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Boiling point, melting point, viscosity, surface tension and vapor pressure as consequences of the forces between molecules — with the one property that runs the other way.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to rank substances by boiling point, viscosity, surface tension or vapor pressure from the forces acting between their molecules, and to give the reason as a separate answer from the ranking. You will be able to say which properties get larger as those forces get stronger and which get smaller, why vapor pressure is the one that runs the other way, and what to look at when two substances have the same formula, the same mass and the same forces.
From the last lesson you can look at a small molecule and say which forces act between one of them and the next: dispersion always, dipole-dipole when the molecule has a permanent dipole, and hydrogen bonding when a hydrogen sits on nitrogen, oxygen or fluorine. This lesson does not add a new force. It spends the one you already have, on six properties that all follow from it.
| Term | What it means |
|---|---|
| Boiling point | The temperature at which molecules break away throughout the liquid, not only at its surface. |
| Vapor pressure | The pressure of molecules escaped from a liquid into the space above it. |
| Viscosity | How much a liquid resists flowing. |
| Surface tension | How much a liquid's surface resists being broken. |
| Volatile | Having a high vapor pressure at room temperature. |
Every property in this lesson is a way of asking the same question: how hard is it to get a molecule away from its neighbors?
Five of them get larger when the forces get stronger, because each one is measuring the work needed to separate molecules:
| Property | What it measures | Stronger forces |
|---|---|---|
| melting point | loosening the molecules enough to slide | goes up |
| boiling point | separating the molecules altogether | goes up |
| energy to boil one mole | the same thing, in kilojoules | goes up |
| viscosity | how much the molecules cling as they slide past | goes up |
| surface tension | how hard the surface is to break | goes up |
One gets smaller:
| Property | What it measures | Stronger forces |
|---|---|---|
| vapor pressure | how many molecules have already escaped | goes down |
And how fast a puddle dries follows vapor pressure, because drying is escape from a surface.
That is the entire structure of the lesson. The one thing to check, every time, is which list the property is on — and the way to check is to ask what the number is counting. If it counts the work of separating, stronger forces make it bigger. If it counts what has already got out, stronger forces make it smaller.
Another way: picture
Picture a crowd holding hands. Melting is letting go enough to shuffle past each other; boiling is letting go altogether and walking off. Both are harder when the grip is tighter, and that is five of the six properties. Vapor pressure is a count of how many people have already left the room — and a tighter grip means fewer of them have, which is why that one number goes down as all the others go up.
Another way: steps
To predict any of these for two similar substances:
Name the forces. For each substance, decide which of dispersion, dipole-dipole and hydrogen bonding act.
Decide which is stronger. Different forces: hydrogen bonding beats dipole-dipole beats dispersion alone, as a rule. Same force: more electrons win, and for identical formulas, the less branched shape wins.
Ask what the property counts. The work of pulling molecules apart, or molecules already escaped.
Read off the direction. Work-of-separating properties rise with the forces; vapor pressure and drying rate fall.
Check the work. If boiling points are given, do they agree with your force ranking? Does your vapor-pressure ranking run exactly opposite to your boiling-point ranking? And have you given the reason separately from the ranking — a reason that would have produced the same order for heavier molecules by accident is not the reason.
Treating boiling point as a measure of the forces is allowed because boiling means giving molecules enough energy to escape their neighbors entirely. The harder they hold on, the more energy, and the higher the temperature needed.
Reading vapor pressure the other way is allowed because at any temperature the molecules in a liquid have a spread of energies. Only the most energetic break free. With weak forces, a larger share of them have enough, so more are in the vapor and the pressure is higher.
Using shape when everything else is equal is allowed because dispersion forces act where molecules touch. A straight chain lies alongside its neighbors over its whole length; a compact, branched molecule touches them at fewer points, so the total attraction is smaller.
Keeping the reason separate from the ranking is allowed, and needed, because one reason can give the right order by luck. Only a reason that names the force can be carried to a substance not on the list. That is the test to apply to any explanation you write: would it still give the right answer for a pair of substances you have never seen, where the heavier one happens to have the weaker forces?
It is possible to rank three liquids correctly by boiling point for a reason that is wrong, and to do it often enough that nobody notices.
