Science · Year 7 · Mixtures and Pure Substances · Term 1

Separation Techniques: Chromatography

Students will investigate chromatography as a method for separating components of a mixture based on their differential movement through a stationary phase.

• ACARA Content Descriptions
• AC9S7U06

About This Topic

Chromatography separates mixtures by exploiting differences in how components interact with a stationary phase, such as filter paper, and a mobile phase, like water or alcohol. Year 7 students investigate this technique with everyday materials: they spot inks or plant extracts on paper strips, suspend them in solvent, and observe coloured bands forming as substances move at different rates. This reveals that mixtures contain distinct components with varying solubilities and affinities, directly addressing key questions on principles, experiment design, and property influences.

Aligned with AC9S7U06 in the Australian Curriculum, chromatography connects to the Mixtures and Pure Substances unit by distinguishing pure substances from mixtures and emphasising testable properties. Students practice fair testing, precise measurement of Rf values, and data interpretation, fostering scientific inquiry skills essential for chemical sciences.

Active learning shines here because chromatography demands hands-on manipulation: students design, execute, and troubleshoot their own setups. When they compare results from different inks or solvents in small groups, they grasp abstract concepts through visible evidence, boosting retention and enthusiasm for experimentation.

Key Questions

1. Explain the principle behind chromatography for separating mixtures.
2. Design a simple paper chromatography experiment to separate ink colours.
3. Analyze how different properties of substances affect their separation in chromatography.

Learning Objectives

• Explain the fundamental principle of chromatography based on differential partitioning between stationary and mobile phases.
• Design and conduct a simple paper chromatography experiment to separate the components of a colored ink.
• Calculate Rf values for separated components in a chromatography experiment and compare them.
• Analyze how the properties of different solvents (mobile phase) affect the separation of ink components.
• Compare the results of chromatography experiments using different types of inks or plant pigments.

Before You Start

Properties of Matter

Why: Students need to understand basic properties like color and solubility to explain why substances separate differently.

Mixtures and Solutions

Why: Understanding that mixtures are composed of different substances that can potentially be separated is foundational for this topic.

Key Vocabulary

Chromatography	A laboratory technique used to separate mixtures into their individual components based on their differing affinities for a stationary phase and a mobile phase.
Stationary Phase	The immobile phase in chromatography, typically a solid material like filter paper or a gel, which the mobile phase moves through.
Mobile Phase	The phase that moves through the stationary phase in chromatography, carrying the components of the mixture. This is usually a liquid or gas.
Rf Value	The ratio of the distance traveled by a component of a mixture to the distance traveled by the solvent front in chromatography. It is a characteristic value for a given substance under specific conditions.
Solubility	The ability of a substance (solute) to dissolve in a solvent to form a solution. Higher solubility in the mobile phase generally leads to greater movement.

Watch Out for These Misconceptions

Common Misconception

All colours in ink move at the same speed.

What to Teach Instead

Colours separate due to different solubilities in the solvent and attractions to paper. Hands-on trials with varied inks let students measure distances and calculate Rf values, revealing patterns through peer comparison.

Common Misconception

Chromatography creates new colours.

What to Teach Instead

Existing components in the mixture become visible as they migrate differently. Group experiments with known dye mixes help students track unchanged colours, using discussions to refine models.

Common Misconception

Water works equally well for all separations.

What to Teach Instead

Solvent choice affects movement based on substance polarity. Testing multiple solvents in rotations shows students how properties influence results, building experimental design skills.

Active Learning Ideas

Demonstration: Basic Ink Separation

Demonstrate by drawing a dot of black ink on filter paper, rolling it into a cylinder, and placing the bottom in water. Students observe and sketch band formation over 20 minutes. Discuss why colours separate based on solubility.

30 min·Whole Class

Pairs Lab: Marker Chromatography

Pairs select washable markers, spot paper strips, and test in water or alcohol. Measure distances travelled by each colour to calculate Rf values. Compare results across pairs to identify patterns.

45 min·Pairs

Small Groups: Plant Pigment Extraction

Groups crush leaves in alcohol, spot the extract on paper, and run chromatography. Identify green, yellow, orange bands. Record observations and link to photosynthesis pigments.

50 min·Small Groups

Individual Challenge: Mystery Ink

Each student tests an unknown ink mixture on paper with different solvents. Predict and verify component colours. Share findings in a class gallery walk.

35 min·Individual

Real-World Connections

• Forensic scientists use chromatography to analyze trace evidence, such as identifying the specific dyes in fibers found at a crime scene or separating components of unknown substances.
• Pharmaceutical companies employ chromatography extensively in drug development and quality control to purify active ingredients and detect impurities in medications.
• Environmental chemists use chromatography to monitor air and water quality, identifying and quantifying pollutants like pesticides or industrial chemicals in samples.

Assessment Ideas

Exit Ticket

On a small card, ask students to draw a simple diagram of their paper chromatography setup. Have them label the stationary phase, mobile phase, and at least one separated ink color. Then, ask them to write one sentence explaining why the colors separated.

Quick Check

Present students with a pre-made chromatogram showing separated ink colors and their corresponding Rf values. Ask them to identify which ink was the most soluble in the mobile phase and justify their answer based on the Rf values.

Discussion Prompt

Pose the question: 'Imagine you are trying to separate the pigments in a leaf. What factors would you need to consider when choosing your stationary and mobile phases? How might these choices affect your results?' Facilitate a brief class discussion.

Frequently Asked Questions

What materials are needed for a simple paper chromatography experiment?

Gather filter paper, soluble inks or markers, water or rubbing alcohol as solvent, pencils, rulers, and jars. Students draw a baseline, add sample dots, roll paper, and suspend in solvent without touching. This setup, costing under $10 per class, allows clear observation of separations in 20-30 minutes.

How do you calculate Rf values in chromatography?

Rf value equals distance travelled by solute divided by distance travelled by solvent front. Students measure both with rulers after drying, recording in tables. This quantitative step helps analyse why components separate, linking to properties like solubility, and prepares for data graphing.

Why does chromatography separate ink colours?

Ink dyes have different solubilities and affinities for paper versus solvent, causing differential migration. More soluble dyes travel farther. Experiments with black pens show hidden colours, helping students connect molecular properties to observable bands.

How can active learning help students understand chromatography?

Active approaches like designing personal experiments with inks and solvents give students ownership, turning theory into visible results. Small group rotations for testing variables build collaboration and troubleshooting skills. Tracking Rf values collectively reveals patterns, making abstract principles concrete and memorable over lectures.