A compound is a pure substance that contains 2 or more elements chemically combined. It is made up of different elements chemically combined in a fixed ratio (will digress on this point later).
So the practical of today.
Part 1: Investigating a mixture of elements
In this practical, we're using the two elements, iron and sulfur. (I forgot to take pics of them individually. >< but you should know how they look like)
Appearance of iron filings: Silvery grey, powder form
Appearance of sulfur: Yellow, powder form
Placing a spatula of iron filings and another spatula of sulfur on a piece of filter paper, we mix them together to form a mixture:
And so we try to use the first method of separation: Using a magnet to attract the iron filings:
The iron filings are attracted to the magnet underneath the filter paper. We were able to use the magnetic property of iron and non-magnetic property of sulfur to separate the iron filings from the sulfur.
Next: Adding the iron and sulfur mixture into a test tube of water, we carried out the next method of separation.
As you can see, we are making use of the densities of the elements to separate the mixture. Iron is denser than sulfur, hence, sinking to the bottom of the test tube and sulfur floating on the surface of the water. Although they do separate, there is still some sulfur that remains unseparated from the iron filings at the bottom.
Learning points:
-The substances in a mixture can be mixed in any proportion.
-When a mixture is formed, heat and light are not usually taken in or given out.
-The mixture can be separated by physical means.
-The mixture has the properties of its constituent substance.
Part 2: Compounds
1. Mix one spatula of sulfur and half a spatula of iron filings in a crucible.
2. Cover the crucible with a lid and heat for 10 mins using a strong flame.
(This is where lighting the Bunsen Burner gets handy.)
Note: Sulfur is flammable and produces a poisonous gas (sulfur dioxide). It is, on its own, poisonous so beware when handling. (If you've been to a geothermal park, you'll know that sulfur can smell quite bad.)
And so the wait begins...
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3. After 10 minutes, allow the crucible to cool for a while before removing it. (Use tongs!)
Open the lid and examine the residue left.
The residue left should no longer be in powder form, but solid. Do some tests on it. Does the residue separate into iron and sulfur with a magnet? Does the residue separate when in water? If you have done the experiment properly, your answers should be no, as now, the iron and sulfur have become a compound called iron sulfide (you can see where the name came from).
(Closer look)
Now, you may notice the grey powder. That is excess iron filings. As promised, I shall digress. Compounds are made up of elements chemically combined in a fixed ratio. As there is excess iron filings, this proves that all the sulfur has already been spent reacting with the iron filings to make iron sulfide, and the excess, unreacted iron filings are left behind. Hence, proving that compounds are formed of elements combined in a fixed ratio.
The chemical reaction takes place when a compound is formed due to an energy change, in this case, is heating.
My thoughts on the practical:
This practical has enhanced my understanding on how compounds are formed, and even solid elements can become compounds (I used to think that only liquid chemicals can form compounds. XD sounds VERY silly now. Very. Very. Silly. )
[And I'm sort of glad that I had a blocked nose then. XD I went to a geothermal park in New Zealand once, and you could smell the sulfur from the car. It was really cool though.]
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