The reason that usually does it is the heavier one boils higher. It gets the right answer whenever the three substances have the same kind of force between their molecules, which is most of the time, because a heavier molecule usually has more electrons and so larger dispersion forces. It then fails completely the first time the forces differ:
| Substance | Molar mass | Boiling point |
|---|---|---|
| $\mathrm{H_2S}$ | 34 | $-60$ °C |
| $\mathrm{H_2O}$ | 18 | $100$ °C |
The lighter substance boils a hundred and sixty degrees higher, because only one of the two can hydrogen bond.
That is why the items in this lesson ask the ranking and the reason separately, and why getting the ranking is not enough. A ranking you cannot justify is a ranking you cannot extend to a substance that was not on the list.
Three comparisons worth having in mind, because each of them breaks a rule somebody has half-learned.
Iodine boils at 184 °C on dispersion forces alone, which is eighty-four degrees above water. Dispersion is not the negligible case: iodine has a hundred and six electrons and they make a very large momentary dipole.
Ethanol and methoxymethane are the same atoms, the same mass and a hundred and three degrees apart, at $78$ °C and $-25$ °C. Nothing but the position of one hydrogen.
Pentane, 2-methylbutane and 2,2-dimethylpropane are all $\mathrm{C_5H_{12}}$, boiling at $36$, $28$ and $10$ °C. Same formula, same mass, same forty-two electrons, same force acting — so the twenty-six degree spread is shape and nothing else. A straight chain lies alongside its neighbors over its whole length; a branched molecule is closer to a sphere and touches them at fewer points.
The third of these is the one to remember when a question gives two substances with identical formulas: if mass, electrons and force are all equal, look at the shape.
American refiners change the recipe of gasoline twice a year, and vapor pressure is the reason. Gasoline is a mixture of hydrocarbons with dispersion forces only, and the lighter ones, such as butane with 34 electrons and a boiling point of $-1$ °C, have high vapor pressures. They are what lets an engine start: a cold engine needs some fuel to evaporate before the spark can ignite it.
In a Minnesota January, refiners add more butane so that enough fuel evaporates to start a cold car. In summer, the same blend would evaporate far too easily in a hot tank and fuel lines, releasing vapors that form smog. So from June to mid-September the Environmental Protection Agency limits gasoline's vapor pressure, and refiners take out much of the butane and use heavier, lower-vapor-pressure components instead.
The rule they are applying is this lesson's: weaker forces between molecules mean a higher vapor pressure. A single extra carbon raises a hydrocarbon's boiling point by about thirty degrees and lowers its vapor pressure accordingly, so adjusting the share of four-carbon and five-carbon molecules lets a refinery in Texas tune the fuel to the season it will be burned in.
Acetone, the main ingredient in most nail polish remover, has dipole-dipole forces but no hydrogen bonding, so it has a high vapor pressure and evaporates quickly. The molecules that escape carry energy away, which is why it feels cold on the skin.
Vapor pressure ranked the same way as boiling point. They run in opposite directions. A liquid with a high boiling point has a low vapor pressure at room temperature, and the two statements are the same statement.
Heavier assumed to boil higher. True when the same force acts in both substances, and false as soon as one of them can hydrogen bond. Check the force first and the mass second.
Bonds confused with forces. A bond and an intermolecular force are not the same size of thing. Boiling water pulls whole molecules apart from each other and leaves every $\mathrm{O-H}$ bond exactly where it was, which is why water boils at a hundred degrees and does not decompose there. A substance with strong bonds inside its molecules can still boil low, and the question which forces act between the molecules is a different question from what holds each molecule together.
Volatility treated as a separate idea. A volatile liquid is one with a high vapor pressure, which is one with weak forces between its molecules, which is one with a low boiling point. Four ways of saying one thing.
Shape forgotten when everything else is equal. If two substances have the same formula, the same mass and the same forces, the only thing left that can differ is how closely their molecules can lie against each other.
Name propanone's strongest force.
$\text{dipole-dipole}$
A permanent dipole, no hydrogen on oxygen.
Name water's strongest force.
$\text{hydrogen bonding}$
Hydrogen on oxygen.
Compare the forces.
$\text{water's are far stronger}$
Compare the forces before the property.
Ask what drying follows.
$\text{vapor pressure, against the forces}$
Drying is escape from a surface.
