Science · Grade 7 · Pure Substances and Mixtures · Term 3

Chromatography

Exploring how chromatography separates mixtures based on differential adhesion and solubility.

• Ontario Curriculum Expectations
• MS-PS1-2

About This Topic

Chromatography separates mixtures based on differences in solubility and adhesion to a stationary phase, such as paper, versus a mobile phase, like water or alcohol. Grade 7 students use paper chromatography to separate inks and markers, spotting samples on filter paper strips, suspending them in solvent, and observing pigments travel at different rates. This reveals that black ink contains multiple colors and connects to key questions about separation mechanisms and solvent effects.

In the Pure Substances and Mixtures unit, chromatography distinguishes mixtures from pure substances by showing variable component movement. Students predict and analyze how changing solvents alters separation, building skills in evidence-based reasoning and aligning with Ontario curriculum standards for investigating techniques like filtration and distillation.

Active learning benefits this topic greatly since students perform hands-on separations, compare results across solvents, and discuss patterns in small groups. These experiences make differential adhesion tangible, encourage prediction-testing cycles, and foster persistence when results vary, turning abstract chemistry into collaborative discovery.

Key Questions

1. Explain how chromatography separates components of a mixture.
2. Analyze how different pigments in ink can be separated using paper chromatography.
3. Predict how changing the solvent might affect the separation in chromatography.

Learning Objectives

• Analyze the separation of ink pigments using paper chromatography based on their differing solubilities and adhesion.
• Explain the role of the stationary phase and mobile phase in separating components of a mixture.
• Compare the results of chromatography experiments using different solvents to predict solvent effects on separation.
• Identify the components of a mixture by observing their movement patterns on a chromatography strip.

Before You Start

Properties of Pure Substances and Mixtures

Why: Students need to understand the difference between a pure substance and a mixture to appreciate why chromatography separates components.

Solubility and Dissolving

Why: Understanding that some substances dissolve in certain liquids is fundamental to grasping how chromatography works.

Key Vocabulary

Chromatography	A laboratory technique used to separate mixtures into their individual components based on differences in their physical or chemical properties.
Stationary Phase	The solid or liquid material that remains fixed in place during chromatography, such as the paper in paper chromatography.
Mobile Phase	The substance that moves through the stationary phase, carrying the mixture components with it, such as a solvent.
Solubility	The ability of a substance to dissolve in a solvent, a key factor in how far a component travels up the chromatography paper.
Adhesion	The attraction between different substances, in this case, between the ink pigments and the paper (stationary phase).

Watch Out for These Misconceptions

Common Misconception

All colors in ink are pure single substances.

What to Teach Instead

Inks contain mixtures of pigments with varying solubilities. Hands-on chromatography lets students see multiple bands emerge from one spot, prompting peer comparisons that clarify mixtures versus pure substances.

Common Misconception

The solvent choice has no effect on separation.

What to Teach Instead

Solvents interact differently with pigments based on polarity. Group experiments testing water versus alcohol reveal faster travel in matching solvents, helping students predict and explain through shared data analysis.

Common Misconception

Components separate by size alone.

What to Teach Instead

Separation depends on solubility and adhesion combined. Station rotations allow students to test varied mixtures, observe counterexamples like small fast-moving dyes, and refine models via discussion.

Active Learning Ideas

Lab Rotation: Ink Separation Stations

Prepare stations with filter paper, black markers, water, and alcohol solvents. Students spot ink at the base, place paper in solvent jars, and measure pigment distances after 20 minutes. Groups rotate to compare solvents and record Rf values.

45 min·Small Groups

Pairs Challenge: Plant Pigment Chromatography

Pairs crush spinach leaves in alcohol, spot extract on paper, and run chromatography. They identify chlorophyll bands by color and distance traveled. Discuss how solubility affects separation rates.

35 min·Pairs

Whole Class: Solvent Prediction Demo

Display three solvents; class predicts separation of candy dyes on paper strips. Run demos simultaneously, then vote and measure results. Debrief on adhesion and solubility patterns.

30 min·Whole Class

Individual: Mystery Mixture Analysis

Provide unknown ink mixtures; students run chromatography alone, sketch results, and identify components by matching known samples. Share findings in a gallery walk.

40 min·Individual

Real-World Connections

• Forensic scientists use chromatography to analyze trace evidence, such as identifying the components of a suspect's clothing fibers or analyzing unknown substances found at a crime scene.
• Pharmaceutical companies employ chromatography to purify drugs and ensure the quality and consistency of medications, separating active ingredients from impurities.
• Environmental chemists use chromatography to detect and measure pollutants in air and water samples, helping to monitor environmental quality and identify sources of contamination.

Assessment Ideas

Quick Check

After students complete a paper chromatography experiment, ask them to draw their results and label the different colors they observe. Then, ask: 'Which color traveled the furthest and why?'

Discussion Prompt

Pose the question: 'Imagine you are testing a new type of marker. How would you use chromatography to determine if it is a pure substance or a mixture? What would you expect to see if it were a mixture?'

Exit Ticket

Provide students with a scenario: 'A student used water as a solvent for black ink and alcohol as a solvent for blue ink. Both inks separated into multiple colors. What is one difference you might expect between the two results, and why?'

Frequently Asked Questions

How does paper chromatography work in Grade 7 science?

Paper acts as the stationary phase where pigments adhere differently; solvent is the mobile phase carrying them based on solubility. Students spot ink on paper, dip in solvent, and measure travel distances to calculate Rf values, analyzing why components separate at unique rates for mixtures like black ink.

What materials are needed for simple chromatography experiments?

Gather filter paper or coffee filters, washable markers or black ink, solvents like water, rubbing alcohol, or salt water, jars, and pencils for marking. Optional: leaves for natural pigments or candy dyes. These common items support quick setups for separating mixtures effectively.

How can I teach chromatography to predict solvent effects?

Start with baseline water runs, then test alcohol; have students hypothesize based on polarity before observing. Use class charts to track Rf changes, reinforcing that solvents dissolve pigments matching their properties best, aligning with Ontario expectations for prediction and analysis.

What active learning strategies work best for chromatography?

Station rotations and solvent comparison labs engage students in predicting, testing, and measuring separations firsthand. Small group discussions of Rf values build collaboration, while individual mystery challenges promote independence. These approaches make solubility visible, reduce cognitive load, and boost retention through repeated observation cycles.