Aluminum is a light metal but it can be strengthened by adding other elements such as magnesium, zinc, and silicon.
Phew, it was hard not to say or type "aluminium" during this lesson/blog entry. ;)
Some very high-end bicycles and cars are made with carbon fibre which is a composite of epoxy (another polymer) and graphite. Carbon fibre can also be reinforced with aluminum or Kevlar (polymer!)
To finish up class we discussed oil and its solubility in water. The students knew oil and water did not mix but we did some fun labs to prove this (and learned some new words [immiscible and emulsion] in the process!)
Floating Spheres Lab - In this lab we floated spheres of food coloring between layers of water and oil.
We poured 1/4 cup of water and 1/4 cup of cooking oil into a plastic cup. The water is heavier than the oil so it sank to the bottom. We then added a few drops of food coloring to the cup and looked to see where the drops ended up. The balls of food coloring floated in the oily layer.
Here's an extra part of this lab to try at home. Follow the directions as above (add about 5 drops of food coloring) but try pushing the drops of food coloring into the water layer. They should break apart and dissolve in the water.
Oil and water are immiscible which is a fancy science word meaning they don't mix. No matter how much you stir or shake, they will separate with water on the bottom and the lighter oil on the top. Food coloring will not dissolve in the oil so those little spheres of color will remain. Since the oil and water won't mix, the oil also prevents the food coloring from touching the water (unless of course you push them with a pencil!)
Immiscible Lab - This lab showed the separation of oil and water into an emulsion (a combination of two immiscible liquids).
We poured 1/2 cup of water into a jar and added a few drops of blue food coloring. We then added 1/4 cup of cooking oil. I closed the lid of the jar tightly and then had the students shake the jar to attempt to mix the oil, food coloring, and water.
As we know, oil and water are immiscible. When the students shook the jar, the oil, water, and food coloring mixed temporarily but immediately started to separate once we put the jar down. This also proves that food coloring is water soluble but is not soluble in the oil. At first, both the oil and water had a blue tint from the food coloring. However, as the jar sat undisturbed, the oil started to lose its blue coloring.
Here's a link to a lab to try at home. This is another one that shows the decomposition of H2O2 (hydrogen peroxide). This will create water and oxygen gas. Yeast acts as a catalyst in this lab.
The Decomposition of Hydrogen Peroxide
More information on catalysts.
More on solutions.
More on alloys (with info. on amalgams and emulsions)
Next Week: Acids and Bases
References:
The labs we completed today are all from Janice VanCleave's Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
Red Velvet Cupcakes
I wanted to include a little bit about red velvet during today's class since these cupcakes (along with most cooking) includes some pretty cool chemistry. Red velvet cake is a Southern tradition. It started out as a chocolate cake with a reddish hue. Acids and bases in the original red velvet recipes react to create a beautiful reddish-brown cake. Unfortunately, these cakes have become a shadow of their former selves; most modern recipes include only a tiny amount of cocoa powder and they almost all use red food coloring to provide the "red" in red velvet.
Apparently the proper thing to do is to slather a good amount of cream cheese frosting on these cakes. Buttercream is a good alternative if you're not a big cream cheese frosting fan.
Here's how the chemistry works:
Natural (not Dutch process) cocoa powder contains pigments that turn red when mixed with acids. Traditional red velvet recipes include acidic ingredients such as buttermilk, Greek yogurt, and/or vinegar (I used all three) to produce the red color.
More information on the chemistry of red velvet.
I made two versions of a red velvet recipe for the students to try. Neither one turned out very "red" but they were both very velvety and moist. Yum! :) I tried adding pureed beets (canned sliced beets that had a good long spin in the food processor) to one batch of cake batter since I'd read that would make a more red cake. For the other batch, I followed the same recipe but left out the beets. I did add a little (1 tbsp.) of vinegar to that second batch. I actually found the beet-free cakes to be the more reddish-brown of the two. Not what I expected!
Here's the recipe I based mine on:
Not so red but oh so velvet cupcakes (from the Home-Ec 101 blog)
Since this recipe uses a liquid fat (no need to cream the butter and sugar), I used what Alton Brown refers to as the "Muffin Method" to make the batter. Combine the dry ingredients (flour, baking soda, baking powder, cocoa, and salt) in one bowl. The wet ingredients (beets/vinegar, yogurt, buttermilk, eggs, oil) and sugar get mixed together in a second bowl. Then mix the wet ingredients into the dry and mix just until they're all combined.
It's a really easy recipe and a fun one for the children to help with.
Hint: To make the 1/4 cup of buttermilk for this recipe, you can just add 1/4 tbsp. vinegar to 1/4 cup of milk. Mix and let it stand for about 10 minutes.
More on food chemistry from Penn State University
Even more kitchen chemistry!
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