Acids and Alkalis

Question 1

What does the pH scale range from and what do the extremes represent?

Answer 1

0 to 14; 0 = strongest acid, 14 = strongest alkali

Question 2

What pH value is neutral?

Answer 2

pH 7

Question 3

What does an indicator do?

Answer 3

Changes colour depending on the pH of a solution

Question 4

What colour is litmus paper in acidic, neutral, and alkaline solutions?

Answer 4

• Red in acidic
• Purple in neutral
• Blue in alkaline

Question 5

What colour does phenolphthalein turn in acidic, neutral and alkaline solutions?

Answer 5

• Colourless in acid
• Colourless in neutral
• Bright pink in alkali

Question 6

What colour does methyl orange turn in acidic, neutral and alkaline solutions?

Answer 6

• Red in acid
• Red in neutral
• Yellow in alkali

Question 7

What is an acid in terms of ions?

Answer 7

A source of hydrogen ions (H⁺), proton donor, pH less than 7

Question 8

What is a base and what is an alkali?

Answer 8

Base neutralises acid, proton acceptor; alkali is a soluble base with OH⁻ ions, pH greater than 7

Question 9

What is neutralisation?

Answer 9

Reaction between acid and base/alkali producing water and salt

Question 10

Write the neutralisation reaction using ions.

Answer 10

H⁺ (aq) + OH⁻ (aq) → H₂O (l)

Question 11

What is produced when an acid reacts with a metal oxide?

Answer 11

Salt + water

Question 12

Give an example of acid + metal oxide reaction.

Answer 12

2HCl + CuO → CuCl₂ + H₂O

Question 13

What is produced when an acid reacts with a metal hydroxide?

Answer 13

Salt + water

Question 14

Give an example of acid + metal hydroxide reaction.

Answer 14

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Question 15

What happens when an acid reacts with ammonia?

Answer 15

Forms an ammonium salt

Question 16

Give an example of acid + ammonia reaction.
HNO₃ + NH₃ → NH₄NO

Answer 16

HNO₃ + NH₃ → NH₄NO₃

Question 17

What ions form when ammonia dissolves in water?

Answer 17

NH₄⁺ and OH⁻ ions

Question 18

What happens when an acid reacts with a metal?

Answer 18

Salt + hydrogen gas

Question 19

Give an example of acid + metal reaction.

Answer 19

2HCl + Mg → MgCl₂ + H₂

Question 20

What happens when an acid reacts with a metal carbonate?

Answer 20

Salt + water + carbon dioxide

Question 21

Give an example of acid + metal carbonate reaction.

Answer 21

2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂

Question 22

What is the purpose of a titration?

Answer 22

To find the exact amount of acid needed to neutralise a quantity of alkali or vice versa

Question 23

What apparatus is used to measure fixed volumes of liquid in titrations?

Answer 23

Pipette and pipette filler

Question 24

What apparatus adds acid or alkali dropwise in titrations?

Answer 24

Burette

Question 25

What is the endpoint in a titration?

Answer 25

The point where the indicator changes colour, showing neutralisation

Question 26

What indicator is colourless in acid and pink in alkali?

Answer 26

Phenolphthalein

Question 27

How do you calculate the number of moles from concentration and volume?

Answer 27

Moles = concentration (mol/dm³) × volume (dm³)

Question 28

In the reaction 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, how many moles of NaOH react with 1 mole of H₂SO₄?

Answer 28

2 moles NaOH per 1 mole H₂SO₄

Question 29

If 0.0025 moles of NaOH react, how many moles of H₂SO₄ are needed?

Answer 29

0.00125 moles

Question 30

How do you find concentration from moles and volume?

Answer 30

Concentration = moles ÷ volume (dm³)

Question 31

How do you carry out a titration to find the concentration of an acid or alkali?

Answer 31

1. Use a pipette and pipette filler to add a measured volume (usually 25 cm³) of alkali to a conical flask. 2. Add 2-3 drops of an indicator (e.g., phenolphthalein) to the alkali. 3. Fill a burette with acid and record the initial volume (read at eye level). 4. Slowly add acid from the burette to the alkali while swirling the flask continuously. 5. Add acid drop by drop as the colour change approaches (the endpoint). 6. The endpoint is when the indicator changes colour, showing neutralisation. 6. Record the final volume of acid used. 7. Repeat the titration several times to get consistent readings for accuracy.