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

Filtration and Decantation

Applying knowledge of physical properties to separate heterogeneous mixtures.

• Ontario Curriculum Expectations
• MS-PS1-2

About This Topic

Filtration and decantation are practical separation techniques for heterogeneous mixtures, relying on physical properties like particle size and density. Filtration uses a porous material, such as filter paper, to trap insoluble solids while liquids pass through. Decantation separates settled solids by carefully pouring off the clearer liquid above. These methods directly address Ontario Grade 7 expectations in the Pure Substances and Mixtures unit, where students explain processes, design procedures like separating sand from water, and compare technique effectiveness.

This topic strengthens understanding of mixtures versus pure substances and connects to real-world applications, such as water treatment or coffee brewing. Students develop skills in observation, procedure design, and data comparison, which support scientific inquiry across the curriculum.

Active learning excels with this content because students achieve visible results quickly from simple setups. Experiments with everyday materials let them test predictions, adjust methods, and discuss why filtration suits fine particles while decantation works for heavier ones. Such hands-on work makes properties tangible and boosts confidence in problem-solving.

Key Questions

1. Explain how filtration separates insoluble solids from liquids.
2. Design a method to separate sand from water using decantation.
3. Compare the effectiveness of filtration and decantation for different mixtures.

Learning Objectives

• Explain the scientific principles behind filtration and decantation for separating heterogeneous mixtures.
• Design and conduct an experiment to separate sand and water using decantation, justifying each step.
• Compare the effectiveness of filtration and decantation in separating different types of insoluble solids from liquids.
• Analyze the role of physical properties, such as particle size and density, in the success of separation techniques.

Before You Start

Properties of Solids, Liquids, and Gases

Why: Students need to understand the basic states of matter and their observable properties to identify components in a mixture.

Classifying Matter: Pure Substances and Mixtures

Why: This topic builds directly on the ability to identify and differentiate between pure substances and various types of mixtures.

Key Vocabulary

Heterogeneous Mixture	A mixture where the different components are not uniformly distributed throughout, meaning you can see distinct parts.
Filtration	A separation technique that uses a porous barrier, like filter paper, to separate insoluble solids from a liquid or gas.
Decantation	A separation technique where a settled solid is separated from a liquid by carefully pouring off the liquid.
Insoluble Solid	A solid that does not dissolve in a liquid, remaining as distinct particles.
Sedimentation	The process where solid particles suspended in a liquid gradually settle to the bottom due to gravity.

Watch Out for These Misconceptions

Common Misconception

Filtration removes all impurities from liquids.

What to Teach Instead

Filtration catches only insoluble solids larger than the filter pores; dissolved substances pass through. Active testing with salt water and food coloring reveals clear filtrate that still tastes salty, prompting students to rethink purity and explore solutions.

Common Misconception

Decantation works equally well for all solids.

What to Teach Instead

It succeeds only with solids that settle quickly due to density; fine particles like clay stay suspended. Timed settling observations in jars help students compare rates and select appropriate methods through trial and discussion.

Common Misconception

Filtration and decantation achieve the same results.

What to Teach Instead

Filtration mechanically traps particles regardless of settling, while decantation relies on gravity separation. Side-by-side trials with the same mixture show differences in speed and completeness, building skills in comparative analysis.

Active Learning Ideas

Stations Rotation: Separation Stations

Set up four stations with mixtures like sand-water, gravel-water, soil-water, and chalk-water. Students test filtration using coffee filters and funnels, then decantation after settling. They record clarity, time, and residue in journals before rotating. Discuss comparisons as a class.

45 min·Small Groups

Design Challenge: Custom Separators

Provide mixtures of sand, salt, and water. In pairs, students design and test a sequence using filtration and decantation. They sketch procedures, predict outcomes, perform trials, and refine based on results. Share best methods with the class.

50 min·Pairs

Filter Material Testing

Give students cloth, paper towels, sand, and sieves. They filter muddy water samples individually, measure filtrate clarity with a scale, and rank materials. Graph results and explain choices based on pore size.

30 min·Individual

Mixture Comparison Demo

As a whole class, prepare large containers of settling mixtures. Demonstrate decantation with a turkey baster, then filter samples. Students vote on best method per mixture and justify with observations.

20 min·Whole Class

Real-World Connections

• Water treatment plants use large-scale filtration systems to remove impurities and sediment from drinking water before it reaches homes.
• Baristas use paper filters to brew coffee, separating the liquid coffee from the solid grounds to create a smooth beverage.
• Geologists use decantation in the lab to separate fine sediment samples from water, helping them analyze soil composition and rock fragments.

Assessment Ideas

Exit Ticket

Provide students with two scenarios: 1) Separating fine sand from water. 2) Separating small pebbles from water. Ask them to choose the best separation method (filtration or decantation) for each scenario and write one sentence explaining why.

Quick Check

During a hands-on activity, circulate and ask students: 'What physical property are you using to separate these components?' and 'How do you know when to stop pouring in decantation?'

Discussion Prompt

Pose the question: 'Imagine you have a mixture of salt and water, and another mixture of sand and water. Which separation technique, filtration or decantation, could you use for each? Explain your reasoning, considering what happens when salt dissolves.'

Frequently Asked Questions

What is the difference between filtration and decantation for grade 7 science?

Filtration uses a physical barrier to separate insoluble solids from liquids based on particle size, while decantation relies on density differences, allowing heavier solids to settle before pouring off the liquid. Students compare them by testing mixtures like sand-water: filtration is faster for fines, decantation simpler for coarse particles. This distinction clarifies heterogeneous mixture separation in the Ontario curriculum.

How to separate sand from water using decantation in class?

Pour sand-water into a clear container and let it settle for 5-10 minutes until sand layers at the bottom. Carefully decant the clear water into another container using a steady tilt or baster. For better results, add steps like initial sieving. Students record before-and-after clarity to assess success and discuss improvements.

How can active learning help students understand filtration and decantation?

Active learning engages students through hands-on trials with household mixtures, making abstract properties like density and pore size observable. Station rotations and design challenges encourage prediction, testing, and refinement, deepening procedural understanding. Peer discussions after experiments clarify comparisons, reducing misconceptions and aligning with inquiry-based Ontario science expectations.

What mixtures are best for teaching filtration and decantation?

Use sand-water for decantation practice, muddy water or soil suspensions for filtration, and gravel-water for both to compare. Include finer mixtures like flour-water to show limits. These everyday examples connect to curriculum key questions, allow quick results, and support data collection for effectiveness analysis.