Acids, bases and salts

Question 1

Acid definition

Answer 1

Forms H+ ions when added to water. (They are H+ ion donors)

Question 2

Base definition

Answer 2

Reacts with acids by accepting an H+ ion and to form a salt and water

Question 3

Alkali definition

Answer 3

A base that dissolves in water to form OH- ions

Question 4

What colour is phenolphthalein in acid neutral and alkali

Answer 4

Acid: Colourless
Neutral: Colourless
Alkali: Pink

Question 5

What colour is methyl orange in acid neutral and alkali

Answer 5

Acid: Red
Neutral: Orange
Alkali: Yellow

Question 6

How do strong acids respond to water

Answer 6

Strong acids fully ionise in water

Question 7

How do weak acids respond to water

Answer 7

Weak acids only partially ionise in water

Question 8

Why are strong acids react in such a way

Answer 8

Strong acids contain more H+ ions and therefore give more vigorous reactions

Question 9

What is formed when a metal and an acid reacts

Answer 9

Metal + Acid -> Salt + hydrogen
(MASH)

Question 10

What is formed when a metal oxide and an acid reacts

Answer 10

Acid + Metal Oxide-> Water + salt
( BAWS)

Question 11

What is formed when a metal hydroxide and an acid reacts

Answer 11

Acid + Metal hydroxide -> Water + Salt
(BAWS)

Question 12

What is formed when a metal carbonate and an acid reacts

Answer 12

Acid + Metal carbonate -> water + salt + carbon dioxide
(CAWCS)

Question 13

What is formed when ammonia and an acid reacts

Answer 13

Acid + Ammonia -> Ammonium salt
(AAAS)

Question 14

Give examples of bases

Answer 14

metal oxide, hydroxides and carbonates

Question 15

Why does the reaction between aluminium and dilute acid start off slowly but then get more vigorous

Answer 15

• Aluminium is a reactive metal so reacts with oxygen in the air producing a hard protective coating of Al2O3.
• The acid has to react with this coating before it can react with the aluminium metal underneath and show its true reactivity. (The coating could be removed by sandpaper before the experiment)

Question 16

Why does the reaction between calcium and sulfuric acid start quickly but then slow down?

Answer 16

Calcium reacts with sulfuric acid to produce calcium sulfate, which is insoluble so covers the surface of the metal preventing further attack by the acid making calcium appear less reactive than it actually is.

Question 17

Crystallisation method

Answer 17

Take a salt solution:​
1. Gently heat to evaporate some water and form a saturated solution​
2. Let hot saturated solution to cool slowly to form hydrated salt crystals​
3. Gently dry crystals

Question 18

What is a saturated solution

Answer 18

A solution that can hold no more solute at a given temperature

Question 19

Why do crystals form as a hot saturated solution cools

Answer 19

• Solubility of salt decreases with temperature so as the solution cools, the solute comes out of solution. ​
• if this is done slowly, the particles have time to form an ordered array and “pull in” water molecules too

Question 20

How do you obtain pure dry crystals

Answer 20

1. Filter the crystals / decant any remaining solution
2. Leave in a warm place to dry slowly/ gently blot with paper towel

Question 21

What would happen if you heated the crystals strongly

Answer 21

it would remove the water of crystallisation and you would be left with a white anhydrous powder

Question 22

Give the method for the excess base method for making crystals

Answer 22

1. Carefully warm the acid while adding base or metal. (to speed up rate of reaction)​
2. Add the base or metal until in excess
3. Filter to remove the excess unreacted base or metal​
4. Gently warm salt solution to evaporate some water and make a saturated solution​
5. Let hot saturated solution cool slowly and form crystals​
6. Dry crystals gently

Question 23

How do you know when all the acid has reacted

Answer 23

when you see a build up of unreacted base

Question 24

What is the method for titration method of making salt

Answer 24

1. Measure 25cm3 of alkali into conical flask using pipette​
2. Add few drops of indicator (name it)​
3. Add the acid from burette until the acid has all been neutralised i.e. indicator just changes colour (state colour change)​
4. Calculate titre by subtracting final and initial volumes​
5. Repeat until concordant titres are achieved (e.g. similar volumes to +/- 0.2cm3)​
6. Calculate average titre​
7. Repeat titration without indicator adding exactly the average titre amount of alkali to make your salt solution​
8. Gently warm salt solution to evaporate some water and make a saturated solution​
9. Let hot saturated solution cool slowly and form crystals​
10. Dry crystals gently

Question 25

In titration when do you know all the acid has been neutralised and you have made a salt solution

Answer 25

Use an indicator

Question 26

In titrations would Universal indicator be a good indicator to use

Answer 26

No, the indicator must have a sharp colour change to give a definite end point to the titration

Question 27

How would you obtain a pure dry sample of salt from the solution formed

Answer 27

Repeat titration without indicator then crystallisation method

Question 28

State all the soluble compounds

Answer 28

- All group 1 and ammonium compounds - All nitrates - All halides e.g. chlorides ( except Ag and Pb (II) halides) - All sulphates ( except Ba, Pb, Ca sulphates)

Question 29

State all the insoluble compounds

Answer 29

- AgCl, AgBr, AgI - PbCl2, PbBr2, PbI2 - BaSO4, PbSO4, CaSO4 - All carbonates (except group 1 and ammonium) - All hydroxides (except group 1 abd ammonium)

Question 30

Precipitate definition

Answer 30

An insoluble solid product the forms from 2 solutions

Question 31

Precipitation definition

Answer 31

The formation of precipitate in the reaction

Question 32

Give method for precipitation method

Answer 32

1. Mix 2 solutions to form the precipitate (the insoluble salt)​ 2. Filter (keeping residue)​ 3. Wash residue with distilled water​ 4. Dry e.g. in an oven, providing it does not decompose on heating

Question 33

What do you wash the residue with distilled water

Answer 33

Tap water contains dissolved impurities so your precipitate would not be pure