Chemistry Classroom Experiments: Four Layer Density Columns

Chemistry classroom experiments: Four Layer Density Columns

Materials needed: Sugar, water, food coloring (4 colors), measuring cup, containers in which to mix and store solutions, plastic cups (4 per group), eye dropper or disposable pipet (1-3 per cup), small test tubes (1 per person), modeling clay (1 marble sized ball per person). One or two plastic buckets, some towels, and a sink (or nearby bathrooms) help a lot with cleanup.

Prepare solution A, which is as concentrated a solution of sugar dissolved in water as possible. Three volumes of sugar to two volumes of water should work (i.e. 3 cups sugar to 2 cups water, or 6 cups sugar to 4 cups water, or any 3:2 ratio). Make sure this is well mixed.

Then prepare two more dilute solutions from solution A. Solution B is two volumes of A mixed with one volume of water. Solution C is one volume of solution A mixed with two volumes of water. Solution D is plain water. Once all four solutions are made, add a few drops of food coloring to each, so that each solution is a different color.

For each group of students prepare four plastic cups, each with one different solution. Fill the cups about half full, and add one or two eye droppers or disposable pipets to each cup. It is helpful to give the students their instructions (directions) first, while making (or at least starting the first two layers of) a density column yourself as a demo. Then pass out the cups to the groups. Also give each student (or have out already) a test tube and small ball of clay. Put the clay on the desk or table and put the bottom of the test tube into it so the test tube stands up straight without having to hold it.

It is easiest to fill the test tubes with the most dense solution (A) first. Make sure you know which one this is by color, and tell the students to put that color in first. Fill the test tube about ¼ full with the first solution, then carefully drip in the second most dense solution (B) on top of the first. Be careful not to squirt B into A – they will mix if this is not done carefully. Repeat by adding solutions C and D.

Tricks to try: 1) Fill D to the top and then pick up the test tube. Put your thumb over the top so that there is no bubble. Carefully and slowly invert the test tube and see what happens. Before you do this, ask the students to predict what will happen (make a hypothesis). I usually flip it back right side up quickly to show that it will mix.

2) Make a density column and carefully set it aside somewhere visible in the classroom. Watch what happens to it over time (make sure no one jiggles or taps or otherwise disturbs it). Over time the solutions will mix slowly.

3) You can try making a five or six or seven layer density column (mix red and blue to make purple, or red and yellow to make orange, or leave one solution clear).

Cleanup – sugar water solutions can go down the drain. Test tubes, cups, clay, and eye droppers can be recycled / reused. Chemistry classroom experiments: Chemistry in a Ziploc bag

Materials needed: Baking soda (sodium bicarbonate), Calcium chloride (Qik Joe ice melt, or Damp Rid), Phenol red (available from pool supply stores) OR red cabbage water, plastic cups (3 per group), teaspoons (2-3 per cup), small Ziploc or other sealable plastic bags (1 per student), small paper cup (1 per student). Candle and matches.

Give each student a Ziploc bag and paper cup. Have the students form small groups of 4 or 5 members each. Give each group one plastic cup half-filled baking soda, one plastic cup half- filled with calcium chloride, one cup half filled with phenol red solution, each with 2-3 spoons.

Stress that the students should not move to the next step before you tell them what to do, and before everyone in their group is done. The first step is to open the bag and add 2 spoonfuls of calcium chloride to it. Have students make observations – what does the calcium chloride look like? Does it do anything in the bag?

Next add one spoonful of baking soda to the bag. Again have the students make observations. Next add two spoonfuls of phenol red solution to the small cup. Once everyone has solution in their cup, have them carefully put the cup into the bag (do not pour the solution into the bag). Get as much air out of the bag as possible and then seal the bag.

Once the bag is sealed, carefully tip the cup and mix the contents. Try to keep the cup away from the mixture (in one corner).

Have the students make observations. As the contents of the bag mix, there will be color changes (Phenol red, white baking soda and calcium chloride mix and change to yellow), some heat is given off (warm to the touch, not hot), and carbon dioxide gas is generated inside the bag (the bag puffs up and you can hear the sound if you hold the bag to your ear). Do not open the bags.

Ask for observations and discuss. Explain that the gas in the bags is carbon dioxide, which will not burn. We exhale it, and plants need it to respire. It is also denser than air and is used in fire extinguishers. Demonstrate the last two points by lighting a candle and asking for a few of the puffiest bags. Hold the bag about 6-8 inches above the flame and open it. The carbon dioxide should pour out and extinguish the flame (by displacing the air and oxygen the flame needs to burn). This can take several tries to work.

Cleanup: Do not open the bags. Simply throw them away. Any spilled material can be wiped up with damp paper towels. Do not eat the materials.

Tweak: Instead of phenol red, red cabbage juice can be used. Both are acid-base indicators, and will change color based on how acidic or basic the solution they are in is. To make the red cabbage juice, buy a small red cabbage, cut it up, and boil the leaves in water for about 10 minutes. Filter the reddish-purple solution (a coffee filter works) and store it in a sealable bottle in a refrigerator.