What is immiscibility?

What’s happening?

Oil and water are two liquids that are immiscible – they will not mix together. Liquids tend to be immiscible when the force of attraction between the molecules of the same liquid is greater than the force of attraction between the two different liquids. In simpler terms - like dissolves like! In the above case, water is referred to as a polar molecule whereas oil is non-polar.

Elements can bond together to form molecules by either sharing electrons between the elements or by one element donating electrons to another. To form water, the hydrogen atoms donate electrons (which have a negative electric charge) to the oxygen atom. This results in a slight positive charge on the hydrogen side of the molecule and a slight negative charge on the oxygen side (i.e. it forms a polar molecule with different charges at each end). In oil, the electrons are shared and distributed evenly throughout the molecule, so there is no electrical charge at either end (a non-polar molecule).

Food colouring is water-based and is a polar molecule; therefore it will not dissolve in the oil. Initially, when food colouring is added to the container, the food colouring does not dissolve but forms spheres. This reduces the contact area between the colouring and the oil. Stirring the mixture allows the food colouring to come into contact with water - another polar molecule - so it dissolves in the water layer, as can be seen by the changing color of the water.

Why does it matter?

Mixing different substances together is something we do every day. The polarity of a substance determines how it will dissolve into another substance. Food scientists, chefs and cooks who are developing new formulations of foods have to be aware of these properties of liquids. This is especially important if processed food products must sit in warehouses and on shelves in a store until they are purchased and used. Products like salad dressing usually have the label directions to “shake before using” because the oil and vinegar (the base of most salad dressings) have different polarities and are immiscible liquids. Products like this also contain another chemical called an emulsifier. Emulsifiers help polar and nonpolar molecules stay mixed together.

For example, a cook can make a salad dressing by adding herbs, spices to equal parts vinegar and oil. If it is poured onto the food without shaking, the first person will get mostly oil and no vinegar; oil (non-polar) and vinegar (polar) do not mix. Shaking will allow them to mix for a very short time. However, the addition of a small amount of mustard will not only enhance the flavour but also improve the ability of these two to mix. This is because the mustard contains an emulsifier.

Polarity also determines how well a substance will work as a solvent. Solvents are substances that are able to dissolve other substances. Good solvents get between the molecules, or mix with the substance you want to get rid of, and dilute it so that you can wipe it away. Water is a really good solvent for other polar molecules including substances that have a water-base (some water in them), like orange juice, maple syrup or latex water-based paint. Oil-based paints need to be cleaned up (or dissolved) with oil-based solvents, like turpentine or varsol (mineral spirits). When selecting a solvent, always remember ‘like dissolves like’!

Investigate further

• Check out the foods in your pantry cupboard or in your fridge for evidence of immiscible liquids. Can you see layers of liquids in certain products? Can you find the ingredients on the labels that might cause the layering? Are there directions to “shake before using” on the label?
• Look for other products, besides food products, that have immiscible layers. What are they used for? (e.g., make-up remover)

For more information on this topic check out these Let's Talk Science resources:

• What happens when we mix liquids? (Hands-on Activity) - Mix it up! What happens when you mix different liquids? Learn about the properties of liquids as you get mixing.
• Properties of Liquids and Solids (Lessons) - Students develop and apply observing, comparing & contrasting and predicting skills as they explore the properties of common liquids and solids.
• Why do oil and water not mix? (Hands-on activity) - What chemistry determines how oil and water behave when mixed? Explore the mixing of oil and water and the density of liquids in this activity.
• Lava Lamps (STEM in Context) - Lava lamps are very interesting to look at. But they are also interesting examples of buoyancy, heat transfer and solubility!