Pure and impure substances
Methods of separation
Name the everyday methods for pulling a mixture apart, and pick the right one for a given mixture.
Almost nothing arrives pure. Grain comes wrapped in husk, tea comes full of leaves, curd holds butter inside it. Each method in this chapter takes hold of one difference between the parts — size, weight, whether it dissolves, whether a magnet cares about it — and uses that difference to pull them apart.
Solid from solid
Handpicking takes out pieces that differ in size, colour or shape. Threshing beats stalks to free the grain. Winnowing blows air so the lighter husk is carried off and the heavier grain drops. Sieving uses a mesh that lets the small through and holds the large. Magnetic separation lifts out whatever a magnet takes hold of.
Solid from liquid
Sedimentation lets the heavier insoluble part settle at the bottom. Decantation then tilts the vessel and pours the liquid off. Filtration passes the liquid through something that holds the solid back. And evaporation drives the liquid off as vapour, which is the one that works when the solid has dissolved.
Have a play
The stages of clearing a pan of tea, and where the leaves end up at each one.
Tap any part of the picture.
Worked example
A mixture of salt and sand has been stirred into water. How would you get the sand out, and then the salt?
Ask what each part does in water: the salt dissolves and disappears; the sand does not.
That difference is the handle. Two solids behaving the same way in water could not be separated like this.
Try it together
Now work through a mixture of iron filings, sawdust and salt, stirred together into water.
Take out one component at a time, naming the method each time.
1.The iron filings are the odd one out — a magnet takes hold of them and nothing else. What is that method called?
Have a go
Have a go on your own. Curd is spun with a churner and butter comes out of it. What is that method called?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.