1. Y8: Knowledge organisers
These slides contain the facts you need to learn for each topic. Of course in your assessments the questions don’t just ask you recall based questions. Most of your questions will ask you to apply knowledge or evaluate procedures however you cannot do these kinds of questions without first knowing the facts.

2. Chemical and physical changes:
In a physical change no new substances are made.
In a chemical change the atoms in the reacting substances are split up and rearranged into the products. New substances are made.
Physical change
Chemical change
State changes eg, melting
Combustion
Dissolving
Neutralisation

3. Acids and alkalis: definitions
A substance that produces H+ ions when dissolved in water.
Base
A substance that is able to neutralise an acid
Alkali
A soluble base that forms OH- ions in water pH
A measure of how strong/weak and acidic or alkaline substance is
Examples of acids
HCl = hydrochloric acid
HNO3 = nitric acid
H2SO4 = sulfuric acid
Examples of alkalis
NaOH = sodium hydroxide
Examples of bases
Metal oxides
Metal carbonates
CuO = copper oxide
CaCO3 = Calcium carbonate
NH3 = ammonia

4. Acids and alkalis: The pH scale
The pH scale is a numerical scale which shows how strong or weak and acid or alkali is.
0- <4
4- <7 7
>7-10
>10-14
Strong acid
Weak acid
Neutral
Weak alkali
Strong alkali
There are 2 ways to determine the pH of a substance.
Both are used with substances dissolved in water.
pH meter
Indicator
Gives a numerical result on a digital readout.
Changes colour at different pHs.
The colour must be compared to a comparison chart to determine the pH.

5. Acids and alkalis: household substances
Common household substances and their pHs
Acids
Vinegar
2-4
Lemon juice
3-5
Neutral substances
Pure water 7
Salt water
Alkalis
Toothpaste 8
Oven cleaner 10

6. Acids and alkalis: using indicators
Indicators are substances that have different colours at different pHs. They are available in solution and paper form.
Some indicators have general uses whereas others are used for very specific purposes.
Universal indicator
An indicator made from a mixture of different indicators which gives a rainbow of colours at different pH values.
Litmus
An indicator that is blue in alkali and red in acid. Most often found in paper form (red litmus paper or blue litmus paper).
Phenolphthalein
A solution indicator which is colourless in acid and pink in alkali.

7. Acids and alkalis: making a natural indicator
Lots of plants are able to act as indicators as they contain pH sensitive coloured pigments.
Examples: Red cabbage and poinsettia
1. Cut up the plant into small pieces. Put it in a beaker.
2. Pour water over the pieces.
3. Boil the mixture for several minutes until a deep colour is obtained.
4. Filter off the pieces of plant to leave a solution of indicator.
Remember that natural indicators will have unique colours at different pHs, they won’t give the same colours as universal indicator.

8. Acids and alkalis: neutralisation
Bases and alkalis can neutralise acids. The H+ ions that cause the acidity react, and the H atoms become part of a new water molecule. The H+ ions are used up and the acidity ‘cancelled out’ or neutralised.
Ionic equation for neutralisation
H+ + OH-  H2O
This just shows what happens to the ions. When we look at all the substances involved we can write word equations for the different neutralisations. These reactions form soluble salts.
Acid + alkali
Salt + water
Acid + base
Specific examples can be found in the salt formation section.

9. Acids and alkalis: monitoring neutralisation
If we are doing a neutralisation reaction we have to have some way of knowing when the neutralisation process is finished. Otherwise we might add too much alkali.
A pH curve is a graph which shows what happens to pH when acids and alkalis react. The long straight, vertical portion of the graph is the neutralisation point.
Most commonly we use indicators to monitor a neutralisation reaction and stop adding when a particular colour change occurs. Not all indicators are suitable for this purpose
Good
Phenolphthalein
Sharp colour change (colourless to pink or pink to colourless) at neutralisation.
Bad
Universal indicator
Has a range of colours and neutral colour would be green. Difficult to judge if it is a yellow-green (so slightly acidic) or green-blue (slightly alkaline) shade.

10. Reactions of acids: salt formation
General word equations for acid reactions
Reactants
Products
Acid +
Alkali
Salt
Water
Base
Metal carbonate
Carbon dioxide
Reactive metal
Hydrogen
How to work out the name of the salt formed
A salt has 2 parts to its name
A metal part (1st word)
A non metal part (2nd word)
The metal part comes from the base/alkali/carbonate/metal.
Eg, sodium hydroxide gives salts where the 1st word is sodium.
The non metal part comes from the acid.
Hydrochloric acid
~chloride
Sulfuric acid
~sulfate
Nitric acid
~nitrate

11. Experiment: Salt formation from acid and alkali
Measure a volume of acid with a syringe and put in a small beaker
Add 2 drops of phenolphthalein
Using a syringe add small portions of alkali until the solution turns pink
Note the volume of alkali used
Repeat the experiment but leave out the indicator
Evaporate the resulting solution to get crystals of salt

12. Experiment: Salt formation from acid and metal oxide
Measure a volume of acid.
Add the metal oxide to excess (so solid can be seen)
Heat the reaction mixture. If all the solid dissolves add more to ensure an excess is used.
Filter off the excess solid
Evaporate the solution to obtain the salt crystals
Precaution
Reason
Using excess metal oxide
To make sure all the acid is neutralised (so it doesn’t get heated up in the next step) so a high yield of crystals will be obtained.
Heating up
To make sure the reaction is complete (lots of experiments are very slow at room temperature)

13. Chemical formulae Most elements in the Periodic Table are atomic
Fe, Cu, Na, Ne, Pd, Si
Some elements are diatomic
Elements in group 7 F2, Cl2, Br2, I2
Hydrogen, oxygen and nitrogen H2, O2, N2
Common acids
Hydrochloric acid HCl, sulfuric acid H2SO4, nitric acid HNO3
Bases and alkalis
Ammonia NH3, Sodium hydroxide NaOH
Other substances
Carbon dioxide CO2, Water H2O

14. Chemical equations: what they show
A convenient way of describing a chemical reaction is to write an equation. There are 2 types –
Word equations
Balanced symbol equations
When chemical reactions happen the atoms in the reactants (shown on the left of the equation) are broken up and rearranged to make the products (shown on the right of the equation).

