Acids, Alkalis & Titrations

100 Question and Answer flashcards covering acids, alkalis, indicators, the pH scale, and titration procedures based on the Edexcel IGCSE Chemistry Unit 1 transcript.

What colour is litmus in an acidic solution?

red

What colour is litmus in an alkaline solution?

blue

What colour is phenolphthalein in an acid?

colourless

What colour is phenolphthalein in an alkali?

pink

What colour is methyl orange in an acid?

red

What colour is methyl orange in an alkali?

yellow

What are two colour indicators used to distinguish between?

Acids and alkalis

What is the source of the common indicator litmus?

Lichens

What are synthetic indicators defined as in the notes?

Organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis

Name two synthetic indicators frequently used in acid-alkali titrations.

Phenolphthalein and methyl orange

Why are synthetic indicators used to show the endpoint in titrations?

They have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa

Why is litmus considered unsuitable for titrations?

The colour change is not sharp and it goes through a purple transition colour in neutral solutions

What is the transition colour of litmus in neutral solutions?

purple

In what forms does litmus paper come for testing solutions or gases?

Red and blue versions

What is the numerical range of the pH scale?

$$0 - 14$$

What pH values do all acids possess?

Below $7$

What pH values do all alkalis possess?

Above $7$

How does the pH value relate to the acidity of a solution?

The lower the pH then the more acidic the solution is

What is the pH range for a strong acid?

$$pH\,0-3$$

What pH value can extremely acidic substances reach?

Below $1$

What is the pH range for a weak acid?

$$pH\,4-6$$

How does the pH value relate to the alkalinity of a solution?

The higher the pH then the more alkaline the solution is

What is the pH range for a weak alkali?

$$pH\,8-10$$

What is the pH range for a strong alkali?

$$pH\,11-14$$

How is a solution with pH $7$ described?

neutral

What type of indicator is universal indicator described as?

A wide range indicator

Does universal indicator provide an exact or approximate pH value?

approximate

What is universal indicator made from?

A mixture of different plant indicators which operate across a broad pH range

How is the pH of an unknown solution determined using universal indicator?

A few drops are added and the colour is matched with a colour chart

Why are colour charts usually provided for specific universal indicator formulations?

Colors vary slightly between manufacturers

Why is it an error to use universal indicator for an acid-base titration?

A sharp colour change is required to identify the end-point, which universal indicator cannot achieve

Which ions are formed when acids are added to water?

$H^+$ ions (positively charged hydrogen ions)

What specific ion makes a solution acidic?

The presence of $H^+$ ions

Which ions are formed when alkalis are added to water?

$OH^-$ ions (negative hydroxide ions)

What specific ion makes an aqueous solution an alkali?

The presence of the $OH^-$ ions

What does the pH scale measure in terms of ions present in solution?

The amount of hydrogen ions

When does a neutralisation reaction occur?

When an acid reacts with an alkali

Which ions react together in a neutralisation reaction?

$H^+$ ions and $OH^-$ ions

What is the product of the reaction between $H^+$ and $OH^-$ ions?

water ($H_2O$)

Why is the reaction between a metal and an acid not considered a neutralisation?

No water is formed

What are the two general products of a neutralisation reaction between hydrochloric acid and an alkali like sodium hydroxide?

A salt (sodium chloride) and water

What is the net ionic equation for all acid-base neutralisations?

$$H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)$$

Why do neutralisation reactions lead to a neutral solution?

Water has a pH of $7$

Why is neutralisation important in soil treatment?

To raise the pH as some crops cannot tolerate pH levels below $7$

What specific bases are added to soil to raise its pH?

Limestone and quicklime

What is the primary purpose of a titration?

Analysing the concentration of solutions

What can acid-base titrations determine regarding quantities?

Exactly how much alkali is needed to neutralise a quantity of acid (and vice versa)

What three values related to an acid or base might you be asked to calculate from a titration?

Moles present, concentration, or volume

Besides concentration analysis, what can titrations be used to prepare?

salts

Which piece of equipment is used to measure exactly $25\,cm^3$ of sodium hydroxide?

A pipette and pipette filler

Which piece of equipment is filled with hydrochloric acid in the titration method?

