Science Bus 2014Hydrophobicity05/19/14

Hydrophobicity Lesson

Last Week:

We learned about the structure of a water molecule and how to split this water molecule using a process called electrolysis.

-  How many atoms make up a water molecule? (3 atoms)

-  What are they? (2-Hydrogen and 1-Oxygen atom)

-  What was so cool about this experiment? (Student will through out some things they found interesting)

This Week:

Draw a simple soap molecule on the board before the lesson starts and label the head (hydrophilic) and tail (hydrophobic)

-  Ask the students to pronounce the words (hydrophobic & hydrophilic) and split the words into 2 parts so they actually get what they mean (hydro = water; phillic = loving; phobic = fearing)

-  Hydrophobic compounds will only mix (or get attached) with their counterparts & same with hydrophilic compounds

-  Briefly explain the concept of how soap is used to wash off dirt from your hands, clothes, dishes and several different objects.

o  The soap tail gets attached to the dirt on the dish (which is also hydrophobic), isolating it from the water.

o  Once all the dirt has been captured and you rinse the dish, the head (which is hydrophilic) dissolves in the water pulling all the dirt along with it.

o  You are now left with a clean dish

-  You can also draw a schematic of a micelle on the board for better understanding.

Students should be in small groups. 1 tutor to 4-6 students

Demo A: Hydrophobic & Hydrophilic Compounds

Here students will understand the differences between hydrophobic and hydrophilic compound

Supplies:

Cups / Olive Oil
Water (from tap) / Juice (Cranberry)
Vegetable Oil / Magic Sand
Spoons

Procedure:

Before starting ask students to point out which of the compound they think will either be hydrophobic or hydrophilic.

-  Pour water into a cup so it is about 1/3 full

-  Add some vegetable oil in the water and student should be able to see water float on the oil. There will be a line separating the oil from the water

-  In another cup of water (1/3 full), pour in some cranberry juice. Now the water should change color and mix well with the juice

-  In another cup, add some vegetable oil and olive oil together. This two should almost mix perfectly well.

-  Magic Sand: Now pour water into a large cup (more than half full) and add some magic sand into the cup. (What do you observe?)

o  You can use a spoon or your hand to stir it up in the water and then scoop out the sand and it comes out completely dry. (What do you see? Can you explain why?)

o  Cautiously add some magic sand in a cup of oil and observe watch what happens. (Be careful, this could be very messy and greasy)

Explanation For Tutors

Explain to students the difference between the compounds used and why some mix and others do not. Remember like mixes with like.

-  Water and oil don’t like each other.

-  Normal sand doesn’t behave like magic sand. The sand used (hydrophobic sand) is like normal sand but it has been coated with a hydrophobic chemical, which not makes it to behave this way.

Demo B: Milk, Food Coloring, & Soap

Here the students will learn how soap reacts when placed in an environment with a lot of dirt (hydrophobic) like oil, fat & grease.

Supplies:

Whole Milk / Food Coloring Dye (3 diff. colors)
Skimmed Milk / Dish Washing Soap
Almond Milk / Flat Plates
Elmer’s glue / Cotton swabs

Procedure:

-  Pour some milk in a plate and count to 10 (or 5, just so you give it time to settle)

-  Add a few drop of food coloring dye. Add all the different colors around each other or slightly apart, but all drops should be very close to the center of the plate

-  Dip one end of a cotton swab into the soap (you don’t need too much on it) and then touch the food dye. (What do you observe?)

-  You can continue using the same swab to touch the dye at different points on the plate until the reaction slows down significantly and stops.

-  Use a clean plate each time and repeat the experiment for the different types of milk (Not Elmer’s glue yet)

-  Fun Time: Every student should grab a small plate and tutors will pour in a small amount of Elmer’s glue & a few drops of the food coloring dye.

-  Then students can drop in dish soap in the center and watch what happens. (Elmer’s glue doesn’t have fat in it but has polar and non-polar sides so tends to act as milk does).

o  Let the glue dry up and you can peel it off the plates for students to take the now decorated glue home with them.

Background For Tutors

-  Milk is mostly water but contains fats and proteins which are hydrophobic

-  The reaction should work best for whole milk because it contains more fat than the others.

-  Skimmed & almond milk reactions should show a much slower reaction because of less fat (or other hydrophobic) molecules present in it.

-  The food coloring has nothing to do with reaction but works as a visual aid to help see what is going on in the solution as the soap molecules run around trying to get attached to all the fat molecules.

-  At some point the reaction stops because all fat molecules have been “trapped” and separated out of the solution.

-  Soap also destroys the surface tension of the milk and this also causes the reaction to stop.

Reference:

https://www.youtube.com/watch?v=mc5ljuG4FYE

http://www.stevespanglerscience.com/lab/experiments/milk-color-explosion

https://www.youtube.com/watch?v=mc5ljuG4FYE

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