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To identify mixtures using chromatography

Science - Particles

Mrs Marshall and Mr Richardson

This science lesson is about identifying mixtures using chromatography

G’day scientists, Mr Richardson here with some assistance from Mrs Marshall. today In this lesson we are going to explore how to separate a homogeneous mixture (one that can’t be separated easily) using something called chromatography. Specifically paper chromatography.

Fun fact: A Russian botanist invented chromatography in 1903. His name was M.S. Tswett and discovered it While studying the coloring materials in plant life. Chromatography is such an important technique that two nobel prizes have been awarded to chromatographers. Over 60% of chemical analysis worldwide is currently done with chromatography or a variation of the technique Including crime scene investigations, so stand aside sherlock holmes, theres some new detectives in town..

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Key principles

The I do phase involves the explanation - indicates where a teacher would be explaining and modelling a concept.

Moving from I do to We do involves guided practice - moving from teacher-led instruction to student-led practice.

Check for Understanding - Formative instruction questions, posed to check whether students have understood and to inform next instructional steps.

You do - Practice - indicates places where students will attempt questions independently.

At the expected level for the lesson; all students should be able to do these questions.

A small increase in difficulty from the expected level, eg larger numbers or a more complex calculation.

Extension - increased difficulty; suitable for a small number of students.

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I

CfU

W

Y

Support - support materials, such as additional templates or questions with fewer steps.

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Engagement icons - to go in top right corner

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Whiteboard response

All students respond to the teacher’s question, showing their responses at the same time.

Non-volunteers

The teacher asks a question, gives wait time, then calls on individual students to respond.

Written response

Students respond to a prompt in a template, their exercise book or booklet.

Pair share

Students discuss with the person next to them.

Choral response

Students respond orally all together.

Thumbs up, thumbs down

Students indicate whether they agree or disagree with a statement, or whether something is an example or non-example.

Concrete materials

Teacher might choose to illustrate this teaching point using concrete materials.

Class discussion or activity

Students discuss as a class and share ideas

Calculator

Students will need a calculator to perform the task on this slide.

Spreadsheet

Students will need access to spreadsheet software to perform the task on this slide.

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Exercise book or paper

Pencil

Image credit: Pixabay

In this lesson, you will need:

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Starter quiz

What is chromatography

Paper chromatography

Chromatograms

Exit quiz

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Keywords

Chromatography: a technique for separating a solution by passing it through a substance.

Solubility: the ability of a substance, the solute, to form a solution with another substance, the solvent. How well it dissolves.

Chromatograms: the trace left by the components in a solution as the solution moves a substance.

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Keywords

Solute: the thing being dissolved.

Solvent: the substance that dissolves the solute. Usually a liquid but can be a gas or even solid.

Miscible: forming a homogeneous mixture when added together.

Homogeneous: a mixture that is mixed or blended together so well that all of the ingredients will not separate out, even over time.

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What is chromatography?

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Homogeneous mixtures

Some mixtures are so well combined they form homogeneous mixtures.

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Credit: Pixabay

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Chromatography

Higher solubility - further distance travelled

Lower solubility - less distance travelled

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Credit: FreeSVG

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Chromatography

Chromato - colour

Graphy - writing or producing images

Must have:

solvent to carry mixture
matrix for solvent to flow through

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Credit: FreeSVG

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Copy into your notebook and fill in the blanks

Some mixtures are so well combined they form ________ mixtures. We can ________ these mixtures by dissolving them in an appropriate liquid or ______ and allowing them to move through an absorbent ______, like paper, and see them deposited according to their ________.

Y

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Copy into your notebook and fill in the blanks

Some mixtures are so well combined they form homogeneous mixtures. We can separate these mixtures by dissolving them in an appropriate liquid or solvent and allowing them to move through an absorbent matrix, like paper, and see them deposited according to their solubility.

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Paper chromatography

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Stationary phase

Paper (or solid matrix) is the stationary phase in paper chromatography.

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Mobile phase

Water (or solvent) is the mobile phase in paper chromatography.

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We’ve got our stationary phase with our sample.

Now lets dip this paper into some water. What is going to happen? Have think about what happens when you get paper wet, even if it seems to defy gravity! ….

Thats right © the water would be absorbed into the paper and will spread up through the paper. The reason it spreads like that, is because of something called capillary action which is kind of like tiny highways in the paper that encourage the water to spread across the paper. This also takes the sample with it but not all at once. Different parts move at different speeds so we get a separation of the different components of the mixture, so this green sample © separates into blue and yellow which gets carried up by the water.

This is why we call it the mobile phase because the solvent moves the different parts of the mixture across the stationary phase.

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Why the separation?

