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Chapter 12: Problem 147

When soap molecules are dissolved in water in a pool, the soap molecules migrate to the surface of the water and actually coat the surface, with the polar heads being dissolved in the water and the non-polar tails sticking out into the air above the surface of the water. (a) Draw the scenario just described. (b) Using the rule of thumb "like dissolves like," explain why the soap molecules at the water's surface align as described. (c) Water bugs can "walk" on the surface of a pool's water, but if you put soap in the water, they fall through. Why do you think this is?

Short Answer

Expert verified
(a) To draw the scenario, imagine a horizontal line representing the water's surface with soap molecules on top. Draw the soap molecules as small objects with a circle (the polar head) and a line (the non-polar tail). The polar heads touch the water surface, and the non-polar tails extend into the air above. (b) The rule "like dissolves like" implies substances with similar polarity tend to dissolve each other. Water attracts the soap's polar heads, while the non-polar tails are repelled by water. This causes the alignment of soap molecules with their polar heads in the water and non-polar tails in the air. (c) Water bugs walk on water due to surface tension caused by hydrogen bonding between water molecules. Soap disrupts the hydrogen bonding, decreases surface tension, and weakens the surface's supporting "skin." As a result, water bugs can no longer walk on the surface and fall through.

Step by step solution

01

Draw the scenario

First, imagine soap molecules at the water's surface. Soap molecules have a polar head and a non-polar tail. When dissolved in water, the polar heads will be attracted to the water, while the non-polar tails will be repelled by the water and stick out into the air above the water's surface. To represent this, draw the water's surface (a horizontal line) with soap molecules on top. Draw the soap molecules as small objects with a circle (the polar head) and a line (the non-polar tail). Ensure that the polar heads are touching the water surface, and the non-polar tails are extending into the air above the water surface.
02

Explain the alignment using "like dissolves like"

The rule of thumb "like dissolves like" means that substances with similar polarity tend to dissolve each other. Water is a polar substance due to its molecular structure, so it will attract the polar heads of the soap molecules. On the other hand, the non-polar tails of the soap molecules are repelled by the polar water molecules. Because the polar heads are attracted to the water and the non-polar tails are repelled, the soap molecules align with their polar heads in the water and their non-polar tails in the air.
03

Discuss the effect on water bugs

Water bugs can walk on the surface of the water due to the water's surface tension, which is caused by the hydrogen bonding between water molecules. The surface tension creates a thin, elastic "skin" that supports the water bug's weight without breaking. When soap is added to the water, the soap molecules disrupt the hydrogen bonding between the water molecules. This decreases the surface tension and weakens the "skin" on the surface. As a result, water bugs can no longer walk on the surface, because the weakened surface tension cannot support their weight, causing them to fall through.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Polar and Non-Polar Molecules
In chemistry, molecules have different types of bonds which result in varying polarity. Polar molecules, like water, have areas of positive and negative charge due to an uneven distribution of electrons. This creates a dipole moment, with both a negative and positive pole.

Non-polar molecules, on the other hand, have electrons distributed more evenly and do not have distinct poles. They are electrically neutral overall with no dipole moment. The difference in polarity affects how substances interact with each other. Polar substances tend to interact and mix well with other polar substances, whereas non-polar substances mix well with other non-polar substances.
Surface Tension
Surface tension is the elastic tendency of liquids which makes them acquire the least surface area possible. This phenomenon occurs because molecules at the surface of a liquid are attracted more strongly to each other than they are to the air above, resulting in an inward force. This inward force creates a sort of 'skin' at the liquid's boundary, capable of supporting light objects.

Surface tension is why certain insects can 'walk on water'; the water's surface tension is strong enough to hold their weight without sinking. It's like a stretched trampoline – the surface is firm enough to support the items on top due to the cohesive forces between the liquid molecules.
Like Dissolves Like Principle
The 'like dissolves like' principle is a basic rule in chemistry that is used to predict and explain why certain substances are soluble in others. It states that polar substances tend to dissolve in polar solvents, while non-polar substances dissolve in non-polar solvents.

For example, oil, which is non-polar, will not mix with water, a polar liquid. This principle is essential for understanding why soap molecules behave the way they do in water – the polar heads of soap are soluble in water due to the alignment of charges, while the non-polar tails are not, causing them to stick out away from the water.
Hydrogen Bonding
Hydrogen bonding is a special type of attraction that occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom such as oxygen or nitrogen, experiences the electrical pull from another electronegative atom nearby. This creates a bond that, while much weaker than a covalent bond, is stronger than other intermolecular forces such as van der Waals forces.

Hydrogen bonds are responsible for many of the unusual and vital properties of water, such as its high boiling point and its ability to expand upon freezing. They also contribute to the surface tension in water by holding the water molecules tightly together, forming a barrier that small insects can walk on.

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Most popular questions from this chapter

You have two solutions, one \(0.650 \mathrm{M}\) iron(III) nitrate and the other \(1.500 \mathrm{M}\) ammonium carbonate. (a) Write a net ionic equation for the precipitation reaction that occurs when these solutions are combined. (b) If you pour \(200.0 \mathrm{~mL}\) of each solution into the same flask, what is the theoretical yield of the precipitate in grams? (c) What is the molar concentration of the excess reactant ion?

The ionic lattice in which compound requires the most energy to break: \(\mathrm{KCl}, \mathrm{Mg}(\mathrm{OH})_{2}\), or \(\mathrm{NaNO}_{3}\) ? Explain your answer.

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The dissolving of a solute into a solvent can be thought of as a three-step process. The drinking alcohol, ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{OH}\right)\), is infinitely soluble in water. However, another alcohol called pentanol, with the molecular formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH}\), is nearly insoluble in water. (a) Which of the three steps in the solubility model is most responsible for this difference in solubility? Describe the step you choose (specify its name). (b) Give a BRIEF explanation for your choice.

Calculate the molarity of pure water at \(4.0^{\circ} \mathrm{C}\). The density of water at \(4.0^{\circ} \mathrm{C}\) is \(1.0000 \mathrm{~g} / \mathrm{mL}\).
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