1. COMPOUNDS AND MIXTURES
In This Section We Will Be Studying These Topics:
Elements & compounds.
How do compounds differ from their elements?
Is it a chemical reaction?
Do compounds react chemically?
Compound or mixture?

2. ELEMENTS & COMPOUNDS
Elements contain just one type of atom e.g. oxygen has only atoms of oxygen.
Compounds are made up of at least two different elements.
Water, a compound, is made up of the elements hydrogen and oxygen.
There are millions of different combinations of the elements and each combination makes a different compound. Also, an element atom can occur more than once in a compound.
Hydrogen Atoms.
Oxygen Atom.
Water is a good example of a compound where an element occurs more than once.
Hydrogen Atoms.
Carbon Atom.
In methane (natural gas) there are even more hydrogen atoms.

3. ELEMENTS & COMPOUNDS C4H10
Instead of writing the name of an element, we can use symbols e.g. oxygen = O, hydrogen = H, carbon = C
So, for the compound water with two hydrogen and one oxygen atom, we could use the symbol HHO and methane would be CHHHH. H C H O
This method would soon get very complicated for compounds with lots of atoms like butane (bottled gas) CCCCHHHHHHHHHH! So scientists use a shorthand way of writing down formulae.
Rather than write HHO it is written H2O and instead of CHHHH for methane we write CH4. The small 2 and 4 are called subscripts. The subscript shows the number of atoms of an element in a compound and it always comes after the symbol for the element.
H2O and CH4 are the chemical formulae of water and methane.
What would be the chemical formula for butane?
C4H10

4. ELEMENTS & COMPOUNDS NaHCO3
Am I right in saying that if we know the formula we can work out the atoms in a compound?
Yes, right again Moley!
Sodium bicarbonate, baking powder has the formula NaHCO3. It is used to make cakes rise. Let us see what atoms make up this compound.
NaHCO3
3 atoms of Oxygen (O).
1 atom of Sodium (Na).
1 atom of Hydrogen (H).
1 atom of Carbon (C).
What are the atoms in …….?
H2SO4
2 atoms H, 1 atom S, 4 atoms O
CO2
1 atom C, 2 atoms O
C2H5OH
6 atoms H, 1 atom O, 2 atoms C
ZnCO3
1 atoms Zn, 1 atom C, 3 atoms O

5. IS IT A CHEMICAL REACTION?
What clues are there that a chemical reaction has taken place?
Is there a change of colour?
Can you see anything happening. Are there gas bubbles?
Is there a new smell?
Has there been a rise or fall in temperature? + =
Has there been a change of state? Are the products different to the reactants? +
If you mix materials can you ‘un-mix’ them? ?

6. IS IT A CHEMICAL REACTION?
All these clues are usually easy to detect in a laboratory but care should be taken when testing for them. Taking a big sniff of an unknown gas can be unpleasant or even dangerous.
Not all the clues are there in every reaction. In fact, only one is enough to confirm a reaction is taking place. For instance, there may only be bubbles of gas given off.
The more clues there are, the more likely it is that a reaction is occurring or has taken place.
Iron sulphate solution.
Copper.
Heat.
Iron filings.
Copper sulphate solution.
Here are three clues:
The blue liquid turns green.
The grey powder turns reddish.
The temperature of the liquid rises.

7. DO COMPOUNDS REACT CHEMICALLY?
So far we have talked about elements reacting to produce compounds, but do compounds react chemically?
If so, do they produce elements or compounds?
I think that compounds do react chemically, but I’m not sure what they produce. So I’ll say both elements and compounds.
What do you think the answers to the questions are?
Let’s have a look then.
If we put a piece of magnesium (an element) into hydrochloric acid (a compound) bubbles start to form and a gas is given off, so we know a chemical reaction has taken place. Both the element and the compound have changed. + Mg Cl H Mg Cl
A new compound – magnesium chloride. H +
The element hydrogen. →

8. DO COMPOUNDS REACT CHEMICALLY?
Reacting an element with a compound does not always produce another element.
When natural gas (methane) burns in oxygen, it produces carbon dioxide and water, both of which are compounds.
Now we will look at what happens when baking powder and hydrochloric acid react. Na O C
Sodium bicarbonate (baking powder) H O C
Carbon dioxide H + O
Water → + Cl H
Hydrochloric acid Cl Na +
Sodium chloride
In this reaction two compounds react to make three compounds.
There would be a lot of bubbles produced.
There would be a change of temperature.
How would you know that a chemical reaction was taking place?

