Water Cohesion & Adhesion (Araceli Barriga)

Araceli Barriga-Pimentel

SED 525S

Student Demonstrations

Title: Water Cohesion -Penny Droplets

Principle(s) Investigated: Water cohesion and adhesion from Van der Waals interactions.

Standards: Chemistry Standard(9^th-12^th grade)-2. h: Students know how to identify solids and liquids held together by van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/ melting point temperatures.

Materials:

· An eye-dropper (medicine section at Wal*mart or Target)

· an American penny

· liquid dish-soap (any general or grocery store),

· a glass of water (can be from the faucet)

Procedure:

· Penny and water

o Set the penny face up on the glass counter top.

o While counting the drops, add as many drops as you can without spilling water off of the sides.

o Record the amount of drops it took to make the water spill from the penny.

o Repeat the experiment three times to reduce experimental error and record the number of water drops each time.

· Penny and Soap

o Add a tablespoon of dish soap to the glass of water. Stir gently to not make too many bubbles.

o Add drops one by one to the face of the penny to see how many drops you can make.

o Record the amount of drops it took to make the water spill from the penny.

o Repeat the experiment three times to reduce experimental error and record the number of soapy drops each time.

Student prior knowledge: Students need to know that oxygen is more electronegative than hydrogen. The water molecule is a polar molecule, where the oxygen has a partial negative charge (δ-) and the hydrogen atoms have a slightly positive charge (δ+).

Explanation: The demonstration shows how water molecules are attracted to each other’s partial charges. The partial negative charge from the water molecule’s oxygen is attracted to the partial positive charge from another water molecule’s hydrogen and vice versa. This occurs throughout the water droplets causing the water molecules on the penny to “hold on to each other” via hydrogen bonds as you add more and more drops. This type of interaction between water molecules creates a cohesive force.

Adhesion forces help keep the water molecules on the coin. Eventually the big drop accumulated on top of the penny reaches a certain threshold and the water spills over the penny. In this case the force of gravity was greater than the forces holding the water molecules together so the water spilled over.

When water is mixed with the liquid dish soap the drops coming out of the dropper are smaller than when it was water by itself. The soap molecule has a hydrophobic hydrocarbon tail that repels charged or partially charged molecules or atoms, like water. The soap molecules also have a hydrophilic head that is attracted polar and charged molecules, such as water. Due to the hydrophobic tail of the soap molecules the water-soap drops that come out of the dropper are smaller than the water drops. The amount of the water and soap solute on the penny before it spills over is less than the amount of the amount of water that is able to stay together on top of the penny before it spills over because the soap weakens the cohesive force of the water molecules.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

1. Water molecules are polar, where the oxygen has a partial negative charge and the hydrogen atoms have a slightly positive charge, so they have a covalent force that holds them together. Predict what would happen to water on a window screen on a rainy day.

a. When it rains water collects on the window screen because the water molecules are attracted to each other partial charges.

2. Draw a diagram of how a soap and water bubble would look like. Explain what is happening in your diagram.

a. Students may draw something different, but the point is to get students thinking on how water molecules and soap/lipids interact. The diagram below shows how the hydrophilic heads of the soap molecules are attracted to the water molecule and the hydrophobic tails repel the water molecules.

b.

3. Describe how oil, a water repelling molecule can be removed from an oily pan with soap and water.

a. The soap removes the oil by surrounding the oil molecule with its hydrophobic molecules facing the oil molecules and its hydrophilic head facing the outside or water. The hydrophobic tail interacts with the oil because they are both water hating and nonpolar. The hydrophilic tail interacts with the water molecules.

Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.

1. Cohesion happens during cloud formation. In the air and the earth’s surface evaporate to form clouds. These evaporated molecules condense due to the cohesive property of water molecules and form clouds.

2. The transport of water in plants is facilitated with the cohesiveness and adhesiveness of water through the xylem vessels in a plant. The water molecules are absorbed by the plants roots and go up to through the xylem and exit the plant via the stomata when the water molecules absorb enough energy to evaporate. The water molecules below the evaporated molecules take the place of the evaporated molecules due to adhesive and cohesive forces by moving up while the roots absorb more water to take the place of the water lost.

3. Water striders are able to stay afloat on water. Water striders are small enough to stay afloat on water with the help of water repelling hairs on their legs.

a. http://www.bugsurvey.nsw.gov.au/html/popups/bpedia_09_tol_wa-st.html

Photographs:

Videos:

References:

(2010). Home Demos: Penny Droplets. <http://www.billnye.com/for-kids-teachers/home-demo-details/?homedemo=Penny+Droplets&start=3&category=Chemistry> 2010, November, 15.