Science · Year 7

Active learning ideas

Separating Mixtures: Filtration and Evaporation

Students retain separation techniques better when they see particles behave under real conditions rather than read about them. Hands-on mixing, filtering, and heating let learners connect particle size and solubility to practical outcomes, turning abstract science into concrete evidence they can touch and discuss.

• National Curriculum Attainment Targets
• KS3: Science - Pure and Impure Substances

• 20–45 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Problem-Based Learning
• 30 min · Pairs

Pairs: Sand-Salt Filtration Race

Students mix sand, salt, and water, then filter to separate insoluble sand. They test filtrate with silver nitrate to check for dissolved salt. Pairs time each other and compare clarity of filtrates.

Explain how filtration separates insoluble solids from liquids.

Facilitation Tip

During the Sand-Salt Filtration Race, circulate with a timer so pairs see how speed does not replace accuracy in trapping sand while letting salt pass.

What to look for

Provide students with two scenarios: 1. Separating sand from water. 2. Separating salt from water. Ask them to write which method (filtration or evaporation) is best for each and briefly explain why.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 02

• Problem-Based Learning
• 45 min · Small Groups

Small Groups: Evaporation Crystal Hunt

Groups prepare salt and sugar solutions of equal concentration. They heat in evaporating dishes over warm water baths, observing vapor and crystal formation. Weigh recovered solids to calculate yields.

Analyze the process of evaporation to obtain a soluble solid from a solution.

Facilitation Tip

In the Evaporation Crystal Hunt, set hot plates to low heat so students observe crystal growth over minutes, not instant disappearance of water.

What to look for

During a practical activity, ask students to hold up their filter paper and filtrate. Ask: 'What is the residue on your filter paper?' and 'What is the filtrate you have collected?' Observe their responses to gauge understanding of terms.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 03

• Problem-Based Learning
• 40 min · Whole Class

Whole Class: Method Match-Up Demo

Demonstrate filtration and evaporation on three mixtures: sand-water, salt-water, soil-water. Class predicts outcomes, votes on best method, then verifies with mini-trials at desks.

Compare the effectiveness of filtration and evaporation for different mixtures.

Facilitation Tip

For the Method Match-Up Demo, prepare labeled beakers in advance so students focus on matching labels to separation reasons rather than setup delays.

What to look for

Pose the question: 'Imagine you have a mixture of iron filings, salt, and water. How would you separate all three components? Which techniques would you use, and in what order?' Facilitate a class discussion comparing different student approaches.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 04

• Problem-Based Learning
• 20 min · Individual

Individual: Prediction and Reflection Sheets

Students draw before-and-after diagrams for four mixtures, select methods, and note expected results. After class experiments, they revise sheets and explain changes.

Explain how filtration separates insoluble solids from liquids.

What to look for

Provide students with two scenarios: 1. Separating sand from water. 2. Separating salt from water. Ask them to write which method (filtration or evaporation) is best for each and briefly explain why.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

A few notes on teaching this unit

Teach filtration first with visible sand so students feel the physical barrier that stops large particles. Follow immediately with evaporation using salt so they contrast the dissolving behavior. Avoid mixing methods until each is mastered separately; research shows early confusion between insoluble and soluble behaviors persists if not isolated. Emphasize slow heating to prevent splattering and crystal loss, modeling lab safety while building patience in observation.

By the end of the series, students will confidently choose filtration for insoluble solids and evaporation for soluble solids, explain why the wrong method fails, and use correct vocabulary like residue, filtrate, and crystals when describing their results.

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Watch Out for These Misconceptions

• During the Sand-Salt Filtration Race, watch for students who believe their filter paper catches all particles, including salt.

  After the race, have students taste the filtrate to confirm salt remains dissolved; then prompt them to explain why filter paper pores only trap undissolved sand.

• During the Evaporation Crystal Hunt, watch for students who think water absorbs into the dish and salt disappears.

  Ask students to weigh the empty dish before adding solution and after crystals form, showing mass stays constant except for water loss; relate this to boiling points to correct the idea of disappearance.

• During the Method Match-Up Demo, watch for students who claim both filtration and evaporation work equally well on any mixture.

  Hand out mixed samples and challenge groups to try both methods; when one fails, ask them to explain particle behavior that caused the failure before retrying.

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Methods used in this brief

• Problem-Based Learning