9. COMPOUND OR MIXTURE? Properties Of Mixtures Examples Compounds
This table shows the properties of Mixtures and Compounds that make them different.
Properties Of
Mixtures
Examples
Compounds
The substances in a mixture are not joined together
Sugar and sand stirred together
The atoms of the reactants are chemically joined
Hydrogen and oxygen reacting to make water
The substances in a mixture can be easily separated.
Sugar and sand can be separated by mixing with water and filtering the sand out.
The atoms in a compound are very difficult to separate.
It takes lots of energy to split the hydrogen and oxygen atoms in water apart.
Mixtures can have any combination of elements or compounds or both.
Water and salt and iron = 2 compounds + 1 element.
Compounds must have atoms of at least two different elements.
Water has two different atoms, baking powder has four.
Mixtures do not have fixed quantities.
The amount of sugar you put in tea depends on how sweet you like it.
Compounds have fixed ratios of atoms
Hydrogen peroxide has an hydrogen to oxygen ratio of 2:2.

10. How can we tell it is a mixture?
COMPOUND OR MIXTURE?
The air around us is an important mixture. It is mainly made up of nitrogen and oxygen with very small amounts of other gases.
The Composition of Air
Nitrogen
78%
Other
Gases 1%
Oxygen
21%
In our lungs oxygen is easily separated from the other gases when we breathe.
How can we tell it is a mixture?

11. COMPOUND OR MIXTURE?
All elements and compounds have a melting point temperature at which they turn from a solid to a liquid. They also have a boiling point temperature at which they turn from a liquid to a gas.
For pure substances, ones not mixed with anything else, these temperatures are fixed. Scientists and engineers have created accurate lists of these temperatures.
Liquid
Gas
100oC
The melting point of water is 0oC. This is the temperature at which water turns to ice, or ice turns to water.
Liquid
Solid
0oC
The boiling point of water is 100oC. This is the temperature at which water turns to steam, or steam turns to water.
If we mix salt with water and freeze the mixture the temperature at which it turns to ice can be as low as -20oC.
If we heat the salt and water mix, we would find that the boiling point rises to about 104oC.

12. You can see that these are very cold temperatures!
COMPOUND OR MIXTURE?
Knowing the melting points and boiling points for the various gases found in air, we can produce pure gases for use in hospitals and factories.
Here is a table of the melting and boiling points for some of the gases found in air.
Gas
Melting Point (oC)
Boiling Point (oC)
Argon
-189
-186
Carbon dioxide
Changes straight from a solid to gas
-79
Helium
-272
-269
Neon
-249
-246
Nitrogen
-210
-196
Oxygen
-218
-183
You can see that these are very cold temperatures!

13. COMPOUND OR MIXTURE? Are the following mixtures? .
If they are mixtures, are they mixtures of elements or compounds?
Not a mixture –element.
Mixture – element & compound.
Not a mixture – one compound.
Mixture of elements.
Mixture of compounds.
Mixture of elements & compounds.

14. DO COMPOUNDS REACT CHEMICALLY?
Well Moley what did you think of that topic?
Quite interesting. There were some important chemical facts.
Compounds can react to produce other compounds and elements or just compounds.
A mixture can affect the freezing and boiling points of pure substances.
There are up to five signs indicating that a chemical reaction has taken place.
The atmosphere is a mixture of gases.
There is a special way of writing chemical formulae.
Compounds are made up of at least two different elements.