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General Chemistry · States of Matter and Intermolecular Forces

Intermolecular Forces

Why does water boil at 100 degrees Celsius while neon boils near minus 246, and salt not until well over a thousand? The answer is the strength of the forces between particles. Identify the dominant force in a wide range of substances, from London dispersion through dipole-dipole, hydrogen bonding, ion-dipole, and ionic attraction, judge its strength, and use it to predict and compare boiling points and the forces that raise them. In every part you predict the answer yourself, then reveal it and compare.

Theory — The Forces Between Particles

Within a molecule the atoms are held by strong covalent bonds, but the forces between particles decide whether a substance is a gas, a liquid, or a solid. The stronger these forces, the more energy is needed to pull the particles apart, and so the higher the melting and boiling points.

The kinds of attraction, from weakest to strongest

Types of intermolecular forceLondon dispersion: present in all substances and the only force between nonpolar molecules and noble gases; it grows stronger with more electrons and larger, more polarizable particles.
Dipole-dipole: between polar molecules, the positive end of one attracted to the negative end of another.
Hydrogen bonding: an especially strong dipole-dipole force when hydrogen is bonded to nitrogen, oxygen, or fluorine.
Ion-dipole: between an ion and a polar molecule, as when an ionic compound dissolves in water.
Ionic (ion-ion): the very strong attraction between oppositely charged ions in an ionic solid such as sodium chloride.
Stronger forces give higher melting and boiling points

A substance can show several of these at once. A polar molecule always has London dispersion as well as dipole-dipole, and a hydrogen-bonding substance such as water has all three. We name a substance by its dominant force, the strongest one present, because that is what mainly sets its boiling point.

Strength and boiling point

The order of strength runs roughly: London dispersion (weakest, but larger molecules make it stronger), then dipole-dipole, then hydrogen bonding, then ion-dipole, and finally full ionic attraction (strongest). The boiling points climb in the same order, from helium at 4 K through water at 373 K to molten sodium chloride above 1700 K.

Dominant forceExampleApprox. boiling point
London dispersionNeon, argon, methane27 to 112 K
Dipole-dipoleHCl, H₂S, acetone188 to 334 K
Hydrogen bondingAmmonia, ethanol, water240 to 391 K
Ion-dipoleNaCl dissolved in waterstrong; controls dissolving
Ionic (ion-ion)Sodium chloride, KBrabout 1700 K

Name the strongest

A substance may have several forces; the dominant, strongest one mainly sets its boiling point.

Size builds dispersion

For nonpolar substances, larger and more polarizable molecules have stronger London dispersion forces.

Ions are strongest

Hydrogen bonding beats ordinary dipoles, ion-dipole is stronger still, and full ionic attraction is strongest of all.

Apparatus

The equipment a real intermolecular-forces experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.

measures temperature
Thermometer
Tracks cooling from evaporation, which probes the force strength.
evaporates samples
Watch glass
Holds a drop of liquid as it evaporates.
reagent solutions
Reagent bottles
Dispense liquids of differing polarity and volatility.
holds solutions
Beaker
Holds liquids for boiling-point and miscibility tests.
heats and stirs
Hot plate
Compares boiling points linked to the forces present.
0.000 gmeasures mass
Analytical balance
Tracks mass loss during evaporation.

Instructions — Running the Virtual Experiment

This is a predict, reveal, and compare lab. For each substance you predict the answer yourself, enter it, and only then does the simulation reveal the result so you can compare. Work through at least eight substances spanning the different force types, and record every prediction in your worksheet.

Part A — Identify the Dominant Force (Forces tab)
1
Open Simulation → Forces and choose a substance from the list.
2
Predict whether it is nonpolar, polar, or ionic, and which intermolecular force dominates (London dispersion, dipole-dipole, hydrogen bonding, ion-dipole, or ionic), enter both, and click Check. The reveal also lists every force present.
Part B — Strength of the Force (Strength tab)
1
Open Strength. Predict how strong the dominant force is, from very weak to very strong, click Check, and read the revealed strength and boiling point. Stronger forces give higher boiling points.
Part C — Boiling Point and Its Cause (Compare tab)
1
Open Compare. Choose two substances and predict which boils at the higher temperature, or whether they are about the same.
2
Then predict which force raises the boiling point of the higher one, and click Check. The boiling points are revealed on a chart, and you confirm both your ranking and the force responsible. Work through enough pairs to compare all the force types.
For your reportInclude a table of the dominant force, its strength, and the boiling point for each substance, screenshots, and a discussion of how the type and strength of the intermolecular force set the boiling point.

