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.

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.

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

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.

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?

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.

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."