Science · Year 6 · Reversible and Irreversible Changes · Term 2

The Science of Solutions and Mixtures

Investigating how substances dissolve and how they can be recovered from solutions.

• ACARA Content Descriptions
• AC9S6U04

About This Topic

The science of solutes and solutions explores what happens at a molecular level when substances mix. Year 6 students investigate how certain solids (solutes) dissolve into liquids (solvents) to create solutions. They will explore factors that affect solubility, such as temperature and stirring, and learn techniques for separating these mixtures, such as evaporation and filtration. This is a key component of the ACARA Chemical Science strand.

This topic has practical applications in many Australian industries, from salt harvesting in Western Australia to water purification in our cities. It also provides an opportunity to discuss how First Nations Australians have used filtration and leaching techniques to remove toxins from bush foods like cycad seeds. Students grasp this concept faster through collaborative investigations where they can compete to see who can dissolve a substance the fastest.

Key Questions

1. Explain the process by which a solid substance appears to 'disappear' when dissolved in a liquid.
2. Analyze the relationship between temperature and the rate at which a solid dissolves in a solvent.
3. Design the most efficient method to separate a complex mixture containing sand, salt, and water.

Learning Objectives

• Explain the process of dissolution at a particle level, describing how solute particles interact with solvent particles.
• Analyze the effect of temperature on the rate of dissolution for a given solute and solvent.
• Design an experiment to compare the efficiency of different separation techniques (e.g., evaporation, filtration) for a salt and sand mixture.
• Compare and contrast the properties of solutions and heterogeneous mixtures.
• Predict the outcome of dissolving a given substance in a specified solvent based on solubility principles.

Before You Start

Properties of Solids, Liquids and Gases

Why: Students need to understand the basic characteristics of these states of matter to comprehend how substances behave when mixed.

Observing and Describing Changes

Why: This foundational skill allows students to accurately record and articulate what they observe during dissolution and separation experiments.

Key Vocabulary

Solute	The substance that dissolves in a solvent to form a solution. For example, salt is the solute when it dissolves in water.
Solvent	The substance that dissolves a solute to form a solution. Water is a common solvent.
Solution	A homogeneous mixture where one substance (solute) is dissolved completely into another substance (solvent).
Dissolution	The process where a solute breaks down into smaller particles and disperses evenly within a solvent.
Filtration	A separation technique used to separate insoluble solids from liquids by passing the mixture through a filter medium.

Watch Out for These Misconceptions

Common Misconception

The solute 'melts' into the water.

What to Teach Instead

Students often use 'melt' and 'dissolve' interchangeably. Use peer discussion to clarify that melting requires heat to change state, while dissolving involves a substance being broken down and spread throughout a liquid.

Common Misconception

A solution is no longer 'heavy' once the solid dissolves.

What to Teach Instead

Many students think the mass disappears. A simple hands-on experiment using a balance scale to weigh the water and salt before and after mixing can quickly correct this error.

Active Learning Ideas

Inquiry Circle: The Great Dissolve-Off

Groups are given a sugar cube and must find the fastest way to dissolve it using different variables (hot water, cold water, stirring, crushing). They must record their times and present their 'winning' method to the class.

40 min·Small Groups

Stations Rotation: Separation Station

Set up stations with different mixtures (sand and water, salt and water, iron filings and flour). Students must use different tools (filters, magnets, heat) to separate the components and recover the original substances.

50 min·Small Groups

Think-Pair-Share: Where Did It Go?

Show a video of salt dissolving. Students think about where the salt particles are 'hiding,' then pair up to draw a diagram of the water molecules surrounding the salt particles, explaining why we can't see them anymore.

20 min·Pairs

Real-World Connections

• Salt producers in places like the Pink Lake in Victoria use solar evaporation to separate salt from seawater, a process directly related to understanding solutions and recovery.
• Water treatment plants across Australia utilize filtration and chemical processes to remove impurities and create safe drinking water from natural sources, demonstrating the practical application of separating mixtures.
• Chefs and bakers frequently work with solutions and mixtures, understanding how ingredients like sugar dissolve in liquids or how to create stable emulsions in cooking.

Assessment Ideas

Quick Check

Provide students with small samples of sand, salt, and water. Ask them to write down the steps they would take to separate the sand from the salt water, and then to recover the salt from the water. Collect their written procedures for review.

Discussion Prompt

Pose the question: 'Imagine you have a cup of warm water and a cup of cold water. Which would you use to dissolve sugar faster, and why?' Facilitate a class discussion where students explain their reasoning, referencing particle movement and energy.

Exit Ticket

On an index card, ask students to draw a simple diagram showing what happens when salt dissolves in water at a particle level. They should label the solute particles and solvent particles.

Frequently Asked Questions

What is the difference between a solute and a solvent?

The solute is the substance that gets dissolved (like salt or sugar). The solvent is the liquid that does the dissolving (like water). Together, they make a solution!

Why does hot water dissolve sugar faster than cold water?

In hot water, the water molecules are moving much faster and have more energy. They bump into the sugar particles more often and with more force, breaking them apart and spreading them through the water more quickly.

How do we get salt from the ocean?

In places like Shark Bay in WA, large shallow ponds are filled with seawater. The sun evaporates the water (a physical change), leaving the solid salt behind to be collected. This is a great example of recovering a solute.

How can active learning help students understand solutes?

Active learning allows students to manipulate the variables of dissolving. By physically stirring, heating, or crushing solutes, they see the immediate effect on the rate of reaction. This 'trial and error' approach helps them build a much stronger understanding of solubility than simply reading a list of factors.