Cohesion and Adhesion- Properties of Water (Veronica Salcedo)
Title: Traveling Water Experiment
Principle(s) Investigated: Cohesion
Adhesion
Standards: HS-PS1-3 Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles
Materials: 2 cups
a long string (cotton twine or strings that absorb water)
food coloring (optional)
water
scotch tape
Procedure:
- Tape one end of the string to the inside bottom of a cup.
- Fill the other cup with water (and food coloring if you want) and put the free end of the string in the water (not taped).
- Lift the cup with water up above the other, but not directly over it. Hold it far enough that the string is taut. Be careful not to pull the string out of either cup.
- Slowly pour the water out of the top cup onto the string. Remember to keep it taut at all times. Observe what happens.
- Now, soak the entire string in water and try again. You should be able to pour the water straight from the top cup to the bottom cup through the string.
Student prior knowledge:
Explanation: This traveling water experiment has demonstrated two important properties of water.
Cohesion: Water molecules are attracted to one another and so they tend to stick together.
Adhesion: Water molecules are also attracted to molecules of other objects and so they tend to stick to objects such as a string.
When you first poured the water, although water molecules were attracted to the string, adhesion wasn’t as strong as gravity pulling the water down.
But when you tried again after soaking the string in water, water molecules were not only attracted to the string, but they were also attracted to the water molecules in the string.
Adhesion and cohesion together prevented the water from dripping straight down.
Instead, the water flowed along the string down to the bottom cup.
Questions & Answers:
1. The property of water that contributes to its ability to stick to certain surfaces is called ____________________.
a. gravity
b. adhesion
c. polymerization
d. parsimony
e. cohesion
Answer: adhesion
Water is a polar molecule, and thus can adhere to different surfaces; thus, adhesion is the correct answer here. Cohesion is close, as cohesion describes the ability of water to stick to itself due to its polarity. We want the property that allows water to stick to toher surfaces, not to itself. Polymerization involves chains of similar molecules, and does not occur in water. Parsimony is the principle that the simplest explanation is sually the reality of a situation. Gravity does not play into the properties of water. (Varisty Tutors)
2. What causes water's high heat capacity?
a. low boiling point
b. adhesion
c. cohesion
d. low density as s solid
Answer: cohesion
Cohesion is the result of increased strength of hydrogen bonding between water molecules. This increased strength requires a great amount of heat in order to break the hydrogen bonds between molecules, in order for these molecules to become vapor. Cohesion and hydrogen bonding are the cause for water's low vapor pressure, high boiling point, and high heat capacity. (Varsity Tutors)
3. Which phase of water is the most dense?
a. liquid
b. solid
c. gas
Answer: liquid
Water, unlike many other compounds, has several special properties due to its hydrogen bonding between molecules. The hydrogen bonds are relatively strong, leading water to have a very low vapor pressure and high surface tension. A side effect of the hydrogen bonding, however, is that when water crystallizes, the molecules will inevitably align so that the hydrogen bonds are maintained. The solid lattice structure of water molecules is, thus, not very tightly packed. The structure is ideal to maximize the intermolecular forces, rather than space and volume. The density of the solid (ice) is thus less than the density of the liquid water. (Varsity Tutors)
Applications to Everyday Life:
Cohesion is the term for molecules of a substance sticking together. One of the most common examples is water beading up on a hydrophobic surface. Water molecules are what are called dipoles: they have an electric 'pole' at each end of the molecule with opposite charges because the electrons in the molecule tend to congregate near the oxygen atom and away from the hydrogen atoms. Thus the negative part of one water molecule will attract the positive parts of other, nearby molecules. This is why water falls from the sky as raindrops, and not individual molecules, or why water tends to bead up on the hood of your freshly waxed car, or why you can cause water to bulge out over the rim of a glass if you fill it carefully; the molecules are all pulling together.
Water molecules are not only attracted to each other, but to any molecule with positive or negative charges. When a molecule attracts to a different substance, this is termed adhesion. Think about what happens when you dip one end of a piece of paper towel into a glass of water. The water will climb up the fibers of the paper, getting it wet above the level of the water in the glass. We know gravity is pulling down on the water, so why do they move up? Because the water molecules' positive and negative charges are attracted to the positive and negative charges in the cellulose molecules in the paper.
Note that both the examples above have both cohesion and adhesion occuring but one is stronger than the other. If the water molecules are more strongly attracted to each other than to the surrounding material, they bead up and try to get as close to each other as possible. If there is a stronger attraction to some other material, they spread out and try to get close to the other material.
Photographs: Include photos and diagrams that illustrate the how the investigation is performed.
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