15. Word equations:
Word equations are useful for remembering the general patterns of reactions
Eg, you remember that acid + alkali gives salt + water
You can then apply this knowledge to a specific example given in a question instead of learning lots of individual pairs of reactants and their products.
Apart from this they are not very useful because chemists acros the world have different names for substances.

16. Law of conservation of mass
“The law of conservation of mass states that mass is neither created nor destroyed. In a closed system, the mass of the reactants is equal to the mass of the products.”
In simple terms:
1. During any chemical reaction no particles are created or destroyed: the atoms are simply rearranged from the reactants to the products.
2. Mass is never lost or gained in chemical or physical changes (although it may seem that it is from measurements taken during these processes).

17. Conservation of mass in chemical change
Mass is conserved in chemical change
Some reactions seem like they don’t conserve mass, these tend to be those that produce gases (so the gases float off and aren’t measured by the balance)

18. Conservation of mass in physical change
Melting
A 2g ice cube weighs the same when it is melted
2g water
Dissolving
1g sugar cubes
50g water
Sugar solution – 51g

19. Chemical formulae
A chemical formula shows the number of atoms of each elements in a substance. Each element is represented using its symbol (found in the Periodic Table). Remember that these atom symbols also start with a capital letter. Some may be just a capital letter e.g. S = sulfur others may be a capital letter and a lowercase letter e.g. Si = silicon
If one atom of an element is present then there is no number in the formula.
E.g. NaOCl
In this formula there is one atom of each Na (sodium), O (oxygen) and Cl (chlorine)
When there is more than one atom of an element, a subscript (smaller and lower down) number is put to the right of the symbol. This multiplies the atom to the left of it.
E.g. H2O
In this formula there are 2 atoms of hydrogen and one of oxygen.
There may also be brackets in a formula. In this case the subscript number multiplies everything in the brackets to the left.
E.g. Mg(OH)2
E.g. Cu(NO3)2
In this formula there is one atom of magnesium and 2 each of oxygen and hydrogen
In this formula there is one atom of copper, 2 atoms of nitrogen and (2 x 3 = 6) atoms of oxygen

20. Conservation of mass: balanced chemical equations
A symbol equation shows how reactants turn into products. To obey the law of conservation of mass they must have the same number of atoms of each element on both the left and right hand side of the equation.
Na + Cl NaCl
Count the atoms on the left and right hand sides of the equation given above. This equation is unbalanced. There are 2 atoms of chlorine on the left and one on the right.
We must balance the equation. You might think the easiest thing to do is to make the chlorine into just Cl rather than Cl2 but this isn’t allowed, we can’t just change the formula of a substance to make our balancing easier!
RULES: To balance an equation we put big numbers in front of a formula. These multiply the whole formula to the right.
Going back to our equation – to make 2 Cl on the right hand side we put a 2 in front of the NaCl
Na + Cl NaCl
But this also changes the number of Na atoms so this isn’t yet finished. So we need a 2 in front of the Na as well.
The balanced equation is : 2Na + Cl NaCl

21. Some equations to practice with…

22. Combustion – reactions with oxygen
When a substance is burned it combines with oxygen in the air.
The new compound is called an oxide.
Air is a mixture of gases, only around 18% is oxygen, the majority of air is the unreactive gas nitrogen.
Combustion reactions can take place in room air or in pure oxygen.
The observations may be different depending on this.

23. Metal and non metals: where we find them on the Periodic Table
We draw a stepped line under boron.
All the metals are to the left and all the non-metals to the right of this line.

24. Metals and non metals: properties
Typical metal properties
Typical non-metal properties
Shiny
Dull
High melting point (tend to be solid at room temperature)
Lower melting points
Conduct electricity
Not usually conductors, often insulators
Conduct heat
Tend not to conduct heat
Sonorous

25. Metalloids and elements with inbetween properties
Some elements have atypical (not typical) properties for their classification.
E.g. Mercury is classed as a metal but it is liquid at room temperature, it has a low melting point
E.g. carbon graphite is classed as a non metal but it is shiny and can conduct electricity
The elements either side of the stepped line we draw on the Periodic Table are sometimes called metalloids or semi-metals. They tend to have a mix of properties, some that would be typical for metals and some that would be typical for non-metals.

26. Metals and non metals: Burning metals in air

27. Metals and non metals: burning metals in excess oxygen
Observation
Magnesium
Burns with a bright white flame
Sodium
Burns with a bright yellow/orange flame
Potassium
Burns with a lilac flame

28. Metals and non metals: burning non metals in oxygen
Observation
Carbon
Bright yellow flash
Sulfur
Burns with a blue flame

29. Metals and non metals: acidic and basic properties of the oxides
Metal oxides are alkaline
Non metal oxides are acidic
(Remember that we need to dissolve the oxide to test its pH, either with a pH meter or with a suitable indicator.)

30. Metals and non metals: Reactive metals and water