Find the Difference

Here is a mixture nobody has taught you: iron filings stirred into salt water.

You have no memorised method for that. And yet you can almost certainly work it out, because you already know the only question that matters.

What is different about the two parts?

Iron is magnetic and salt water is not, so a magnet lifts the iron straight out. What is left is salt water, and you know that one: boil it dry for the salt, or distil it if you want the water too.

That is the entire skill. There are eight separation methods in this lesson and you will meet all of them, but they are not eight things to memorise — they are eight differences worth knowing about. Every one of them works the same way: find a property where the two parts do not match, and use it to pull them apart.

You have done this before without noticing. In lesson 18 you asked three questions to identify a mixture — is it clear, does it settle, does a filter catch anything? Those same three properties are what separate it. Settling was a test; now it is a method called decanting. Filtering was a test; now it is filtration. Same physics, used twice: once to find out what you have, once to take it apart.

By the end of this lesson you'll be able to:

• name the eight separation methods and the difference each one exploits
• choose a method by looking at the mixture rather than by remembering
• explain why the same mixture can need two different methods depending on what you want to keep
• handle a mixture that needs more than one step