Simulation — The Intermolecular Forces Bench

Intermolecular Forces Virtual LabChoose a substance, predict, then reveal and compare
SubstanceYour typeTypeYour dominant forceDominant force
No rows yet — choose a substance, predict, and check.

Substance

Selected: Neon
Type— hidden
Dominant force— hidden
All forces present— hidden

Substance

Selected: Neon
Dominant force— hidden
Strength— hidden
Boiling point— hidden

Compare two substances

Reasoning
Higher boiling point— hidden
Force responsible— hidden

Team Questions

Question 1. Which intermolecular force is the only one acting between noble-gas atoms such as neon and argon? (two words)
Question 2. Which force forms when an ionic compound such as NaCl dissolves in water, between the ions and the water molecules? (two words, hyphenated)
Question 3. What special force gives water and ethanol their unusually high boiling points? (two words)
Question 4. Why does bromine (Br₂) boil at a higher temperature than chlorine (Cl₂), even though both are nonpolar? Because bromine is larger and more ___ (one word).
Question 5. Which has the higher boiling point, sodium chloride (NaCl) or water (H₂O)? (formula)
Question 6. A molecule of HCl is polar. Which force dominates between HCl molecules? (two words, hyphenated)
Question 7 — Challenge. Rank these forces from weakest to strongest: hydrogen bonding, London dispersion, ionic. Give the weakest one. (two words)

Example Lab Report

A worked example showing the expected format and the predict, reveal, and compare workflow.

Intermolecular Forces

Chemistry | Section: [Your Section] | Date: [Date]

Lab Members: [Names of all members present]

Objective

To identify the dominant intermolecular force in a range of substances, to judge its strength, and to predict and compare boiling points, confirming that the type and strength of the force set the boiling point.

Results Table (worked example)

SubstanceDominant forceStrengthBoiling point (K)
NeonLondon dispersionVery weak27.1
ChlorineLondon dispersionModerate239.1
HClDipole-dipoleModerate188.1
EthanolHydrogen bondingStrong351.4
WaterHydrogen bondingStrong373.2
Sodium chlorideIonic (ion-ion)Very strong1738

The boiling points rose with the strength of the dominant force: weak dispersion in neon, stronger dispersion in chlorine, hydrogen bonding in ethanol and water, and full ionic attraction in sodium chloride. Comparing water and methane, hydrogen bonding was the force that raised water's boiling point.

Discussion and Conclusion

Every prediction agreed with the simulation. Nonpolar substances held only by London dispersion boiled lowest, and their boiling points rose with size; polar molecules with dipole-dipole forces boiled higher; hydrogen bonding pushed water, ethanol, and ammonia higher still; and the full ionic attraction in sodium chloride gave by far the highest boiling point. In each pair, the force that raised the boiling point was the dominant force of the higher-boiling substance.

Practice Questions

Question 1
Rank neon, chlorine, ethanol, and sodium chloride from lowest to highest boiling point and name the dominant force in each.
Hint: neon (London, 27 K) < chlorine (London, 239 K) < ethanol (hydrogen bonding, 351 K) < sodium chloride (ionic, 1738 K).
Question 2
Methane (CH₄) and water (H₂O) are both small molecules, yet water boils about 260 K higher. Which force is responsible?
Hint: hydrogen bonding in water, which methane (nonpolar, London only) cannot form.
Question 3
What force lets table salt dissolve in water, and between which particles does it act?
Hint: ion-dipole, between the Na⁺ and Cl⁻ ions and the polar water molecules.
Question 4
HCl is polar and Ar is nonpolar, with similar sizes. Which would you expect to boil higher and why?
Hint: HCl, because its dipole-dipole force adds to dispersion; HCl boils at 188 K versus argon at 87 K.
Question 5 — Challenge
Argon (87 K) and oxygen (90 K) have almost identical boiling points. Explain why, and why this differs from comparing argon with water.
Hint: argon and oxygen are both nonpolar with similar dispersion forces, so their boiling points are nearly equal; water has hydrogen bonding, far stronger, so it boils much higher.