Dissolving, Solutions and Separating Mixtures
In 2010, Sydney's Kurnell Desalination Plant began pushing 250 megalitres of seawater through membranes each day — and the salt never disappeared, just separated from the water.
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● Know
- A mixture is two or more substances physically combined.
- A solution is a special clear mixture where one substance dissolves in another.
- Mixtures can be separated using physical methods — no reactions needed.
● Understand
- Dissolving is a physical change — the solute is still there, just spread through the solvent.
- Solutions look uniform because the solute particles are spread evenly.
- Which separation method works depends on the physical property you can exploit (size, boiling point, magnetism, density).
● Can do
- Identify the solute, solvent and solution in everyday mixtures.
- Choose a sensible separation method for a given mixture.
- Explain why dissolving is a physical change, not a chemical one.
When a solid disappears into a liquid, it has not been destroyed — it has dissolved. The substance that dissolves is called the solute, and the substance doing the dissolving is the solvent. Together they form a solution: a homogeneous mixture where the solute particles are evenly spread among the solvent particles. You cannot see them, but they are still there.
Dissolving is a physical change, not a chemical one. The solute can be recovered unchanged by evaporating the solvent. At the particle level, dissolving happens when solvent molecules surround solute particles and pull them away from the crystal lattice into the liquid.
Stir a teaspoon of table salt into a glass of water. The sodium and chloride ions separate and spread evenly through the water. If you leave the glass on a windowsill for several days, the water evaporates and tiny salt crystals reappear at the bottom — proof the salt was never destroyed.
BlueScope Steel, one of Australia's largest manufacturers, uses filtration and distillation in its industrial processes. Separating mixtures efficiently is not just a classroom exercise — it is essential for producing clean water, recovering valuable chemicals, and protecting the environment.
Many students think that when something dissolves, it is destroyed or ceases to exist. This is false. Dissolving is simply particle-level mixing. The solute is still present; its particles are just too small to see and too spread out to settle out.
Because dissolving is a physical change, the solute can be recovered using separation techniques that exploit differences in physical properties. Filtration separates insoluble solids from liquids by passing the mixture through a porous barrier. Evaporation removes the solvent by heating, leaving the dissolved solid behind. Distillation separates liquids with different boiling points by boiling one off and condensing the vapour. Chromatography separates mixtures based on how fast components travel through a medium.
Choosing the right method depends on the properties of the mixture. Ask yourself: is the solid soluble? Are the liquids miscible? Do the components have different boiling points? The answer tells you which technique to use.
To separate sand and salt mixed in water, first use filtration to trap the insoluble sand. Then heat the saltwater filtrate to evaporate the water. The salt crystals that remain are the same substance you started with — no chemical reaction has occurred.
The Australian Synchrotron near Melbourne uses advanced separation techniques in materials science research. Scientists there analyse complex mixtures to develop new medicines, stronger alloys, and more efficient catalysts — all built on the same principles of solubility and separation.
A surprisingly common error is thinking that saltwater is a compound. It is not. Saltwater is a mixture because the salt and water are physically combined and can be separated by evaporation. A compound requires chemical bonding in a fixed ratio, which saltwater does not have.
Selecting the correct separation method is a practical skill that scientists use every day. The key is to match the technique to the property you want to exploit. If the solid particles are too large to pass through filter paper, use filtration. If the solid has dissolved, use evaporation or crystallisation to recover it. If you need to separate two liquids with different boiling points, distillation is the answer. For mixtures of coloured pigments, chromatography reveals the hidden components.
An immiscible liquid mixture — such as oil and water — can be separated with a separating funnel because the two liquids form distinct layers and have different densities.
Forensic scientists use chromatography to analyse ink from a forged document. A tiny spot of ink is placed on special paper and solvent is allowed to move up it. Different pigments travel at different speeds, separating into a pattern of colours that can be matched to a specific pen.
The Great Barrier Reef Marine Park Authority regularly monitors water quality around the reef. Filtration and chromatography help scientists detect pollutants, excess nutrients, and microplastics in seawater, guiding conservation efforts for one of Australia's most precious ecosystems.
Students often believe that filtration can separate dissolved substances from water. It cannot. Dissolved particles such as salt ions are far smaller than the pores in filter paper and pass straight through. To recover a dissolved solid, you must change the state of the solvent through evaporation.
- Sand in water
- Salt dissolved in water
- Ink colours
- Alcohol and water
- Oil and water
- Separating funnel
- Chromatography
- Distillation
- Evaporation
- Filtration
One of the most important ideas in this topic is that dissolving does not create a new substance. The solute and solvent retain their individual chemical identities; they are simply mixed at the particle level. This is why solutions are classified as mixtures, not compounds. If you had a compound, you would need a chemical reaction to break it apart.
Another common point of confusion is the difference between soluble and insoluble. A soluble substance disappears into the solvent to form a clear solution. An insoluble substance does not dissolve and can be seen floating or settling at the bottom. The same substance can be soluble in one solvent and insoluble in another.
Sugar dissolves easily in hot tea but does not dissolve in cooking oil. Chalk is insoluble in water but reacts with acid. Knowing which category a substance falls into is the first step in choosing the right separation technique.
CSIRO leads world-class research into desalination technologies for drought-prone Australia. By understanding how salt dissolves in seawater and how to separate it efficiently, CSIRO engineers design plants that provide fresh water to communities across the country.
Many students confuse a solution with a pure substance because it looks clear and uniform. A solution is actually a mixture. The solute is still present, just evenly spread out. You can prove this by evaporating the solvent and watching the solute reappear.
You have a mixture of sand, salt, and water. Predict which separation methods you would use and in what order.
How close was your prediction?
Nice calibration — your intuition is good for this kind of problem.
Good — being surprised is the point. This answer is worth remembering.
Separating mixtures is not just an academic exercise — it is a foundation of modern science and industry. Every separation method exploits a difference in physical properties. Filtration uses particle size. Evaporation uses the difference in boiling points between solute and solvent. Distillation uses differences in boiling points between two liquids. Chromatography uses differences in how strongly components stick to a medium.
When you are faced with a complex mixture, break the problem down step by step. Remove the easiest component first, then choose the next method for what remains. Good experimental design is as much about planning as it is about execution.
To separate a mixture of sand, salt, iron filings, and water, you would use four different properties. A magnet removes the iron filings by magnetism. Filtration removes the sand by particle size. Evaporation recovers the salt by boiling point difference. Each step isolates one component using a unique physical property.
The Bureau of Meteorology distils water to precise purity standards for calibrating sensitive atmospheric instruments. Even trace impurities can skew measurements, so understanding separation at a deep level ensures accurate weather forecasts for Australian farmers and emergency services.
Some students think that dissolving creates a new substance with new properties. It does not. Sugar dissolved in water still tastes sweet, and saltwater still conducts electricity because both original substances are present unchanged. A new substance would require a chemical reaction.
At the start of this lesson, you thought about how the Sydney Desalination Plant separates salt from seawater even though the salt seems to have completely vanished.
Now that you've explored dissolving and solutions, revisit your first ideas. Did you think dissolving was physical or chemical? What evidence from today's lesson confirmed or changed your mind?
1. In a saltwater solution, which is the solvent?
2. Which method would you use to separate sand from water?
3. What type of change is dissolving sugar in water?
4. Which of the following is a solution?
5. How could you recover salt from saltwater?