Density & Buoyancy (Avni Thakkar)

Wash you face: oil and water do not mix. That is why when you wash your face, which may have dirt in the form of oil or grease, with just water, chances are it will do no good! However, if you add a little bit of soap to this activity, you will get better results! Soap is a unique substance of potassium fatty acid salts, produced through a chemical reaction called saponification. Its molecules are made up of a hydrocarbon chain, which is non-polar, as well as a carboxylate molecule, which is polar. Therefore, the non-polar part of the soap – the hydrocarbon chain, is not attracted to water but to oil (lipophilic). On the other hand, the carboxylate molecules, which are negatively charged, are attracted to the positively charged water molecules (hydrophilic).

2. Oil spills in the ocean: When oil spills in the ocean, you will see it layer at the

top of the water and will not mix with the water in the ocean. This is because of the density differences between the two. Additionally, water is polar and oil is non-polar. As stated above, this is also a reason that the two do not mix.

3. Digestion problems: Tummy aches? Well this is why we use Alka-Seltzer. Because it is a base, it helps to soothe the stomach lining by neutralizing some of the stomach's acid. It also contains a little aspirin, which is a mild pain reliever.

4. Glaciers: Because ice is less dense than water, the world is not under water. If the glaciers were more dense than water and were to sink to the bottom of the ocean, sea levels would rise substantially,causing many cities like Los Angeles to drown. Remember, ice doesn't melt when it gets to the bottom, it actually is colder down there and much of the ocean would wind up freezing… that is why the sea levels would rise.

What else can I teach with this experiment?

CHEMICAL REACTIONS: After doing the experiment:

So what happened in what we just saw? How would you describe it to someone? We put something into something and then something happened. What do we call this process? It is a reaction. Chemical reactions happen all around us all the time and that is what is responsible for creating all the things we see today. A chemical reaction consists of two parts: the reactants and the products. What are the reactants? They are the things that come together to make a reaction happen. So what were our reactants? The Alka-Seltzer tablet and the water. How do we know it was not the oil? Watch again closely and you will see that when the tablet hits the oil, nothing happens. The reaction starts when the tablet touches the water.

And as a result of this reaction what is formed? That is much harder to tell. We see some sort of gas, but that’s about all we can tell. So far we have this then:

Alka-Seltzer + Water -> Gas

This is a basic chemical reaction. On the left side we write the reactants and on the right side we write the products. However, if you remember from our earlier lesson on Atoms and Compounds, there is a scientific way to express a particular substance. We do this by writing the various atoms that it consists of. So how do we represent water in chemistry? What atoms make up water? H20. Similarly, Alka-Seltzer has a chemical called sodium bicarbonate, which is written as NaHCO3.

Now, Alka-Seltzer is a unique chemical. Does anyone know what it is for? People take it when they have an upset stomach. When your stomach has too much acid in it, it becomes upset. What exactly does it mean that it is acidic? It means that the mixture of liquids in the stomach has a unique property -- it has too many protons. Who remembers what a proton is? A proton is the part of the atom that has a positive charge. But how can we tell this? Can we see it? Absolutely not. Instead, we can use fancy equipment to figure it out, or we can use a little bit of our own experience. When have you heard the word “acidic”? What comes to mind? Citric fruits are acidic. One way is taste - sour, sharp tasting.

So going back to our reaction, the Alka-Seltzer is reacting with the acid to make it neutral. What is neutral? Water. So we are forming water and a gas, which we know as Carbon Dioxide. So the chemical reaction for this process is:

NaHC03 + H3O+ -> Na+ + 2H20 + CO2

Now let’s try this. Count up all the Na’s on both sides, and all the Hs, and Cs and Os. What do you get? Na=1, H=4, O=4, C=1. So in reality we have the same thing on both sides, but just in a different combination. That is an important lesson about chemical reactions. Things do not just disappear into thin air, and elements cannot be lost.

Alka-Seltzer is a famous antacid. When the mixture of foods in your stomach has excess acid, an antacid helps eliminate the discomfort by reacting with it. So how can we tell if something is acidic or not? Look at this glass of water. Is it acidic? No, it is neutral. So what makes one liquid acidic and another not? A simple way to understand this is that an acid is a substance that, when added to water, loses a proton and makes the water have too many protons. Who remembers what a proton is? A proton is the part of the atom that has a positive charge? So if something is acidic it has a positive charge. But can you see this? No. So what kinds of things do we think of usually when we think of acidic? Sour, sharp tasting - lemon juice, orange juice, any citrus fruit is acidic. The opposite of an acid is a base. A base, when added to water, gains a proton and decreases H+ ions in the water. Can anyone think of an example of a base? Bleach is a base, so is soap.

So what can we do to figure out which is which? One way is using the Litmus test. There are special strips of paper called Litmus paper. Depending on the nature of the mixture they change colors. When you put them in an acidic mixture, the paper turns red.

Alka-Seltzer contains one main chemicals in it: sodium bicarbonate (NaHCO3). In an acidic solution, NaHCO3 reacts with H3O+ to form H2O + CO2 (G). Let’s now take a look and see this reaction in action. (Do the experiment.

MEMBRANES

You can also make this an activity to introduce lipid membranes and hydrophobic and hydrophilic interactions!

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Google won't let me rotate my images (even though they are perfectly fine on my laptop) so sorry about the weird orientation :(