Less soluble = less distance traveled in mobile phase

More soluble = more distance travelled in mobile phase

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Why does the yellow get carried in the mobile phase or the water for longer than than the blue? Well, the separation we see is essentially two forces pulling the green mixture apart. The forces that make the green dissolve in the mobile phase are trying to carry the whole mixture up the paper using that capillary action we talked about earlier. BUT the stationary phase or paper is making it more difficult for things to pass through it. So we get this separation according to how well the particular component dissolves into the mobile phase. Generally © the less soluble or larger the molecules of a material, the less it will travel in the mobile phase. © this also applies in the reverse, the more the component of the mixture dissolves into the water or mobile phase, the further it travels up the stationary phase and can fight that pull of the stationary phase. Generally these are smaller molecules so can get carried farther and dont get caught in the stationary phase as quickly as larger molecules.

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All methods of chromatography have a:

stationary phase

mobile and solvent phase

mobile and stationary phase

moving and still phase

CfU

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The paper is the:

stationary phase

mobile and solvent phase

mobile and stationary phase

moving and still phase

CfU

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The more soluble the substance the further it will move in the mobile phase:

True

False

CfU

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Chromatograms

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Chromatography 101

1. A miscible sample is placed onto the paper (stationary phase) with a base line.

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2. The paper is placed into water (mobile phase) not touching the sample.

3. Over time the water moves up the paper, separating the substances and when dried creates a chromatogram.

Just to recap, how we carry out chromatography is © we start with a miscible sample (which just means a sample that will be carried by the solvent, essentially a liquid) with a line that shows the starting point of the sample on the stationary phase. © this is then placed in water with just the bottom part of the paper placed in the water by suspending it on a paddle pop stick or something and then placed into a beaker with just enough water. We don’t touch the sample because that will stop the mobile phase from carrying the sample up the stationary phase, it’ll just sit there if its fully immersed © then we wait, as the water or mobile phase moves up the stationary phase the mixture gets separated into different components according to their solubility. We let it dry and then we have our chromatogram.

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Task: A method for a chromatography experiment

Write out the steps of how you would create a chromatogram using paper chromatography.

Y

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Task: A method for a chromatography experiment

1. A miscible sample is placed onto the paper (stationary phase) with a base line.

2. The paper is placed into water (mobile phase) not touching the sample.

3. Over time the water moves up the paper, separating the substances and when dried creates a chromatogram.

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Real life chromatograms

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Analysing chromatograms

Four different inks are separated using chromatography.

How many coloured dyes make up the green ink?

One

How many coloured dyes make up the brown ink?

Three

What colour dyes make up the brown ink?

Blue, Red and Yellow

What colour dyes make up the yellow ink?

Yellow

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Detective work

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Who most likely did the crime?

Suspect 3

CfU

Your turn now. Someone in this share house has taken the last timtam in the fridge without asking and has left the empty packet sitting in the fridge with a note. What cheek! Each one of the suspects is a mathematician at uni and has their own special pen that they keep in their top pocket to write down any maths solutions they might solve while they are doing boring things like doing the dishes. Which usually only happens once a month because it’s a share house full of mathematicians. Lets use chromatography to try and figure out who dunnit. © Pause the video and take a look at the chromatogram. Theres a bigger version in work samples if you want to take a look at it in more detail. When you’re ready to share your results, press play.

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Detective work

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2. Who could have taken the second TimTam and added to the note?

Suspects:

1

6

8

9

CfU

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Detective work

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3. What property is unique about the fourth suspect’s pen?

The ink is not miscible, or does not dissolve into the mobile phase.

CfU

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Answers (worksheet)

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Match steps of paper chromatography to their correct order

A miscible sample is placed onto the paper on the baseline.

The paper is placed into water (mobile phase) not touching the sample.

Over time the water moves up the paper, separating the substances.

When dried creates a chromatogram.

Paper is prepared (stationary phase) with a baseline.

1

2

5

4

3

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Solubility

What is the least soluble component of the sample?

Pink (1)

Which two components are most similar in properties?

Green and orange (2 and 3)

How would you describe the solubility of component 4?

The most soluble component in the mobile phase.

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Chromatography in the real world

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What two heavy metals are present in the river?

Lead and Cadmium

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Allergy test

Which lollies are safe to eat?

1, 3, 5, 6

Rank the lollies in order of most likely to cause a reaction to least likely to cause a reaction. Write a sentence to justify your order of the top 3 most allergic.

4: has both dyes

2/7: has one dye

7/2: has one dye

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3

1

5

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Allergy test

3. Someone who is allergic to only purple dye should avoid which lollies?

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4. What happened with the chromatography of lolly #6? Use the data and what you know about chromatography to predict what went wrong in the method.

The Red#107 dye sequence occurs in #6 but has not travelled as far as the other samples. Likely, it has not been in mobile phase long enough.

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