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.

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

Learning Objectives

1. Explain the scientific principles behind separating insoluble solids from liquids using filtration.
2. Analyze the process of evaporation for recovering soluble solids from a solution.
3. Compare and contrast the suitability of filtration and evaporation for separating different types of mixtures.
4. Identify the pure substances obtained from specific impure mixtures using filtration and evaporation.

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.

Prepare & details

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.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

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

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.

Prepare & details

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.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

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

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.

Prepare & details

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.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

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

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.

Prepare & details

Explain how filtration separates insoluble solids from liquids.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

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

Teaching This Topic

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.

What to Expect

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.

Watch Out for These Misconceptions

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Assessment Ideas

Exit Ticket

After the Sand-Salt Filtration Race, give students two scenarios: separating sand from water and separating salt from water. Ask them to write which method they would use and justify with one sentence referencing particle size or solubility.

Quick Check

During the Evaporation Crystal Hunt, ask students to hold up their crystals and tell you the name of the solid they recovered and the process that formed it. Listen for accurate use of terms residue and evaporation.

Discussion Prompt

After the Method Match-Up Demo, pose this scenario: 'You have iron filings, salt, and water. How would you separate all three?' Ask students to share their step-by-step order and explain which method they would use at each stage, listening for correct sequencing and vocabulary.

Extensions & Scaffolding

• Challenge early finishers to design a two-step separation for a mixture of sand, salt, and iron filings using provided tools.
• Scaffolding for struggling learners: provide pre-labeled diagrams of filter paper folds and hot plate settings to reduce cognitive load during setup.
• Deeper exploration: ask students to calculate percent recovery of salt crystals by measuring initial salt mass and final crystal mass, then graph results across groups.

Key Vocabulary

Filtration	A separation technique used to separate insoluble solids from a liquid or gas by passing the mixture through a filter medium.
Evaporation	A process where a liquid changes into a gas or vapor, often used to separate a soluble solid from a solvent.
Soluble	Able to be dissolved in a particular solvent, such as salt dissolving in water.
Insoluble	Not able to be dissolved in a particular solvent, such as sand in water.
Residue	The solid material that remains on the filter paper after filtration.
Filtrate	The liquid that has passed through the filter paper during filtration.