Separation Techniques: Filtration and Decantation

Active learning lets students test ideas immediately with real mixtures, turning abstract ideas about particle size and solubility into tangible results. When students handle sand, water, and filter paper themselves, they see why some particles pass through and others get trapped, building lasting understanding.

• Year 7
• Science
• 4 activities
• 20 min–45 min

Learning Objectives

1. Compare the mechanisms of filtration and decantation for separating insoluble solids from liquids.
2. Design an experimental procedure to separate a mixture of sand and water using filtration.
3. Evaluate the effectiveness of filtration in removing impurities from a sample of muddy water.
4. Explain the role of a filter medium in the process of filtration.

Inquiry Circle

⏱ 25 min·Pairs

Pairs: Sand-Water Filtration Race

Pairs mix sand and water in beakers, then filter using coffee filters over funnels into clean containers. They time the process and rate the filtrate clarity on a scale of 1-5. Compare results with a partner who uses decantation instead.

Prepare & details

Differentiate between filtration and decantation as separation techniques.

Facilitation Tip: During Sand-Water Filtration Race, circulate to ensure pairs fold filter paper correctly and pour slowly to avoid spills, preventing lost time and wasted materials.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 35 min·Small Groups

Small Groups: Decantation Design Challenge

Groups layer different-sized sand in water, let settle for 5 minutes, then decant into test tubes. They predict settling times, pour slowly to avoid disturbance, and measure solid residue left behind. Discuss why larger particles settle faster.

Prepare & details

Design an experimental procedure to separate sand from water using appropriate techniques.

Facilitation Tip: In Decantation Design Challenge, remind groups to sketch their plan before testing so they connect theory to measurable outcomes like settling time and clarity.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 45 min·Whole Class

Whole Class: Purification Effectiveness Test

Class prepares contaminated water samples with soil and leaves. Each student filters a sample, tests pH and turbidity before/after, and shares data on a class chart. Vote on the best method for clean drinking water.

Prepare & details

Evaluate the effectiveness of filtration in purifying contaminated water.

Facilitation Tip: For the Purification Effectiveness Test, prepare two identical muddy water samples so every group compares results directly, reinforcing the idea of controlled variables.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 20 min·Individual

Individual: Procedure Planning Worksheet

Students draw and label steps to separate gravel, sand, and salt water using both techniques. They list materials, safety steps, and success criteria, then test their plan with teacher-provided mixtures.

Prepare & details

Differentiate between filtration and decantation as separation techniques.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Teaching This Topic

Start with a quick demo of pouring dirty water through a coffee filter to show immediate visual results, then have students predict what will get caught. Avoid long lectures on solubility; instead, let evidence from their own tests drive concept development. Research shows hands-on mixing and observing outperforms diagrams alone for Year 7 learners.

What to Expect

Students will confidently choose the right technique based on solid properties, describe how each method works, and explain their reasoning using precise terms like residue, filtrate, and insoluble. You’ll notice clear links between procedure choices and outcomes during their explanations.

Watch Out for These Misconceptions

Common Misconception

During Sand-Water Filtration Race, watch for students assuming dyed salt water filters cleanly.

What to Teach Instead

Have pairs test dyed salt water at their station after filtering sand-water mixtures, then discuss why color remains in the filtrate and what this shows about dissolved substances.

Common Misconception

During Decantation Design Challenge, watch for students believing decantation works instantly regardless of particle size.

What to Teach Instead

Ask groups to time how long fine sand vs coarse sand takes to settle, then adjust their pouring speed and record observations to connect particle behavior with method effectiveness.

Common Misconception

During Purification Effectiveness Test, watch for students thinking filtration and decantation work the same way for all solids.

What to Teach Instead

Set up stations with fine clay, coarse sand, and gravel so students compare clogging rates and clarity differences, prompting them to articulate when to choose each method.

Assessment Ideas

Exit Ticket

After Sand-Water Filtration Race, give each student a mini whiteboard to sketch the residue and filtrate from their sample and label each part, then hold up responses for quick visual confirmation.

Quick Check

After Decantation Design Challenge, display images of a tea bag, a sieve with pasta, and an oil-vinegar mixture. Ask students to write on sticky notes whether each shows filtration, decantation, or neither, then sort them under headings on the board.

Discussion Prompt

During Purification Effectiveness Test, pause after results are noted and ask groups to explain why filtration failed to remove salt, guiding them to connect particle size with solubility before concluding the session.

Extensions & Scaffolding

• Challenge early finishers to design a two-step process using both filtration and decantation to separate a mixture of sand, salt, and water, recording each step and outcome.
• For struggling students, provide pre-measured sand-water samples and a simple checklist of steps to follow during the Sand-Water Filtration Race.
• Deeper exploration: Ask students to research how water treatment plants use similar principles, then compare their lab methods to industrial processes and present findings to the class.

Key Vocabulary

Filtration	A separation technique that uses a porous material to separate insoluble solids from a liquid or gas. The solid particles are trapped by the filter, while the liquid or gas passes through.
Decantation	A separation technique used to separate a liquid from a solid that has settled at the bottom of a container. The liquid is carefully poured off, leaving the solid behind.
Insoluble	Describes a substance that does not dissolve in a solvent. For example, sand is insoluble in water.
Filter medium	The material used in filtration that allows the liquid or gas to pass through but traps the solid particles. Examples include filter paper or cloth.
Residue	The solid material that remains on the filter paper or in the container after filtration or decantation.
Filtrate	The liquid that has passed through the filter paper during filtration.