Acids - Paper 1

Question 1

Define acid

Answer 1

An acid in solution is a source of H⁺ ions.

Question 2

Define alkali

in terms of ions

Answer 2

An alkali in solution is a source of ^-OH ions.

Question 3

How can the pH scale identify acids and alkalis?

Answer 3

• Neutral solutions have pH 7.
• Acidic solutions have pH <7.
• Alkaline solutions have pH >7.

Question 4

How does litmus paper operate as an indicator?

Answer 4

Acidic: red
Neutral: purple
Alkaline: blue

Question 5

How does methyl orange operate as an indicator?

Answer 5

Acidic: red
Neutral: yellow
Alkaline: yellow

Question 6

How does phenolphthalein operate as an indicator?

Answer 6

Acidic: colourless
Neutral: colourless
Alkaline: pink

Question 7

How is the pH of a substance quantified?

Answer 7

As H⁺ concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.

Question 8

What is the concentration of H⁺ in a substance with pH 1?

Answer 8

0.1 mol/dm³

Question 9

What is the difference between dilute and concentrated?

Answer 9

A dilute acid has a small amount of acid in a given volume of water, while a concentrated acid has a large amount of acid in the same volume.

Question 10

What is the difference between a strong and weak acid?

Answer 10

Strong acids fully dissociate in water.
Weak acids only partially dissociate.

Question 11

Define base

Answer 11

A base is any substance which reacts with an acid to form a salt and water only.

Question 12

Define alkali

as a subset of bases

Answer 12

An alkali is a soluble base.

Question 13

Describe the chemical test for hydrogen.

Answer 13

Put a lighted splint near the mouth of the test tube containing the gas. Hydrogen would ignite with a squeaky pop.

Question 14

Describe the chemical test for carbon dioxide.

Answer 14

Bubble the gas through limewater. Carbon dioxide would turn the limewater milky or cloudy white.

Question 15

Define neutralisation

Answer 15

A neutralisation reaction is a reaction between an acid and a base.

Question 16

Give the ionic equation for neutralisation.

Answer 16

H⁺_(aq) + ^-OH_(aq) → H₂O_(l)

Question 17

List the general rules for the solubility of common types of substance in water.

Answer 17

• All common sodium, potassium and ammonium salts are soluble.
• All nitrates are soluble.
• Common chlorides are soluble except those of silver and lead.
• Common sulfates are soluble except those of lead, barium and calcium.
• Common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium.

Question 18

Describe a method for obtaining a pure, dry sample of an insoluble salt.

Answer 18

• Produce a precipitate XY by reacting X nitrate with sodium Y.
• Use filtration to separate the precipitate as a residue from the solution.
• Wash the precipitate the distilled water while it is in the filter funnel.
• Leave the washed precipitate aside or in a warm oven to dry.