Place the followingsubstances in order of decreasing boiling point
Understanding how to arrange chemical substances by their boiling points is a fundamental skill in chemistry, chemical engineering, and everyday laboratory work. Boiling point reflects the temperature at which a liquid’s vapor pressure equals the surrounding atmospheric pressure, and it is governed by intermolecular forces, molecular weight, polarity, and molecular shape. Practically speaking, this article walks you through the concepts, provides a step‑by‑step method for ranking substances, and illustrates the process with a concrete example list. By the end, you will be able to confidently place any set of substances in order of decreasing boiling point and explain why the order makes sense Easy to understand, harder to ignore..
1. What Determines a Substance’s Boiling Point?
Before ranking, it helps to know the key factors that raise or lower a liquid’s boiling temperature.
| Factor | How It Influences Boiling Point | Typical Examples |
|---|---|---|
| Intermolecular forces (IMFs) | Stronger IMFs → higher boiling point. Still, | |
| Molecular shape & symmetry | Linear or less‑branched molecules pack more efficiently, increasing surface contact and LDF strength → higher bp. neopentane (bp ≈ 9.And | n‑Pentane (bp ≈ 36 °C) vs. 5 °C). |
| Molecular weight / size | Larger, heavier molecules have more electrons → stronger London dispersion forces → higher boiling point (when IMFs are similar). Hydrogen bonding > dipole‑dipole > London dispersion. Worth adding: | |
| Presence of hydrogen bonding | H‑bonding is a special, strong dipole‑dipole interaction that can dramatically increase bp. But branching lowers bp. Practically speaking, | |
| Polarity | Polar molecules engage in dipole‑dipole interactions, raising bp relative to non‑polar analogues of similar mass. Consider this: | Ethanol (bp ≈ 78 °C) vs. n‑Butane (C₄H₁₀, bp ≈ –0.But |
Understanding these contributors lets you predict trends without needing experimental data for every compound Not complicated — just consistent..
2. Step‑by‑Step Method to Order Substances by Decreasing Boiling Point
Follow this systematic approach whenever you need to rank a group of liquids or solids that melt before boiling.
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List the substances and note their molecular formulas.
Write down each compound’s formula; this gives you immediate clues about size and possible functional groups Most people skip this — try not to.. -
Identify the dominant intermolecular force for each substance.
- Does it contain O‑H, N‑H, or F‑H bonds? → hydrogen bonding likely.
- Does it have a carbonyl, nitrile, or halogen substituent? → dipole‑dipole possible.
- Is it a hydrocarbon or a symmetric noble gas? → mainly London dispersion.
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Compare molecular weights when IMFs are similar.
If two substances both rely only on LDF, the heavier one will have the higher bp Not complicated — just consistent.. -
Account for shape and branching.
For comparable weights, choose the less‑branched (more linear) isomer as the higher‑boiling candidate. -
Rank from strongest to weakest overall IMF contribution. Place the substance with the strongest net IMF at the top (highest bp) and work downwards Easy to understand, harder to ignore..
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Verify with known data (if available).
Check a reliable source (textbook, NIST database) for a few borderline cases to confirm your reasoning. -
Write the final ordered list, using “>” to indicate “has a higher boiling point than.”
Example: Substance A > Substance B > Substance C Small thing, real impact..
3. Example: Ranking Six Common Substances
Let’s apply the method to the following set (the “following substances” the title refers to):
- Water (H₂O)
- Ethanol (C₂H₅OH)
- Acetone (CH₃COCH₃)
- n‑Hexane (C₆H₁₄)
- Carbon dioxide (CO₂)
- Methane (CH₄)
Step 1: Formulas & Basic Info
| Substance | Formula | Approx. Molar Mass (g/mol) |
|---|---|---|
| Water | H₂O | 18 |
| Ethanol | C₂H₅OH | 46 |
| Acetone | CH₃COCH₃ | 58 |
| n‑Hexane | C₆H₁₄ | 86 |
| Carbon dioxide | CO₂ | 44 |
| Methane | CH₄ | 16 |
Step 2: Dominant IMF
- Water – extensive hydrogen bonding (each molecule can donate two H‑bonds and accept two).
- Ethanol – hydrogen bonding (one –OH group) plus LDF.
- Acetone – dipole‑dipole (carbonyl group) + LDF; no H‑bond donor.
- n‑Hexane – only London dispersion forces (non‑polar hydrocarbon).
- Carbon dioxide – linear, non‑polar; primarily LDF (though quadrupole interactions exist, they are weaker than H‑bonding).
- Methane – only LDF, very small electron cloud.
Step 3: Compare Weights Where IMFs Similar
- Among the LDF‑only substances (n‑hexane, CO₂, CH₄), boiling point rises with molar mass: CH₄ (16) < CO₂ (44) < n‑hexane (86).
- Acetone (58) and ethanol (46) have comparable masses but differ in IMF type.
Step 4: Shape & Branching
- n‑Hexane is linear; no branching to consider.
- CO₂ is linear; CH₄ is tetrahedral but both are small, so shape effects are minor compared to mass.
Step 5: Overall IMF Strength Ranking
- Water – strongest IMF (hydrogen bonding network). 2. Ethanol – hydrogen bonding
