Acids, Bases, and Salts

Question 1

Characteristics of bases:

Answer 1

• produces hydroxide ions when dissolved in water
• accepts hydrogen ions when reacting with acids
• proton acceptor
• bitter taste
• turns red litmus to blue
• turns the universal indicator to blue/violet

Question 2

Definition of a strong and a weak acid/base:

Answer 2

Strong: completely dissociate when dissolved in water.
Weak: partially dissociate when dissolved in water to leave some undissociated molecules of acids/bases.

Question 3

How to control soil acidity:

Answer 3

By adding bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide) to raise soil pH. Alkalis are not used because they seep into the soil, raising the pH by too much.

Question 4

Define alkali:

Answer 4

A base that dissolves in water.

Question 5

Define amphoteric oxide:

Answer 5

A substance that reacts both as an acid and as a base. Examples include aluminium, zinc, and lead (II).

Question 6

Characteristics of acidic oxides:

Answer 6

• non-metal oxides
• dissolves readily in water to produce acids
• reacts with alkalis to produce salt and water

Question 7

Characteristics of basic oxides:

Answer 7

• metallic oxides
• mostly insoluble in water; exceptions are called alkalis
• solids at room temp
• reacts with acids to produce salt and water

Question 8

Define neutral oxide:

Answer 8

Nonmetal oxides that don’t react with both acids and bases. They are insoluble in water. Examples include water, carbon monoxide, and nitrogen monoxide.

Question 9

Characteristics of acids:

Answer 9

• produces hydrogen ions when dissolved in water
• accepts hydroxide ions when reacting with bases
• proton donor
• sour taste
• turns blue litmus to red
• turns the universal indicator to red/orange/yellow

Question 10

Describe the reaction of HCl with metals

Answer 10

Only reacts with metals lower than hydrogen in reactivity series. From Mg-Fe, produces H2 as a product.
In dilute HCl, no reaction with Cu downwards, but in concentrated HCl, reacts very slowly with Pb, and Cu onwards.

Question 11

Describe the reaction of H2SO4 with metals

Answer 11

Dilute H2SO4 will react with all metals below hydrogen up to iron, to produce H2. It has a very slow reaction with Pb, and no reaction with Cu onwards.
Hot concentrated H2SO4 will react with all metals and produce SO2.

Question 12

Describe the reaction of HNO3 with metals.

Answer 12

Reacts with all metals to produce NO2. No H2 gas is produced because HNO3 is a very strong oxidising agent.

Question 13

Preparation of acids:

Answer 13

• dissolving a non-metallic acid in water
• heating a sodium/potassium salt of a volatile acid with sulphuric acid. The volatile acid will evaporate and may be condensed.

Question 14

Define alkali

Answer 14

Bases that are soluble in water

Question 15

Precipitation of insoluble metal hydroxides

Answer 15

Use solutions of alkalis to precipitate insoluble metal hydroxides from solutions of their salts

Question 16

Preparation of soluble salts from group I and ammonium

Answer 16

Acid + soluble base –> salt + water
The acid and base must each contain an element from the targeted salt.

1. Put the base in the burette
2. Use a pipette to add 25cm3 of HCl to the conical flask.
3. Add 1-2 drops of phenolphthalein indicator.
4. Add the base from the burette until the solution turns from colourless to light pink.
5. Evaporate the neutralised solution until it becomes saturated.
6. Wait for it to cool
7. Filter out the crystals and dry between filter paper.

Question 17

Preparation of soluble salts other than group I and ammonium

Answer 17

Acid + insoluble base –> salt + water

1. Add the base to the acid until no more will dissolve
2. Filter to remove excess base
3. Partially evaporate the filtrate
4. Cool the solution and filter out the crystals, then dry between filter paper

Question 18

Distinction between partially evaporate and fully evaporate

Answer 18

For a hydrated salt, “partially evaporate” is preferred, while for an anhydrous salt, “fully evaporate” is preferred.

Question 19

Preparation of insoluble salts

Answer 19

Double decomposition reaction is used.
Soluble salt + soluble salt –> insoluble salt + soluble salt.
A displacement reaction occurs here. :) What we really need is the first salt’s cation and the second salt’s anion. The second salt’s cation should (ideally) be a group 1 salt.
1. Mix both solutions
2. Filter to remove precipitate
3. Wash the precipitate with distilled water
4. Dry the precipitate between filter paper