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A short class with Sarah and her students. The other ways are under More.
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Oxygen is an element and water is a compound. What is the basic difference between them?
Oxygen has only one kind of atom, but water has hydrogen and oxygen atoms joined together.
Right. Can we break water into simpler substances?
Yes, by electrolysis. It splits into hydrogen and oxygen.
And can we break oxygen down further by any chemical method?
No, because it is already an element. That is why we write it with a symbol, O, and water with a formula, H₂O.
I have mixed iron filings with sulphur powder. Watch what happens when I bring a magnet close.
The iron filings jump onto the magnet and the yellow sulphur is left behind!
So in a mixture, each part keeps its own properties. Now I will heat some of the mixture strongly.
It is glowing red, and it keeps glowing even after you take it out of the flame.
That glow shows energy being given out. The black solid formed is iron sulphide. Let me try the magnet again.
Nothing happens. The iron has combined with the sulphur, so it is a new substance, a compound.
Two beakers hold clear, colourless liquids. One is pure water and one is salt water. How could we tell them apart without tasting?
Boil them. Pure water boils at 100 degrees Celsius at normal pressure.
Good. And the salt water?
It boils at a higher temperature, and that temperature depends on how much salt is in it.
Exactly. Compounds have fixed boiling and melting points, but mixtures do not. Is there another quick test?
Evaporate a few drops on a watch glass. If salt is left behind, that liquid was a mixture.
elements and compounds have a fixed composition, while mixtures can vary.
an element has one kind of atom and a symbol; a compound has chemically combined elements and a formula.
a compound has its elements in a fixed ratio by mass, such as 1:8 for hydrogen and oxygen in water.
a compound has new properties, while each part of a mixture keeps its own.
compounds need chemical methods, but mixtures can be separated by physical methods.
pure substances melt and boil at fixed temperatures, while mixtures do not.
Water is a compound: hydrogen and oxygen are always in the ratio 1:8 by mass, filtering cannot separate them, but electrolysis splits water into the two gases.
A mixture of iron filings and sulphur can be separated with a magnet, but after strong heating it becomes iron sulphide, a compound that a magnet does not attract.
Air is a mixture: the amount of water vapour and dust in it varies, its oxygen still supports burning, and its gases can be separated by fractional distillation of liquid air.
Brass, an alloy of copper and zinc, is a homogeneous mixture, while common salt is a compound whose properties are completely different from those of sodium and chlorine.
Compounds are fixed and change their face; mixtures vary and keep their place.
Iron and Sulphur went to a party and stood side by side, so a magnet could still pull Iron away. Then the party got very hot, they joined hands and became Iron Sulphide, and no magnet could separate them any more.
"Mixtures Mix, Compounds Combine": mixed parts keep their properties, combined parts form something new.
F.N.C.F. - Fixed ratio, New properties, Chemical separation, Fixed melting and boiling points: the four signs of a compound.
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Name two pure substances that are different kinds of matter.
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How is a compound broken into its elements?
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In what ratio by mass are hydrogen and oxygen combined in water?
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Why is air a mixture and not a compound?
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What shows that iron sulphide is a compound?
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A substance with a fixed composition and definite properties; either an element or a compound.
A pure substance made of only one kind of atom, which cannot be broken into simpler substances by chemical methods.
A pure substance formed when two or more elements combine chemically in a fixed ratio by mass.
Two or more substances mixed in any proportion, each keeping its own properties, separable by physical methods.
A mixture with the same composition throughout, such as salt water or air.
A mixture whose parts can be seen separately, such as sand and iron filings.
A homogeneous mixture of a metal with other metals or non-metals, such as brass, which is made of copper and zinc.
Breaking down a compound by passing an electric current through it, as when water is split into hydrogen and oxygen.
Elements, compounds and mixtures differ in what they are made of, whether their composition is fixed, whether they keep or change the properties of their parts, and how they can be separated.
This lesson brings the three kinds of matter side by side. It first separates pure substances (elements and compounds) from mixtures, then compares an element with a compound, and a compound with a mixture, point by point: composition, properties, method of separation, energy change, melting and boiling points, and how each is represented. The classic experiment with iron filings and sulphur shows all these differences at once, and simple tests, such as checking a boiling point, show how a chemist can tell a pure compound from a mixture.
Matter can be divided into pure substances and mixtures. A pure substance has a fixed composition and definite properties; elements and compounds are both pure substances. A mixture contains two or more substances that are simply mixed together, so its composition can vary and it is sometimes called an impure substance.
For example, passing an electric current through water (with a little acid added) splits it into hydrogen and oxygen, which are elements and cannot be split further by chemical means.
Two examples show the idea of fixed composition. In water, hydrogen and oxygen are always combined in the ratio 1:8 by mass, and in carbon dioxide, carbon and oxygen are always in the ratio 3:8. A sugar solution, by contrast, can be made weak or strong.
Mix iron filings and powdered sulphur. In the mixture, the yellow sulphur and grey iron can still be seen, a magnet pulls out the iron filings, and carbon disulphide dissolves the sulphur and leaves the iron behind. With dilute hydrochloric acid, the iron in the mixture gives off hydrogen, a gas with no smell.
Now heat the mixture strongly. It glows, and the glow spreads through the tube even after the flame is removed, showing that energy is given out. The product is iron sulphide, a black solid formed when iron and sulphur combine in the ratio 7:4 by mass. It is not attracted by a magnet, carbon disulphide cannot dissolve sulphur out of it, and with dilute hydrochloric acid it gives hydrogen sulphide, a gas that smells of rotten eggs. A new substance with new properties has formed, so it is a compound. (This is a teacher demonstration: carbon disulphide is poisonous and catches fire easily, and hydrogen sulphide is a poisonous gas.)