Physical Properties of Ethanol

Appearance and State

Ethanol is a colourless liquid at room temperature. Unlike water which has no smell, ethanol has a characteristic odour that you can easily recognize. This is the smell you notice when you open a bottle of methylated spirit or hand sanitizer.

The liquid is clear and transparent. If you see coloured alcohol drinks like beer (golden), wine (red or white), or whisky (brown), the colour comes from other substances added during production, not from the ethanol itself.

Smell and Taste

Characteristic Smell

Ethanol has a distinct smell that is easy to recognize once you know it. This is the smell of:

• Hand sanitizer used in hospitals and schools
• Methylated spirit (used for cleaning)
• Alcoholic drinks
• Some perfumes and aftershaves

The smell is often described as sharp or pungent. It is not unpleasant but is very noticeable.

Taste

Pure ethanol has a burning taste. This is why strong alcoholic drinks feel hot in your mouth and throat. When mixed with water and other ingredients, the taste becomes milder.

Warning: Industrial ethanol (methylated spirit) should never be drunk because it contains poisonous additives like methanol.

Boiling Point

Ethanol boils at 78°C. This is a very important property for several reasons:

Why 78°C is Important

1. Separation from water: Since water boils at 100°C and ethanol at 78°C, we can separate them by distillation. This is how ogogoro (local gin) makers purify alcohol.
2. Volatility: The low boiling point means ethanol evaporates easily. This is why hand sanitizer dries quickly on your hands.
3. Storage: Bottles of ethanol must be kept sealed or the alcohol will evaporate into the air.

Why is the Boiling Point Relatively High?

You might wonder: ethanol is a small molecule (C₂H₅OH), so why doesn’t it boil at a much lower temperature like methane (CH₄) which boils at -161°C?

The answer is hydrogen bonding.

Ethanol molecules have an -OH group (hydroxyl group). The hydrogen in this group can form a special type of attraction called a hydrogen bond with the oxygen in another ethanol molecule:

C₂H₅O-H ····· O-H-C₂H₅

These hydrogen bonds (shown as ····) are stronger than ordinary intermolecular forces. More energy is needed to break them, so ethanol needs higher temperature to boil.

Comparison with Similar Molecules

Notice that molecules with -OH groups (alcohols) have much higher boiling points than similar molecules without the -OH group. This proves that hydrogen bonding is responsible for the high boiling point.

Solubility in Water

Ethanol is completely miscible with water. This means you can mix ethanol and water in any proportion and they will dissolve completely. You will never see two separate layers.

Why is Ethanol Soluble in Water?

Both ethanol and water have -OH groups. These groups can form hydrogen bonds with each other:

C₂H₅O-H ····· H-O-H

When ethanol molecules break apart from each other and mix with water molecules, they form new hydrogen bonds with water. This releases energy, making the mixing process favourable.

Practical Examples in Nigeria

• When you add water to ogogoro or whisky, they mix completely
• Hand sanitizer (60-70% ethanol) is mixed with water and gel
• Methylated spirit sold in shops is usually 70% ethanol, 30% water
• Palm wine is mostly water with some ethanol dissolved in it

Solubility in Other Solvents

Ethanol is also soluble in:

• Organic solvents: Like ether, chloroform, benzene
• Oils and fats: This is why ethanol is used in perfumes (dissolves fragrant oils)

We say ethanol is both hydrophilic (water-loving) and lipophilic (fat-loving). This makes it a very useful solvent.

Effect on Litmus Paper

Ethanol is neutral to litmus paper. This means:

• Red litmus paper stays red
• Blue litmus paper stays blue
• pH is approximately 7

Ethanol is neither an acid nor a base. It does not produce H⁺ ions or OH^– ions in water.

Why is This Important?

Some students confuse ethanol with ethanoic acid (CH₃COOH) which is acidic. Remember:

• Ethanol (C₂H₅OH): Neutral, no effect on litmus
• Ethanoic acid (CH₃COOH): Acidic, turns blue litmus red

Volatility

Volatile means a liquid evaporates easily at room temperature. Ethanol is very volatile because of its low boiling point (78°C).

