Answer to Question #181226 in General Chemistry for Nonetheless

Question #181226


a. Put water in a glass up to its lid. Let it settle for a while on top of a table.

b. Get a teaspoon of starch and sprinkle it on a top of the water on a glass.

c. Observe the motion of the starch on the water. Record your observation as an answer to Hypothesis 1.

d. Get a glass of water and another glass with oil. Fill in equal amounts of water and oil.

e. Using 2 marbles, drop each marble on both liquids.

f. Observe the travel time of each marble on both liquids until it reaches the bottom. Record the time as your evidence of travel.

g. Use this data to answer Hypothesis 2.

Guide Questions:

1. What happen to starch as you sprinkle it on top of a glass of water?

2. How will you explain this result scientifically?

3. Will your observation be used in describing surface tension as a property of liquid? How?

Expert's answer

1. As the starch is sprinkled on top of the water, it is observed that the starch neither dissolves nor sink, it stays undisturbed on the surface of the water.

2. This observation can be explained scientifically using the kinetic theory of molecules. The intermolecular force of attraction between the starch molecules and the molecules of water is relatively low compared to the cohesive force between the water molecules and those between the starch molecules. For this reason, I here is almost no interaction between the starch molecules and the water molecules.

3. The observation can be used to describe surface tension as the property of liquid. The reason is that, the tangential force (surface tension) acting perpendicularly per unit length across the surface of water makes it behave as if it is covered with a stretched elastic skin which allows the starch to stay on it without sinking.

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