Ch 4 · Separation and Purification · O Level Chemistry 5070
Topic 4 of 5 in Experimental Techniques and Chemical Analysis — O Level Chemistry 5070 — 48 lessons.
Introduction
Chemists rarely make a pure substance directly; they make a mixture and then separate it. Each technique exploits a difference in a physical property such as solubility or boiling point, and an exam question expects you to name the technique, describe it and explain why it works for that mixture. Melting and boiling points then tell you how pure the product is.
Key ideas — Using a solvent, filtration and crystallisation
Dissolving in a suitable solvent separates a soluble solid from an insoluble one. To separate salt from sand, add water and stir: the salt dissolves and the sand does not.
🧑🏫 Teacher's explanation
Imagine you have a bowl of cereal with milk. The cereal pieces are like an insoluble solid (they don't dissolve), and the milk is the liquid. If you pour the mixture through a sieve, the cereal stays in the sieve while the milk goes through. This is exactly how filtration works.
📖 Definitions
🔍 Worked example — Easy
A student has a mixture of sand and salt. Describe the steps to obtain pure, dry salt crystals from this mixture.
🔍 Worked example — Standard
A solution contains 24.0 g of NaCl in 100 g of water. If the solubility of NaCl at 20^°C is 36 g per 100 g of water, how many grams of NaCl will crystallize out if the solution is cooled to 20^°C from a saturated state at 100^°C (assume solubility at 100^°C is 40 g/100 g water)?
🔍 Worked example — Exam-style
A student evaporates a solution to dryness to obtain crystals. Why is this method inferior to slow cooling for obtaining pure crystals?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the process used to remove the sand from the solution. [1] (b) Describe how the student can obtain pure, dry crystals of sodium chloride from the resulting filtrate. [3] [4 marks]
Key ideas — Simple and fractional distillation
Simple distillation separates a solvent from a solution, for example pure water from sea water. Heat the solution in a flask; the solvent boils, and its vapour passes into a condenser, a tube surrounded by a jacket of cold water flowing in at the bottom and out at the top. The vapour cools, condenses and is collected as the distillate; the dissolved solid stays behind because its boiling point is far higher.
🧑🏫 Teacher's explanation
Imagine you are at a crowded party where everyone is talking at once. If you want to hear just one person, you need a way to filter out the noise. Distillation does this using heat. In simple distillation, we use heat to turn a liquid into a gas, leaving the 'noise' (the solid) behind. It is like boiling salt water to get pure water; the salt is too heavy and stubborn to turn into a gas, so it stays in the pot.
📖 Definitions
🔍 Worked example — Easy
A student performs simple distillation to purify water from a salt solution. If the flask contains 50.0 g of salt solution and the salt has a boiling point of 1413 °C, explain why the salt does not evaporate.
🔍 Worked example — Standard
In fractional distillation, a mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) is heated. What temperature should the thermometer at the top of the fractionating column read when pure ethanol is being collected?
🔍 Worked example — Exam-style
A liquid mixture contains substance A (bp 50 °C) and substance B (bp 85 °C). If the mixture is being fractionally distilled, describe what happens to the temperature reading on the thermometer when substance A is being collected, and what happens when substance B begins to distill.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the purpose of the fractionating column. [1] (b) Describe how the distillate is collected to ensure the ethanol is as pure as possible. [2] [3 marks]
Key ideas — Choosing a technique
Ask what states the components are in and which property differs.
🧑🏫 Teacher's explanation
Imagine you are sorting a messy drawer. If you have marbles and sand, you can shake them through a sieve. If you have salt mixed in water, you can't shake it out, so you have to let the water evaporate away. Choosing a separation technique is simply about identifying which physical property—size, solubility, boiling point, or density—the components differ in.
📖 Definitions
🔍 Worked example — Easy
A mixture contains sand (insoluble) and sodium chloride (soluble). Suggest the sequence of techniques to obtain pure, dry sand and pure, dry sodium chloride.
🔍 Worked example — Standard
A mixture consists of ethanol (boiling point 78^° C) and water (boiling point 100^° C). Which technique is best to separate them, and why?
🔍 Worked example — Exam-style
A sample of pure water has a boiling point of 100^° C. A student tests an unknown liquid and finds it boils at 105^° C. What does this indicate about the purity of the liquid?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Purity from melting and boiling points
A pure substance has a sharp, fixed melting point and boiling point; pure water melts at exactly 0 °C and boils at exactly 100 °C at atmospheric pressure. An impure substance melts below the true melting point and over a range of temperatures, and boils above the true boiling point, also over a range. Comparing a measured melting point with a data-book value therefore identifies a substance and assesses its purity.
🧑🏫 Teacher's explanation
Think of a pure substance like a perfectly synchronized marching band where every member moves at the exact same tempo; everything happens at once. Now, imagine a crowd of people walking through a plaza; they all move, but they do so at different speeds and at different times. This is the difference between a pure substance and a mixture.
📖 Definitions
🔍 Worked example — Easy
A sample of white powder is tested. It melts sharply at 115 ^circC. The data book value for the melting point of the suspected substance is 115 ^circC. Is the substance pure or impure?
🔍 Worked example — Standard
A chemist is testing a sample of benzoic acid. The expected melting point is 122 ^circC. The observed melting point starts at 115 C and finishes at 119 C. Determine the purity status.
🔍 Worked example — Exam-style
A liquid is being analyzed for purity. The literature boiling point is 78 C. The experimental boiling point is 82 C and the temperature rises slowly during the phase change. How would you describe this liquid?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State whether the sample is pure or impure. [1] (b) Explain your answer to part (a) using the data provided. [2] [3 marks]
Worked example
Question. A student is given a mixture of copper(II) sulfate crystals and powdered glass. Copper(II) sulfate is soluble in water; glass is not. (a) Describe how the student could obtain pure, dry crystals of copper(II) sulfate from the mixture. [4] (b) The student measures the melting point of a white solid as 115 to 120 °C. Pure benzoic acid melts at 122 °C. State and explain what this tells the student about the solid. [2]
🔍 Worked example — Easy
A mixture contains sand (insoluble in water) and common salt (soluble in water). Describe the steps to obtain pure, dry salt crystals.
🔍 Worked example — Standard
A sample of pure water has a boiling point of 100 °C. A student tests an unknown liquid and finds it boils over a range of 95 °C to 98 °C. What does this indicate?
🔍 Worked example — Exam-style
A student wants to separate a mixture of two liquids, Liquid A (boiling point 56 °C) and Liquid B (boiling point 118 °C). Which technique is most suitable and why?
⚠️ Common errors
Common mistakes
Practice
Answers Simple distillation [B1]: heat the sea water so the water boils and its vapour enters a condenser [B1]; the vapour is cooled, condenses and is collected [B1]; the salt has a much higher boiling point so remains in the flask [B1]. Fractional distillation [B1]; the column is cooler at the top, so water vapour condenses and runs back into the flask [B1] while ethanol vapour, with the lower boiling point, passes to the condenser first [B1]. (a) filtration [B1]; (b) separating funnel [B1]. P [B1]; it has a sharp melting point, whereas Q melts over a range and below the true value, showing it contains impurities [B1]. Impurities could be harmful to the patient, or make the dose of active ingredient uncertain. [B1]