All subjects · Chemistry · WCH13 Practical Skills in Chemistry I

Preparing and purifying an organic liquid (Core Practicals 6 and 7)

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Organic preparation questions give you a method and ask why each step is done, what the apparatus is called, and how to work out the yield. The chemistry is in the Unit 2 lessons on alcohols, oxidation and purification.

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The usual sequence

1. React: heat under reflux, shake at room temperature, or distil out as it forms.

2. Separate: in a separating funnel. Identify the layers by density: the organic layer is usually on top, but halogenoalkanes such as bromoethane are denser than water and form the lower layer. Open the tap to run off the lower layer. Invert and open the tap regularly to release pressure.

3. Wash: with sodium hydrogencarbonate solution to remove acid (CO2 forms, so release the pressure), then with water.

4. Dry: add anhydrous calcium chloride, magnesium sulfate or sodium sulfate until the liquid is clear, not cloudy; decant or filter. These are drying agents (they absorb water), not dehydrating agents.

5. Distil: collect the fraction that boils within a degree or two of the product's boiling temperature.

Core Practicals 6 and 7 in brief

CP6: 2-methylpropan-2-ol shaken with concentrated hydrochloric acid at room temperature gives 2-chloro-2-methylpropane, which separates as an upper layer. Wash with NaHCO3, dry with anhydrous CaCl2, distil.
CP7: propan-1-ol oxidised by acidified dichromate(VI). For propanal, distil it out as it forms so it is not oxidised further (it has the lowest boiling point). For propanoic acid, heat with excess oxidising agent under reflux, then distil.

Yield

Mass of a liquid reactant = volume x density.
Theoretical yield from the reacting ratio; % yield = actual / theoretical x 100.
Yields are low because of incomplete reaction, side reactions, loss of volatile product, and losses in each transfer, separation and distillation.
A product that is still wet gives a mass that is too high.

Worked example

10.0 cm³ of 2-methylpropan-2-ol (density 0.786 g cm^-3, M = 74.0) gives 6.20 g of 2-chloro-2-methylpropane (M = 92.5). Calculate the % yield.

Mass of alcohol = 10.0 x 0.786 = 7.86 g; n = 7.86 / 74.0 = 0.1062 mol
Theoretical mass = 0.1062 x 92.5 = 9.82 g
% yield = 6.20 / 9.82 x 100

Answer: 63.1%

Key idea: React, separate (by density), wash with NaHCO3, dry until clear, distil over a narrow range. Distil propanal out as it forms; reflux for propanoic acid. Mass of a liquid = volume x density.

Check yourself

10.0 cm³ of butan-2-ol (density 0.808 g cm^-3, M = 74.0) gives 2.64 g of 2-chlorobutane (M = 92.5). Calculate the percentage yield.
Show answer
26.1 %. n = 10.0 x 0.808 / 74.0 = 0.1092 mol; theoretical = 10.10 g; % yield = 2.64 / 10.10 x 100 = 26.1%.
Why is the crude product shaken with sodium hydrogencarbonate solution?
- To dry the product
- To oxidise impurities
- To neutralise and remove acid left in the product
- To raise the boiling point
Show answer
To neutralise and remove acid left in the product. Acid reacts to give a salt (which dissolves in water) and CO2.
Bromoethane is denser than water. How is it collected from a separating funnel?
- Open the tap and run off the lower layer
- Pour off the upper layer from the top
- Shake until there is one layer
- Distil the whole funnel
Show answer
Open the tap and run off the lower layer. The denser liquid forms the lower layer (June 2024 report).
How can you tell an organic liquid has been dried by anhydrous calcium chloride?
- The liquid turns blue
- The liquid starts to boil
- The drying agent dissolves completely
- The liquid turns from cloudy to clear
Show answer
The liquid turns from cloudy to clear. Water droplets make the liquid cloudy.
Why is propanal distilled off as soon as it forms?
- Because it is a catalyst
- To stop it being oxidised further to propanoic acid
- To stop it reacting with the alcohol
- Because it decomposes at room temperature
Show answer
To stop it being oxidised further to propanoic acid. Removing the aldehyde stops further oxidation.
Which technique is used to make propanoic acid from propan-1-ol?
- Distil with a limited amount of oxidising agent
- Shake with concentrated hydrochloric acid
- Warm with Fehling's solution
- Heat with excess acidified dichromate(VI) under reflux
Show answer
Heat with excess acidified dichromate(VI) under reflux. Reflux keeps the aldehyde in the flask so it is fully oxidised.
How does the final distillation help to purify the product?
- It removes water by reaction
- Only the liquid boiling over a narrow range near the product's boiling temperature is collected
- It increases the yield
- It neutralises acids
Show answer
Only the liquid boiling over a narrow range near the product's boiling temperature is collected. Impurities with other boiling points are left behind or collected separately.

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Last updated 8 November 2026. Spotted a mistake? Tell us.