Check with the boiling points.
$56 < 100 \ ^\circ\mathrm{C}$
Propanone dries first; the same fact from the other end.
Name the force in butane, pentane and hexane.
$\text{dispersion only}$
Carbon and hydrogen only.
Count the electrons.
$34, \ 42, \ 50$
Same force, so the electron count decides.
Write the ranking.
$-1, \ 36, \ 69 \ ^\circ\mathrm{C}$
About thirty degrees per carbon.
Compare pentane's isomers.
$36, \ 28, \ 10 \ ^\circ\mathrm{C}$
Same formula, electrons and force.
Find the spread.
$36 - 10 = 26$
The electron count cannot explain it.
Name the cause.
$\text{branching reduces contact}$
When the usual reason runs out, look at shape.
Name water's strongest force.
$\text{hydrogen bonding}$
Hydrogen on oxygen.
Name hexane's strongest force.
$\text{dispersion only}$
Carbon and hydrogen only.
Decide which is stronger.
$\text{water's}$
Hydrogen bonds outweigh dispersion here.
Check with the boiling points.
$100 > 69 \ ^\circ\mathrm{C}$
The order agrees.
Predict the surface tension.
$\text{water's is higher}$
A work-of-separating property.
Predict the vapor pressure.
$\text{hexane's is higher}$
An escape property, so it runs the other way.
Predict which puddle dries first.
$\text{hexane}$
Drying follows vapor pressure.
Compare the forces.
$\text{ethanol's are stronger}$
Hydrogen bonding against dispersion only.
Decide the viscosity.
Decide the vapor pressure.
Put these three substances in order of boiling point, lowest first.
Number the steps in order (write the number in the box):
Complete the worked solution: butane boils at $-1$ °C, pentane at $36$ °C and hexane at $69$ °C. All three have dispersion forces only. Find the rise from butane to pentane, from pentane to hexane, and from butane to hexane.
Compare pentane with butane.
$(\text{pentane}) - (\text{butane}) =$ a
One more carbon, more electrons.
Compare hexane with pentane.
$(\text{hexane}) - (\text{pentane}) =$ b
Another carbon, a similar rise.
Compare hexane with butane.
$(\text{hexane}) - (\text{butane}) =$ c
The two rises together.
Two open dishes stand side by side in the same room. One holds a liquid that boils at $36$ °C and the other a liquid that boils at $100$ °C. Which has the higher vapor pressure in the room, and why?
Put these three substances in order of their vapor pressure at room temperature, lowest first.
Number the steps in order (write the number in the box):
pentane and 2,2-dimethylpropane are both $\mathrm{C_5H_{12}}$, with the same mass and the same $42$ electrons. pentane boils at $36$ °C and 2,2-dimethylpropane at $10$ °C. How many degrees apart are they?
Answer:
A nail salon in Los Angeles keeps water, which boils at $100$ °C, and ethanol, which boils at $78$ °C. How many degrees higher does water boil?
The answer: a.
A company that makes printing inks is changing the solvent in a product. The new solvent is like the old one in every way except that the forces between its molecules are stronger. What happens to vapor pressure at room temperature?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
The boiling points of these three substances are given. For each one, say how the forces between its molecules compare with the other two, and how its vapor pressure at room temperature compares. Use each of the three ranks once in each column.
| boiling point, in degrees Celsius | forces between its molecules | vapor pressure at room temperature | |
|---|---|---|---|
| propanone | 56 | ||
| butane | -1 | ||
| propan-1-ol | 97 |
You can go from which force acts to how a substance behaves in bulk, and back. Say out loud why a liquid with a high boiling point has a low vapor pressure, and why three substances with the formula $\mathrm{C_5H_{12}}$ boil twenty-six degrees apart. Next: gases, where the forces between molecules are the thing the simplest model leaves out.
15. Your turn: ethanol boils at $78$ °C and hexane at $69$ °C. Which has the higher vapor pressure at room temperature, and which is the more viscous?, step 2
$\text{ethanol more viscous}$
Viscosity goes with the forces.
15. Your turn: ethanol boils at $78$ °C and hexane at $69$ °C. Which has the higher vapor pressure at room temperature, and which is the more viscous?, step 3
$\text{hexane higher}$
Vapor pressure runs against the forces.