Physical Chemistry : Properties of Liquids

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Properties Of Liquids

An unknown molecule (molecule A), in its solid phase, is found to have a density of . Eight grams of this molecule is added to a cubic container with length of . The container is then heated until all of the solid has melted. What can you conclude from the given information?

Possible Answers:

Solid will not fit in the container

No liquid will overflow

The container will be about halfway full with liquid

Liquid will overflow

Correct answer:

Liquid will overflow

Explanation:

The dimensions of the cubic container are  by  by  (cube has same length, height, and width); therefore, the volume of the cubic container is

Recall that  is the same as ; therefore, the container can contain  of volume. This means that all of the solid will fit into the container. Upon melting, the solid will expand and the volume will increase (as it becomes liquid). This means that the volume of the container will not be sufficient for the liquid and, consequently, lead to an overflow of the liquid.

Example Question #2 : Properties Of Liquids

Which of the following is true regarding the liquid phase?

I. Liquids have lower entropy than gases

II. Compared to solids, more energy is required to separate liquid molecules

III. Liquids have higher intermolecular forces than gases

Possible Answers:

I only 

I, II and III

III only

I and II

Correct answer:

I, II and III

Explanation:

Liquid is an intermediate phase, between solid and gas. Solids are characterized by tightly packed molecules in an organized manner whereas gases are characterized by greatly spread out molecules that are highly disordered; liquids lie somewhere in the middle. This means that they are more disordered than solids (more entropy) and less disordered than gases (less entropy).

The boiling point of a substance is always higher than the melting point; therefore, you always need higher energy to break the interactions between liquid molecules.

The key difference between a liquid and a gas is the distance between the molecules. Liquid molecules are closer together whereas gas molecules are spread apart. Intermolecular forces are the forces between adjacent molecules. Since they are highly spread apart, the gas molecules have lower intermolecular forces.

Example Question #2 : Properties Of Liquids

Which of the following are responsible for the high boiling point of water, compared to molecules of similar size?

Possible Answers:

Dipole-dipole interactions

Positive charge

Hydrogen bonding

Two lone pairs of electrons

Molecular size

Correct answer:

Hydrogen bonding

Explanation:

Hydrogen bonding occurs between water molecules. While intermolecular forces between molecules exist for most molecules, these forces are much weaker than the bond formed between a hydrogen from one molecule and an oxygen, fluorine, and/or nitrogen atom of another molecule. Since the molecules are held more tightly together, more energy is required to break those bonds. This results in a higher temperature required to boil water (boiling breaks bonds between molecules, and this causes the molecules to escape into the gas phase).  

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