Acids, Bases, and Salts

Question 1

What are acids?

Answer 1

Substances that form hydrogen (H+) ions when dissolved in water

Question 2

What are bases?

Answer 2

Any substance that can neutralise an acid
Either soluble or insoluble
Soluble bases release hydroxide (OH-) ions in liquids - these are called alkalis
Insoluble bases are metal oxides and metal carbonates

Question 3

What happens when an acid and alkali react?

Answer 3

Hydrogen ions react with hydroxide ions to form the neutral solution: water
H+ + OH- = H2O

Question 4

What does the strength of an acid depend on?

Answer 4

How much it ionises in water (how many acid molecules break up to form H+ ions)

Question 5

What do weak acids do?

Answer 5

Do not fully ionise in water and so reactions with weak acids are much slower and less exothermic
Most acids are weak
E.g.
Ethanoic acid

Question 6

What do strong acids do?

Answer 6

Fully ionise in water so reactions are faster and more exothermic
Strong acids include:
Hydrochloric acid
Sulphuric acid
Nitric acid

Question 7

What are examples of strong bases?

Answer 7

Aqueous sodium hydroxide + Potassium hydroxide

Question 8

What is an example of a weak base and why is it weak?

Answer 8

Ammonia - Only a few NH3 molecules react with water molecules to form ions

Question 9

What is the difference between strength and concentration of an acid?

Answer 9

Strength = How much of the acid ionises in water
Concentration = How many moles of H+ ions are in a unit volume

Question 10

What are examples of indicators and how can we tell whether something is an acid or alkali by using them?

Answer 10

Litmus paper:
Red in acid
Blue in alkali

Methyl orange:
Red in acid
Yellow in alkali

Question 11

What does a reaction between an acid and a metal produce?

Answer 11

Salt + Hydrogen

Question 12

What does a reaction between an acid and a base produce?

Answer 12

Salt + Water

Question 13

What does a reaction between an acid and a metal carbonate produce?

Answer 13

Salt + Water + Carbon dioxide

Same as bases but with added carbon dioxide, because a carbonate is also a base and so neutralises an acid

Question 14

What are examples of bases that neutralise acids?

Answer 14

Oxides and hydroxides of metals
e.g.
Sodium hydroxide
Potassium hydroxide
Copper (II) oxide

Question 15

What names do different acids give salts?

Answer 15

Sulfuric acid - Sulfate
Hydrochloric acid - Chloride
Nitric acid - Nitrate

Question 16

What is produced when a base is reacted with ammonium salt?

Answer 16

Salt + Ammonia gas + Water

Question 17

How can we solve the problem of a soil’s pH level being too low?

Answer 17

Add a suitable base e.g. slaked lime to control the acidity of the soil so it is the optimum pH for plant growth

Question 18

What can be observed when an acid reacts with a metal carbonate?

Answer 18

Fizzing occurs (from carbon dioxide produced)
Turns limewater milky

Question 19

How would you test for a carbonate?

Answer 19

Add acid - Results in fizzing

Gas turns limewater milky

Question 20

How would you test for an acid?

Answer 20

Add any carbonate - Fizzing occurs

Question 21

How would you test for sulfate ions?

Answer 21

Add barium chloride to a SOLUTION of the sulfate - White precipitate is formed

Question 22

What are redox reactions?

Answer 22

Reactions which involve a reduction reaction (one reactant gains electrons) and an oxidation reaction (one reactant loses electrons)
e.g. In reaction between Mg and HCl, magnesium atoms are oxidised, hydrogen atoms are reduced, and chlorine atoms remain unchanged

Question 23

How do we prepare soluble salts?

Answer 23

Through a reaction between acids and solid insoluble substances like metals, metal oxides, metal hydroxides, or metal carbonates

Question 24

How would you produce blue copper sulfate crystals?

Answer 24

Sulfuric acid + Copper carbonate OR Copper oxide

Copper carbonate is a green powder, copper oxide is a black powder

Question 25

What are the steps to producing a dry sample of a soluble salt? E.g. Making copper sulfate crystals

Answer 25

When using copper carbonate: 1. Keep adding the metal to the acid until fizzing stops and powder is left over (in excess) 2. Filter out excess copper carbonate so filtrate contains only copper sulfate solution 3. Remove water by putting solution in an evaporating dish OR speed up process by removing ~1/3 of the water with heat

Question 26

How would you separate different mixtures?

Answer 26

Mixture of insoluble solid and liquid - Filtration Mixture of soluble solid and liquid (solution) - Crystallisation Mixture of liquids - Simple distillation/ Fractional distillation Mixture of dissolved solids - Chromatography

Question 27

What do the sections on the pH scale mean?

Answer 27

``` 1-6 = Acidic 7 = Neutral 8-14 = Alkaline ```

Question 28

How would you measure the pH of a solution?

Answer 28

Universal indicator - Changes to a particular colour depending on the pH of the solution pH probe - Returns a number which represents the pH of the solution

Question 29

What exactly is the pH scale?

Answer 29

A measure of the concentration of H+ ions in a solution

Question 30

What does titration measure?

Answer 30

The volumes of acid and alkali solutions that are needed in a neutralisation reaction - making us able to work out the concentration of either the acid or the alkali if we know the concentration of the other

Question 31

Which indicators can be used in titration and what is their colour change?

Answer 31

Phenolphthalein: Pink in alkali, colourless in acid | Methyl orange: Yellow in alkali, red in acid

Question 32

Why do we need indicators in titration?

Answer 32

To show the end point of a titration by changing colour when the solution changes from acidic to alkaline or from alkaline to acidic

Question 33

What are the steps in titration?

Answer 33

1. Measure a known volume of alkali into a flask 2. Add indicator 2. Put acid of known concentration into a burette and take reading (doesn't need to be 0) 3. Add acid to alkali until it has neutralised 4. Read burette to discover how much acid it took to neutralise the alkali 5. This is the rough titration, ignore this result and repeat 3 more times and average these results

Question 34

Why do we do a rough titration?

Answer 34

To know roughly where the neutralisation point is so we can be more accurate the next time

Question 35

How would you use titration to make a soluble salt?

Answer 35

1. Carry out titration as normal, however this solution is not pure as it has been contaminated with indicator 2. Repeat with fresh alkali in beaker but do NOT add indicator 3. Add volume of acid you know will neutralise it exactly 4. Evaporate the water to recover dry salt