Acids, Alkalis and Titrations

Question 1

What pH range is classed as a strong acid?

Answer 1

The pH range for a strong acid is 0-3.

Question 2

What pH range is classed as a weak acid?

Answer 2

What pH range is classed as a weak acid?

Question 3

What pH is classed as neutral?

Answer 3

pH 7 is classed as neutral.

Question 4

What pH range is classed as a strongly alkaline?

Answer 4

What pH range is classed as a strongly alkaline?

Question 5

What pH range is classed as a weakly alkaline?

Answer 5

The pH range for a weak base is 8-10.

Question 6

What colour is litmus in acidic and alkaline conditions?

Answer 6

Litmus is red in acid and blue in alkaline conditions.

Question 6

What colour is methyl orange in acidic and alkaline conditions?

Answer 6

Methyl orange is red in acid and yellow in alkaline conditions.

Question 7

What colour is phenolphthalein when added to an acid?

Answer 7

Phenolphthalein is colourless in acid.

Question 7

Describe how to use universal indicator to test the pH of a substance.

Answer 7

To measure the pH of a substance with universal indicator:

Add a few drops of universal indicator solution to the substance.

Check the colour against a colour chart to determine the pH value.

(Universal indicator paper can also be used).

Question 8

Why is universal indicator not used measure pH during titrations?

Answer 8

Universal indicator is not used for titrations as it does not give a sharp colour change which is required to identify the end-point.

Question 8

Which ion causes a substance to become acidic when added to water?

Answer 8

The ion that causes a substance to be acidic is a hydrogen ion, H+

(Proton is also an acceptable answer)

Question 9

What is the method of titration?

Answer 9

• titrations are used to find out concentrations

1) allow you to find out exactly how much acid is need to neutralise a quantity of alkali
2) using a pipette and pipette filler, add some alkali to a conic flask, along with two or three drops of indicator
- fill a burette with the acid (do this below eye level as you don’t want to be looking up if some acid spills over)
- using the burette, add the acid to the alkali a bit at a time, giving the conical flask a regular swirl
- go slowly during the end point (colour change) is about to be reached
- indicator changes colour when the alkali has been neutralised
- record the volume of acid used to neutralise the alkali
- repeat process a few times

Question 10

The pH scale starts at pH 0 and goes up to what pH?

Answer 10

The pH scale goes from pH 0 - 14.

Question 11

Which ion causes a substance to become alkaline when added to water?

Answer 11

The ion that causes a substance to become alkaline is a hydroxide, OH-.

Question 12

State the net ionic equation for neutralisation.

Answer 12

The net ionic equation for neutralisation is:

H+ (aq) + OH- (aq) ⟶ H2O (l)

Question 13

What substances are produced when an acid and an alkali react?

Answer 13

When an acid and alkali react the products are water, H2O, and a salt.

Question 14

True or False?

All reactions of acids are neutralisations.

Answer 14

False.

Not all reactions of acids are neutralisations. For example, when a metal reacts with an acid, although a salt is produced, there is no water formed, so it does not fit the definition of neutralisation.

Question 15

What type of reaction occurs when an acid and an alkali react?

Answer 15

The type of reaction that occurs when an acid and alkali react is neutralisation.

Question 16

True or False?

The lower the pH, the more acidic the solution.

Answer 16

True.

The lower the pH, the more acidic the solution is.

Question 17

How is can soil be made more alkaline in agriculture?

Answer 17

By adding bases to the soil, such as limestone and quicklime.

Neutralisation is important in the treatment of soils to raise the pH, as some crops cannot tolerate pH levels below 7.

Question 18

True or False?

All alkalis have a pH value above 7.

Answer 18

True.

All alkalis have a pH value above 7.

Question 19

State what is meant by concordant results. (Extended Tier Only)

Answer 19

Concordant results are results that are within 0.1 cm3 of each other.

Question 20

What is an acid-base titration used for? (Extended Tier Only)

Answer 20

An acid-base titration is a method used to determine the concentration of an unknown acid or alkali.

Question 21

What is the purpose of the indicator in a titration? (Extended Tier Only)

Answer 21

The indicator is used to identify the endpoint of the titration by changing colour sharply when the reaction is complete.

Question 22

True or False?

Leaving a funnel in the burette during a titration is acceptable. (Extended Tier Only)

Answer 22

False.

The funnel should be removed from the burette before starting the titration, as it can drip liquid into the burette, making the initial reading false.

Question 23

Why is it important to repeat a titration? (Extended Tier Only)

Answer 23

Repeating a titration and achieving concordant results increases the accuracy of the measurement.

Question 24

State what is meant by the term endpoint. (Extended Tier Only)

Answer 24

The point at which an indicator changes colour sharply during a titration, signalling the completion of the reaction.

Question 25

What is the purpose of swirling the flask during a titration? (Extended Tier Only)

Answer 25

Swirling the flask during a titration ensures thorough mixing of the solutions and helps to identify the endpoint more accurately.

Question 26

Which substances are soluble?

Answer 26

• common salts of sodium potassium and ammonium
• nitrates
• common chlorides (except silver & lead chloride)

Question 27

Which substances are insoluble?

Answer 27

• common carbonates (except for sodium, potassium, ammonium)
• common hydroxides (except for sodium, potassium and calcium)