Evidence of Volatility

1. Hand sanitizer dries quickly: When you rub sanitizer on your hands, the ethanol evaporates in seconds, leaving your hands dry.
2. Cooling effect: When ethanol evaporates from your skin, it takes heat energy with it. This is why methylated spirit feels cold when applied to your skin.
3. Storage problems: If you leave a bottle of methylated spirit open, the level goes down as ethanol evaporates into the air.
4. Flammability: The vapour from ethanol catches fire easily. This is why you must keep ethanol away from flames.

Practical Uses of Volatility

• In perfumes – alcohol evaporates quickly, leaving fragrance on skin
• In thermometers – some thermometers use coloured ethanol instead of mercury
• In hand sanitizers – quick drying is convenient
• In cleaning – evaporates without leaving residue

Density

Ethanol has a density of 0.789 g/cm³ at 20°C. This is less than water (1.0 g/cm³), which means ethanol is lighter than water.

However, when you mix ethanol and water, you don’t get two layers (unlike oil and water) because they are miscible.

Melting Point

Pure ethanol freezes at -114°C. This extremely low melting point means ethanol remains liquid even in very cold conditions.

This is why methylated spirit is used in car windscreen washers in cold countries – it won’t freeze in winter like water would.

Viscosity

Ethanol is slightly more viscous (thicker) than water, but still flows easily. When you pour methylated spirit, it flows smoothly like water.

Summary Table of Physical Properties

Practice Questions

Multiple Choice Questions

1. What is the boiling point of pure ethanol?

(a) 65°C
(b) 78°C ✓
(c) 100°C
(d) 110°C

2. The relatively high boiling point of ethanol compared to ethane is due to:

(a) Ionic bonding
(b) Covalent bonding
(c) Hydrogen bonding ✓
(d) Metallic bonding

3. What is the effect of ethanol on litmus paper?

(a) Turns red litmus blue
(b) Turns blue litmus red
(c) No effect on either colour ✓
(d) Bleaches the litmus paper

4. Why is ethanol soluble in water?

(a) It is less dense than water
(b) It has a low boiling point
(c) It can form hydrogen bonds with water ✓
(d) It is colourless

5. Which of the following is NOT a physical property of ethanol?

(a) Colourless liquid
(b) Boiling point of 78°C
(c) Burns with a blue flame ✓
(d) Miscible with water

Essay Questions

1. (a) State three physical properties of ethanol. (3 marks)

(b) Explain why ethanol has a higher boiling point than ethane. (3 marks)

Tips for answering:

• Part (a): Pick any three from: colourless, boiling point 78°C, volatile, miscible with water, neutral to litmus, characteristic smell
• Part (b): Must mention hydrogen bonding in ethanol due to -OH group, while ethane has only weak van der Waals forces. More energy needed to break hydrogen bonds.

2. A student left a beaker containing 50 cm³ of ethanol on the laboratory bench for two hours. When he returned, the volume had decreased to 35 cm³.

(a) Explain this observation. (2 marks)

(b) Name the physical property of ethanol responsible for this observation. (1 mark)

(c) How can the student prevent this from happening in future? (1 mark)

Tips for answering:

• Part (a): Ethanol evaporated into the air because it is volatile and has a low boiling point (78°C)
• Part (b): Volatility
• Part (c): Cover the beaker or use a bottle with a tight stopper

3. (a) Explain why ethanol is miscible with water in all proportions. (3 marks)

(b) State two practical applications of the solubility of ethanol in water. (2 marks)

Tips for answering:

• Part (a): Both molecules have -OH groups. They form hydrogen bonds with each other. This strong intermolecular attraction allows complete mixing.
• Part (b): Production of alcoholic beverages; preparation of tinctures in medicine; making hand sanitizers; etc.

4. Complete the table below showing physical properties of ethanol: (5 marks)

Expected answers: Colourless liquid; 78°C; Completely miscible/soluble; No effect/neutral; Less dense/lighter

Related Topics

Explore more about ethanol and organic chemistry:

• Ethanol – General introduction and overview
• Preparation of Ethanol – How to make ethanol by hydration and fermentation
• Chemical Properties of Ethanol – Reactions with sodium, acids, oxidizing agents
• Uses of Ethanol – Industrial and domestic applications
• Hydrogen Bonding – Understanding this important intermolecular force