Exploring Mixtures and their Separation · Lesson 8 of 13
How Can We Separate the Components of Heterogeneous Mixtures?
“Oil and water finally agree on one thing: they would rather live on separate floors.”
• Explain why immiscible liquids form layers. • Relate layer position to density. • Operate a separating funnel conceptually and safely. • Recognise heterogeneous mixtures containing gases. • Choose physical properties for separating heterogeneous mixtures.
Pour water into a container holding mustard oil and two layers appear. Stirring may divide one liquid into droplets, but the liquids do not become one uniform phase. Their immiscibility and different densities provide a direct route to separation.
Immiscible liquids are liquids that do not mix uniformly and form separate layers when allowed to stand.
Separation of Two Immiscible Liquids
A separating funnel holds the mixture while the two layers settle. The denser liquid forms the lower layer and the less dense liquid forms the upper layer. A stopcock at the bottom allows the lower layer to be drained first without pouring the upper layer through it.
Activity: Let Us Separate
Pour about 5 mL mustard oil and 20 mL water into a separating funnel with the stopcock closed. Insert the stopper and allow the mixture to stand undisturbed until two clear layers form. Identify the lower water layer and upper oil layer.
Remove the stopper before draining. Open the stopcock slowly and collect the lower water layer. Close it as the boundary approaches. Collect a small boundary portion separately because it may contain both liquids, then collect the oil in another vessel.
| Observation or Action | Scientific Reason |
|---|---|
| Two layers form | Oil and water are immiscible |
| Water is below oil | Water is denser than cooking oil |
| Lower layer is drained first | The outlet is at the bottom |
| Boundary portion is collected separately | It may contain droplets of both liquids |
| Stopper is removed while draining | Air must enter so liquid can flow smoothly |
Problem
One litre of cooking oil has a mass of 910 g. Predict its position when mixed with water.
- 1.One litre of water has a mass of about 1000 g under ordinary conditions.
- 2.For the same volume, the oil has less mass and therefore lower density.
- 3.Oil and water are immiscible, so they form layers.
- 4.The less dense oil forms the upper layer.
- 5.A separating funnel can drain the water first and collect the oil separately.
What If …
If two immiscible liquids had exactly the same density, density would no longer determine a stable upper and lower layer in the usual way. A separating funnel would not provide reliable separation based only on vertical layering. Another distinguishing property or a different method would be needed.
Most gas-gas mixtures are homogeneous because gas particles move freely and mix readily. However, smoke contains solid particles in air, fog contains tiny liquid droplets in air, and dusty air contains solid matter spread through a gas. These are heterogeneous mixtures.
Many solid-solid mixtures are also heterogeneous. Iron nails can be removed from sawdust using magnetism. Separation succeeds when one component has a useful property that the other lacks.
Problem
A mixture contains iron filings and sulfur powder. Which property can separate them?
- 1.The components are visible solids in a heterogeneous mixture.
- 2.Iron is attracted by a magnet, whereas sulfur is not.
- 3.Move a magnet over the mixture with suitable protection around it.
- 4.Iron filings are lifted away, leaving sulfur behind.
Support the funnel securely. Remove the stopper while draining, open the stopcock slowly and never use the apparatus for unknown or hazardous liquids without trained supervision.
Quiz
Which description best matches Immiscible Liquids?
Which term matches this description: Immiscible liquids are liquids that do not mix uniformly and form separate layers when allowed to stand.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Explain why oil floats above water.
- Why is a small boundary portion collected separately?
- Give two heterogeneous mixtures with air as one component.
- Why would equal-density immiscible liquids be difficult to separate in a funnel?
Key Takeaways
• Immiscible liquids form separate phases. • The denser liquid usually forms the lower layer. • A separating funnel drains the lower layer first. • Smoke, fog and dusty air are heterogeneous mixtures containing a gas. • Separation depends on a useful difference in physical properties.