Lesson Plan

5. Show how mixtures can be separated using evaporation and distillation

KS3-06-05

Intent

Lesson Intention

  • Develop an understanding of the processes involved in distillation and evaporation and their differences
  • Appreciate the differences between the two methods in the separation of compounds (solute and solvent)

National Curriculum

  • Chemistry - Pure and impure substances Learn about simple techniques for separating mixtures: evaporation and distillation

Working Scientifically

  • Learning how to distil and evaporate mixtures
  • Understand the difference between evaporation and distillation

Learning Outcomes

  • Explain the terms "evaporation" and "distillation" 
  • Describe the differences between the two process
  • Separate mixtures by applying distillation and evaporation of two liquids

Resources

Resources: 

Distilling the essence of apple juice: 50 ml of apple juice; conical flask, delivery tube, beaker, water and a test tube.

Separating the salt from a salt water solution: water, salt and an evaporating dish.

Both tasks: Bunsen burner (or tealights); tripod; safety goggles and the handout. 

Core Handout: A series of comparative questions on evaporation and condensation. 

Rocket words

  • evaporation
  • distillation
  • crystallisation
  • simple distillation
  • fractional distillation

Implementation

Prior Learning: 

Recap on KS2 evaporation learning. 

Starter

Revise the concepts of solution, solute and solvent, saturation and solubility. Ask students to brainstorm the different ways they can separate a mixture of salt and water. List their ideas on the board and briefly discuss each one. Introduce the concepts of evaporation and distillation as methods of separating mixtures.

Main Teaching

Demonstration (10 minutes):

  1. Prepare a salt solution in a beaker or flask by dissolving an appropriate amount of salt in water.
  2. Demonstrate the process of evaporation by slowly heating the solution and showing how the water evaporates, leaving behind the salt.
  3. Demonstrate the process of distillation by setting up a simple distillation apparatus, including a condenser, thermometer, and collection flask, and then heating the solution. Point out to students how the water vapour is condensed back into a liquid in the collection flask, separate from the salt.

Guided Practice (15 minutes):

  1. Distillation of apple juice - see Mission Assignment.

Independent Practice (15 minutes):

  1. Evaporation of salt water - see Missions Assignment.

Career Film: Andy Davies is the Learning Delivery Manager at Rolls Royce. Andy is responsible for the Advanced Engineering Apprenticeships.

Career Film: Professor Mark Searcey is head of chemistry and pharmacy at the University of East Anglia. Mark provides some insight into his career and his love of science.

Mission Assignment

Comparing evaporation and distillation

This lesson has two tasks which the students will carry out. They will then compare the two, pointing out what was similar and dissimilar between the two methods using the handout. 

Distilling the essence of apple juice

Students should pour 50 ml of apple juice into a conical flask, placing a bung in the top with a delivery tube leading out of it. The end of the tube should lead into a test tube that is submerged in water, which will act as a condenser. 

Students should then heat the apple juice in the conical flask. To do this, they could use a Bunsen burner (on a small blue flame, with the collar open and the gas down) or some tea lights on a tripod. The essence of apple juice will heat up and evaporate, travel along the delivery tube, and be cooled back into a liquid in the test tube.

Separating the salt from a salt water solution

Students should firstly make a solution of salt water. The solution should be concentrated, but not saturated. It should then be poured into an evaporating dish and heated, using the Bunsen burner of tealights, like in the previous task. The dish should be left long enough so that the water has completely evaporated, leaving behind only crystals of salt.

Differentiated tasks (Support/Core/Challenge)

Support:

  • Provide clear and concise instructions during the demonstration and guided practice.
  • Use visual aids, such as diagrams or images, to help students understand the process of distillation and evaporation.
  • Have students work in pairs or small groups during the independent practice to provide additional support.
  • Carry out the mission assignment with only evaporation of a salt water solution with different concentrations. 

Core: 

  • Carry out mission assignment of distillation of apple juice.
  • Encourage students to think critically about the process, asking them to consider why certain steps are necessary and what might happen if they were skipped.
  • Provide opportunities for students to ask questions and discuss their thoughts with each other during the conclusion.
  • Encourage students to use a thermometer during the distillation to measure the temperature changes and to observe the results.

Challenge: 

  • Carry out mission assignment of both tasks.
  • Encourage students to think creatively about the process, asking them to consider alternative methods for separating the mixture and to compare the advantages and disadvantages of each method.
  • Challenge students to design and carry out their own distillation experiments, and then present their results and conclusions to the class during the conclusion.
  • Encourage students to think about the real-life applications of distillation and evaporation, and to discuss potential areas of improvement or optimisation of the process.

Impact & Assessment Opportunities

Plenary

Students report on their results with a short verbal report of their tasks and observations. Review what students have learned about evaporation and distillation and their use in separating mixtures. Ask students to think of real-life examples where these processes might be used (e.g. in the production of distilled water or alcohol). Summarise the key points of the lesson and answer any final questions students may have.

Teacher Mastery

A mixture refers to a combination of different elements and compounds which aren't chemically bonded together. The different elements and compounds in a mixture can be isolated or separated from the other components in a number of ways. Mixtures can be separated using heat through the process of evaporation and distillation because individual components in the mixture have different boiling points. 

Evaporation can be used to separate a dissolved solid from the liquid that the solid is dissolved in. In a laboratory, a mixture can be poured into an evaporation dish and then heated from below using a Bunsen burner. If this process was conducted using a mixture of salt (NaCl) and water, the water would boil (turning from a liquid to a gas) and evaporate away to leave behind solid salt crystals. 

Distillation can be used to separate a mixture of liquids with different boiling points. Unlike evaporation, the evaporated liquid is collected through cooling instead of being released into the atmosphere. In a laboratory, distillation can be carried out using a conical flask stoppered with a bung and a delivery tube that feeds into a test tube. The mixture in the conical flask can be heated using a Bunsen burner, allowing the vapour to pass along the delivery tube to the test tube. The vapour can then be condensed (i.e. turned from a gas back into a liquid) if the test tube is cooled (for example, by placing it in a beaker of ice-cold water).

An example of evaporation is heating a sauce when cooking. The water is boiled and evaporates away to leave behind the other ingredients in the saucepan. An example of distillation carried out on an industrial scale is the distillation of crude oil. The distillation process separates the complex mixture of crude oil because different hydrocarbons have different boiling points. This produces simpler mixtures that are more valuable and useful.