Acids, Bases and Salt Preparations

Question 1

Which ionic compounds are always soluble in water?

Answer 1

Compounds containing potassium, ammonium, sodium and all nitrates

There are no exceptions with these. You can remember it with PANS

Question 2

What are the rules for ionic compounds which are chlorides regarding solubility in water?

Answer 2

All chlorides are soluble except for those of silver and lead(II)

Ionic compounds are generally soluble compared to covalent compounds, but there are some exceptions as you can see.

Question 3

What are the rules for ionic compounds which are sulfates regarding solubility in water?

Answer 3

All sulfates are soluble except for those of barium, calcium, and lead(II)

Question 4

What are the rules for ionic compounds which are carbonates regarding solubility in water?

Answer 4

All carbonates are insoluble except for those of sodium, potassium and ammonium (as expected)

Question 5

What are the rules for ionic compounds which are hydroxides regarding solubility in water?

Answer 5

All hydroxides are insoluble except for those of sodium, potassium, ammonium (as expected) and calcium

Calcium hydroxide is only slightly soluble in water

Question 6

What are the rules for ionic compounds which are oxides regarding solubility in water?

Answer 6

Oxides are generally insoluble, but with exceptions (such as PANS)

Question 7

What is the difference between a base and an alkali?

Answer 7

• An alkali is a base which is water soluble
• All alkalis are bases but not all bases are alkalis

Question 8

Are acids seen to be proton donors or proton acceptors?

Answer 8

Proton donors

Question 9

Why are acids seen to be proton donors?

Answer 9

• When dissolved in an aqueous solution they produce H+ ions, which are just protons
• These protons are donated to hydroxide ions

Remember, H+ ions are what make a solution acidic

Question 10

Are bases seen to be proton donors or proton acceptors?

Answer 10

Proton acceptors

Question 11

Why are alkalis seen to be proton acceptors?

Answer 11

• When dissolved in an aqueous solution, they produce OH- ions
• These OH- ions accept H+ ions (protons) from acids

Remember, OH- ions are what make a solution alkaline

Question 12

What are the two products that always form when a base reacts with an acid (neutralisation)?

The base could be a hydroxide, oxide or carbonate

Answer 12

A salt and water

(Apart from ammonia which only produces a salt)

Question 13

What extra product forms when a metal carbonate reacts with an acid?

Answer 13

Carbon Dioxide

These reactions are easily distinguishable because effervescance/bubbling is produced due to the carbon dioxide

Question 14

What happens when an acid reacts with ammonia?

Ammonia is still a base, but acts slightly differently

Answer 14

An ammonium salt is produced, and nothing else

The same happens when ammonia reacts with water, just ammonium hydroxide is produced, nothing else

Question 15

How do you prepare a soluble salt from an insoluble base and acid?

(Using the example of making copper sulfate)

Answer 15

• React a fixed volume of acid (sulfuric acid) with an excess of a solid insoluble base (copper oxide) and heat the solution, stirring with a glass rod until the reaction finishes
• Use filtration with a funnel and filter paper to ensure that only the soluble salt and water remains (the excess insoluble base will be filtered out)
• Crystallise this solution by heating it to evaporate some (around half) of the solvent in the solution so it becomes saturated and then removing the heat and allowing crystals to form as it cools for a few hours
• Filter out the remain solution by decanting it, then collect the crystals and dry them on/with a paper tower or evaporating dish in a warm place (such as a warm oven)

• You could also do the reaction by just reacting a metal with the acid, but it must be reactive enough, but not too reactive that it is dangerous (so this method is not used much)
• Alternatively a hydroxide or carbonate of a metal could be used

Question 16

How do you prepare a soluble salt from an acid and alkali?

Answer 16

• Do a titration to find the exact amount of acid necessary to neutralise an alkali, and then repeat it without the indicator
• Heat the solution so some of the solvent (water) evaporates and it becomes more concentrated (or saturated)
• Remove the heat, allowing pure salt crystals to form as it cools for a few hours, then filter out the remaining solution and dry the crystals
• This salt will be soluble (for example NaCl from HCl and NaOH)

You need to add the exact right amount otherwise there would be excess reactants, making the sample impure

Question 17

How do you prepare an insoluble salt?

(Using the example of making lead(II) sulfate)

Answer 17

• Mix two solutions of soluble salts together (Lead nitrate and another soluble sulfate salt or sulfuric acid ) and stir
• Filter it to remove the precipitate (insoluble salt) from the rest of the solution, wash it with water and then dry it
• You should be left with a pure, dry sample of an insoluble salt, in this case lead(II) sulfate

• The precipitate forms because lead sulfate is insoluble, everything else is in the solution because it is soluble
• If the two soluble salts are in solid form, dissolve them both in water first then continue