A burette

Why should a small portion of acid be run through the burette into an empty beaker before starting?

To remove any air bubbles

To what accuracy should the starting point on the burette be recorded?

To the nearest $0.05\,cm^3$

Where should the conical flask be placed to help see the indicator change?

On a white tile

Where should the tip of the burette be positioned during the titration?

Inside the flask

How much indicator should be added to the conical flask solution?

A few drops

How is a rough titration performed?

By taking a burette reading and running in the solution in $1-3\,cm^3$ portions while swirling vigorously

When should the burette tap be closed during a titration?

When the end-point is reached (sharp colour change)

Where should your eye level be when recording a volume from a burette?

Level with the meniscus

How is the burette solution added as the rough end-point volume is approached in subsequent tests?

One drop at a time until the indicator just changes colour

What are concordant results in a titration?

Two results that are within $0.1\,cm^3$ of each other

What is the purpose of achieving concordant results?

To increase accuracy

What tool is used to fill the burette with acid?

A funnel

Why must the funnel be removed before starting the practical?

It can drip liquid into the burette, making the initial reading false

What was the rough titre value in the provided example results table?

$$15.50\,cm^3$$

What was the value of Titre 1 in the provided example results table?

$$14.90\,cm^3$$

What was the value of Titre 2 in the provided example results table?

$$15.00\,cm^3$$

What was the mean titre calculated in the example results table?

$$14.95\,cm^3$$

What organic substances in plants can act as indicators?

Substances extracted from lichens (litmus)

What is the state symbol for water in the neutralisation ionic equation?

$$(l)$$

What are the common state symbols for hydrogen and hydroxide ions in aqueous solution?

$$(aq)$$

Is the reaction between hydrochloric acid and a metal a neutralisation?

no

What is the pH of a strong acid like extremely acidic substances?

Below $1$

What happens to the alkalinity as pH moves from $8$ to $14$?

It increases (becomes a stronger alkali)

How does universal indicator provide approximate pH values?

By matching colours to a colour chart provided by the manufacturer

Name the specific acid used in the titration method example.

hydrochloric acid

Name the specific alkali used in the titration method example.

sodium hydroxide

What precision is used for all titration volume readings in the results table?

To the nearest $0.05\,cm^3$ (two decimal places)

Which indicator is red in acid and blue in alkali?

litmus

Which indicator is colourless in acid and pink in alkali?

phenolphthalein

Which indicator is red in acid and yellow in alkali?

methyl orange

According to the notes, what is the net ionic equation a lead-in to?

A neutral solution (pH $7$)

What is the specific requirement for identifies the end-point in a titration?

A sharp colour change

How is a rough titration's accuracy described in terms of adding acid?

It is run in $1-3\,cm^3$ portions (less accurate than dropwise)

Define the relationship between pH scale and hydrogen ion amount.

It is a measure of the amount of the hydrogen ions present in solution

What is the pH range for a weak acid vs a weak alkali?

Weak acid: $pH\,4-6$; Weak alkali: $pH\,8-10$

What is the pH range for a strong acid vs a strong alkali?

Strong acid: $pH\,0-3$; Strong alkali: $pH\,11-14$

What is the most common natural indicator mentioned?

litmus

Why should you swirl the flask vigorously during titration?

To ensure the acid and alkali are thoroughly mixed

What is the title of the provided lecture notes?

Acids, alkalis & titrations

Which exam board and specification is this material for?

Edexcel IGCSE Chemistry (Modular)

What unit does this content cover?

Unit 1

What is the copyright year on the documents?

$$2026$$

What is the website provided for more exam resources?

savemyexams.com

What kind of indicators show the endpoint in titrations due to sharp colour changes?

Synthetic indicators

Is universal indicator a single compound or a mixture?

A mixture of different plant indicators

If you calculate the mean titre, which results should you use?

The concordant results (Titre 1 and Titre 2 in the example)

Why is a pipette used instead of a beaker for the alkali in titrations?

To provide an exact volume ($25\,cm^3$)

What does the abbreviation 'aq' stand for in chemical equations?

aqueous (dissolved in water)

What is the primary factor that classifies a solution as acidic?

The presence of $H^+$ ions