Pure substances and mixtures

Distilled water bought for ironing consists only of water, without salts or other substances. What is a material formed by a single substance called? Give another example from your everyday life.

Tap water seems like 'just water', but it contains small amounts of dissolved mineral salts. Is it a pure substance or a mixture? Justify your answer.

Complete the sentence with the missing words: 'A pure substance is formed by a single $\dots$; a mixture, by two or more. If its components cannot be distinguished in a mixture, not even with a magnifying glass or a microscope, it is $\dots$, and if they can be distinguished, it is $\dots$.'

A glass of salt water and a glass of distilled water look exactly the same: both are transparent. However, one is a homogeneous mixture and the other a pure substance. Which is which? Explain the difference between 'seeming to be' a single substance and 'being' a single substance.

Aoife and Seán are arguing about a glass of water with ice cubes like the one in the figure.

Aoife says that it is a heterogeneous mixture because two parts can be distinguished: the liquid and the solid. Seán says that it is a pure substance because everything is water. Give reasons for who is right by thinking about how many different substances are in the glass.

A bottle of mineral water advertises 'pure water' on the label. A classmate says that if it were truly pure, no trace would be left after evaporating a few drops on a glass pane.

1. a) What do you think would actually be seen on the glass pane when the water evaporates?
2. b) What does this result show about the mineral water? Is the label correct?

The air you breathe is a mixture of gases: mainly nitrogen and oxygen. Wherever you look, you do not see some gases on one side and others on another. The figure shows the particles in air:

Is it a homogeneous or heterogeneous mixture? Give reasons for your answer.

Aoife prepares a salad with lettuce, tomato, sweetcorn and tuna. Is it a homogeneous or heterogeneous mixture? Explain what you see to find out.

Bronze in medals is an alloy: copper and tin melted together, mixed so well that you cannot tell one from the other. Is bronze a pure substance or a mixture? Homogeneous or heterogeneous?

Hugo loves orange juice with pulp, in which small pieces of fruit float, as shown in the figure.

Is it a homogeneous or heterogeneous mixture? Give reasons for your answer.

Pick up a handful of sand from the beach and look at it with a magnifying glass: you will see quartz grains, bits of shell and traces of other minerals. What type of mixture is sand? Why, however, does the beach look like it is all the same colour from a distance?

Milk seems to be a perfectly uniform liquid, but under a microscope, tiny droplets of fat can be seen floating in it, as shown in the figure.

Is it a homogeneous or heterogeneous mixture? Explain why it seems to be the opposite with the naked eye.

On a foggy morning, the air is full of tiny water droplets in suspension. Clean air is a homogeneous mixture, but what about foggy air? Justify your answer.

A fizzy drink, newly bought and unopened, seems like a uniform liquid. Upon opening it and leaving it for a while in the glass, it fills with bubbles. When is it a homogeneous mixture and when is it heterogeneous? Explain the change.

Blood seems to be a uniform red liquid, but if it is left to stand in a test tube, it separates into layers: red blood cells at the bottom and a yellowish liquid called plasma at the top, as shown in the figure.

Is blood a homogeneous or heterogeneous mixture? What does the appearance of these layers indicate?

Milk and mayonnaise have something in common: they seem uniform, but under a microscope, tiny fat droplets can be seen distributed inside them.

1. a) What type of mixtures are they?
2. b) What conclusion do you draw about mixtures that 'seem' homogeneous to the naked eye?

You mix oil and vinegar in a jar and shake it vigorously: the mixture becomes cloudy and uniform. After ten minutes, two layers have formed, as shown in the figure.

1. a) At what point is the mixture heterogeneous: when just shaken, after standing, or always? Justify your answer.
2. b) What has changed while standing: the substances or only how they are distributed?

Classify these materials into pure substance, homogeneous mixture or heterogeneous mixture:

1. a) Distilled water.
2. b) The air in a room.
3. c) A yoghurt with strawberry pieces.
4. d) The oxygen from a medical cylinder.

Classify these mixtures as homogeneous or heterogeneous and justify your answer in each case:

1. a) Mountain air.
2. b) Gazpacho.
3. c) A stainless steel spoon.
4. d) A strawberry milkshake with pieces of fruit.

A $24$-carat gold ring is pure gold. A $18$-carat ring contains $75\ \%$ gold mixed with silver and copper, evenly distributed.

1. a) Classify each ring: pure substance, homogeneous mixture or heterogeneous mixture?
2. b) Both rings are yellow and shiny all over. Why is that not enough to claim that both are pure substances?

Look at a granite countertop: it has gray veins, shiny black dots and pinkish areas, because it is made up of several different minerals (quartz, feldspar and mica), as shown in the figure.

Is granite a homogeneous or a heterogeneous mixture? Justify your answer, taking into account that the countertop looks like "a single stone."

On the International Space Station, astronauts breathe a mixture of nitrogen and oxygen similar to Earth's air, evenly distributed throughout the spacecraft.

1. a) Is it a homogeneous or heterogeneous mixture?
2. b) Imagine that it were heterogeneous and the gases separated into layers inside the spacecraft. What problem would the astronauts have? Give reasons why it is so important to them that the mixture is homogeneous.

Find an example of each type of material in your home: a pure substance, a homogeneous mixture and a heterogeneous mixture. Name them and justify each choice by indicating what components you think each one has.

In the ironmonger's, they sell pure copper wire and steel nails, which are an alloy of iron and carbon.

1. a) Which of the two materials is a pure substance?
2. b) Steel is a mixture: homogeneous or heterogeneous? Justify your answers thinking about whether its components can be distinguished.