C
CraftExam
medium difficulty6 min

Physical Properties and Hydrogen Bonding

Hydrogen bonding in alcohols and phenols increases boiling point and supports water solubility, especially for lower members.

In one line: O-H raises boiling point; long carbon chain lowers water friendship.

Practice This Concept

Understand it

  • Alcohols and phenols have polar O-H bonds, so molecules can form intermolecular hydrogen bonds.
  • This makes their boiling points higher than comparable hydrocarbons and ethers of similar molar mass.
  • Lower alcohols are soluble in water because they can hydrogen-bond with water, but solubility decreases as the non-polar hydrocarbon chain becomes larger.
  • Ethers can accept hydrogen bonds from water but cannot donate hydrogen bonds to each other because they lack an O-H bond.

Example

Ethanol is much more soluble in water than pentanol because the ethyl group is small, while the pentyl group has a larger hydrophobic part.

Rule to remember

Trend rule: stronger intermolecular hydrogen bonding gives higher boiling point; water solubility decreases as the alkyl chain length increases.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1Compare water solubility of ethanol and hexan-1-ol.
  2. 2Both contain -OH, but hexan-1-ol has a much larger non-polar chain, so ethanol is more soluble.

Method you can reuse

Check whether the molecule has an O-H bond, decide if it can donate and accept hydrogen bonds, compare alkyl chain size, then state the property trend.

Visual tip

A simple hydrogen-bond sketch between alcohol molecules or between alcohol and water is useful but not essential.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does ethanol have a higher boiling point than dimethyl ether though both have the same molecular formula?

Think first, then show answer
Ethanol molecules form intermolecular hydrogen bonds through O-H groups, while dimethyl ether lacks an O-H bond and cannot self-associate in the same way.

Common wrong answer

Saying ethers are completely unable to hydrogen-bond is incorrect; ethers can accept hydrogen bonds from water but do not self-associate like alcohols.

Don't mix this up

Students often compare boiling points using molar mass only and ignore intermolecular hydrogen bonding.

Exam tip

For boiling point and solubility, always balance two effects: hydrogen bonding increases attraction, while a larger hydrocarbon group reduces water solubility.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions usually ask for boiling point order, solubility trend, or explanation of anomalous physical properties.
  1. 1

    Enough for 1 mark

    Hydrogen bonding in alcohols and phenols increases boiling point and supports water solubility, especially for lower members.

  2. 2

    Add for 2 marks

    Example: Ethanol is much more soluble in water than pentanol because the ethyl group is small, while the pentyl group has a larger hydrophobic part.

  3. 3

    Add for 3 marks

    Alcohols and phenols have polar O-H bonds, so molecules can form intermolecular hydrogen bonds.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results
Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?

Practice This Concept