Science · Grade 3 · Matter and Its Properties · Term 2

Dissolving and Mixtures

Students will investigate how some solids dissolve in liquids to form mixtures, and how these mixtures can sometimes be separated.

• Ontario Curriculum Expectations
• 2-PS1-2

About This Topic

Students investigate dissolving and mixtures by testing solids such as salt, sugar, and sand in water. They observe that some solids dissolve completely to form clear solutions, while others remain visible, creating mixtures that can settle. Key experiments show how stirring accelerates dissolving, and warmer water increases the rate compared to cold water. These activities address differentiating dissolving from non-dissolving substances, temperature effects, and methods like filtering or evaporation to separate mixtures.

This topic aligns with the Matter and Its Properties unit in the Ontario Grade 3 curriculum, building understanding of physical properties and reversible changes. Students practice scientific skills: making predictions, controlling variables in fair tests, recording data, and drawing conclusions from evidence. Everyday examples, like dissolving instant coffee or separating dirt from water, connect concepts to real life and spark interest.

Active learning benefits this topic greatly because hands-on experiments with safe, common materials make invisible processes visible. When students design tests, collaborate on separation challenges, and share findings, they build confidence in inquiry, correct misconceptions through trial and error, and retain concepts longer than through lectures alone.

Key Questions

1. Differentiate between a substance that dissolves and one that does not.
2. Explain how temperature can affect the rate at which a substance dissolves.
3. Design a method to separate a dissolved solid from a liquid.

Learning Objectives

• Classify substances as soluble or insoluble in water based on experimental results.
• Explain the effect of water temperature on the rate of dissolving for common solids.
• Design and describe a method to separate a dissolved solid from a liquid using evaporation or filtration.
• Compare and contrast the properties of a solution and a suspension.
• Predict whether a given solid will dissolve in water based on prior observations.

Before You Start

Properties of Solids and Liquids

Why: Students need to identify solids and liquids before they can investigate how solids interact with liquids.

Observing and Describing Matter

Why: Students must be able to observe and describe changes in matter, such as a solid disappearing, to understand dissolving.

Key Vocabulary

dissolve	When a solid disappears completely into a liquid, forming a clear mixture called a solution.
solution	A clear mixture formed when a solid dissolves completely into a liquid. The solid is no longer visible.
mixture	A combination of two or more substances that are not chemically bonded. Some parts may remain visible.
insoluble	A substance that does not dissolve in a liquid, remaining visible as separate particles.
rate of dissolving	How quickly a solid disappears into a liquid. This can be affected by factors like temperature and stirring.

Watch Out for These Misconceptions

Common Misconception

All solids dissolve in water.

What to Teach Instead

Many solids like sand or chalk do not dissolve but form mixtures. Hands-on testing of various solids helps students see differences firsthand. Group discussions of observations clarify that dissolving means the solid disappears into the liquid.

Common Misconception

Once a solid dissolves, it cannot be separated.

What to Teach Instead

Dissolved solids can be recovered through evaporation. Students experiment with salt water to watch crystals re-form, building evidence against this idea. Active trials reinforce that mixtures are reversible.

Common Misconception

Stirring alone causes dissolving.

What to Teach Instead

Water's properties and temperature matter most. Varying stirring while changing water temperature in experiments reveals true factors. Peer sharing of data corrects overemphasis on stirring.

Active Learning Ideas

Dissolving Race: Temperature Test

Pairs predict and test how quickly salt dissolves in hot, room-temperature, and cold water. Measure water temperatures with thermometers, stir consistently for 1 minute intervals, and time until clear. Record results in a class chart to compare patterns.

30 min·Pairs

Mixture Separation Stations

Set up stations for filtering sand-water mixtures with coffee filters, evaporating salt water in shallow dishes under heat lamps, and settling dirt in jars. Small groups rotate every 10 minutes, sketch methods, and note what works best.

45 min·Small Groups

Design Challenge: Clean Water

Provide mixtures of sand, salt, and water. Groups design and build a separation device using sieves, filters, and evaporation setups. Test prototypes, refine based on results, and present to the class.

50 min·Small Groups

Observation Jars: Mix and Settle

Individuals fill jars with water and add sugar, sand, or chalk powder. Observe over 10 minutes with periodic notes on changes. Shake to stir and compare settling rates.

20 min·Individual

Real-World Connections

• Food scientists use dissolving principles when creating flavored drinks, instant soups, and candy. They adjust ingredients and water temperature to ensure proper dissolving for taste and texture.
• Water treatment plant operators monitor and adjust processes to remove impurities from water. They use filtration to separate undissolved particles and sometimes evaporation to concentrate dissolved substances for removal.

Assessment Ideas

Exit Ticket

Provide students with three small containers: one with salt water (solution), one with sand in water (mixture), and one with plain water. Ask students to label each container and write one sentence explaining why the salt disappeared but the sand did not.

Quick Check

Present students with a scenario: 'Imagine you have a glass of cold water and a glass of hot water. You add one spoonful of sugar to each. Which glass will have dissolved sugar faster? Explain your thinking using the word 'temperature'.

Discussion Prompt

Ask students to think about how they might separate the salt from the water in a saltwater solution. Guide the discussion towards methods like boiling the water to let it evaporate, leaving the salt behind. Prompt them to consider if this would work for sand in water.

Frequently Asked Questions

How to teach dissolving and mixtures in grade 3 Ontario science?

Use simple tests with salt, sugar, and sand in water to show solutions versus mixtures. Explore temperature effects with hot and cold water setups. Teach separation via filtering and evaporation. Fair testing protocols ensure students control one variable at a time for reliable data.

How does temperature affect dissolving rate?

Warmer water molecules move faster, helping solids break apart quicker. Students test salt in hot, room-temp, and cold water, timing results to see patterns. This builds understanding of particle motion without advanced theory, using observable evidence.

What are common student misconceptions about mixtures?

Students often think all solids dissolve or mixtures cannot separate. Address with experiments: non-dissolving sand settles, salt reappears via evaporation. Structured observations and class charts help revise ideas based on evidence.

How can active learning help students understand dissolving and mixtures?

Active experiments like temperature races and separation stations let students manipulate variables directly, making abstract processes concrete. Collaborative design challenges encourage prediction, testing, and refinement. These methods boost engagement, retention, and skills in evidence-based reasoning over passive instruction.