Acids, Alkalis & Titrations (Edexcel IGCSE Science (Double Award): Chemistry): Flashcards
Exam code: 4SD0
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Define indicator.
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Cards in this collection (18)
Define indicator.
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline, used to distinguish between the two.
What colours does phenolphthalein show in acid and in alkali?
Phenolphthalein is colourless in acid and pink in alkali.
Litmus turns .......... in acid and .......... in alkali.
Litmus turns red in acid and blue in alkali.
Why is universal indicator not suitable for titrations?
A sharp colour change is required to identify the titration endpoint. Universal indicator changes colour gradually across a range of pH values and cannot give a precise endpoint.
True or False?
Litmus is a suitable indicator for acid-alkali titrations.
False.
Litmus is not suitable for titrations because its colour change is not sharp and it passes through a purple transition colour near neutral pH, making it difficult to identify the endpoint accurately.
On the pH scale, solutions with a pH of 0–3 are classified as .........., pH 4–6 as .........., pH 7 as .........., pH 8–10 as .......... and pH 11–14 as .......... .
On the pH scale, solutions with a pH of 0–3 are classified as strongly acidic, pH 4–6 as slightly acidic, pH 7 as neutral, pH 8–10 as slightly alkaline and pH 11–14 as strongly alkaline.
What colours does methyl orange show in acid and in alkali?
Methyl orange is red in acid and yellow in alkali.
True or False?
Universal indicator can be used to find the exact pH of a solution.
False.
Universal indicator gives only an approximate pH value. It cannot provide an exact reading because it shows a gradual range of colours rather than a sharp change.
How is universal indicator used to estimate the pH of a solution?
A few drops of universal indicator are added to the solution. The resulting colour is matched against a colour chart to give an approximate pH value.
Define acid (in aqueous solution).
An acid is a substance that, when dissolved in water, is a source of hydrogen ions (H+).
Define alkali (in aqueous solution).
An alkali is a substance that, when dissolved in water, is a source of hydroxide ions (OH-).
Acids in aqueous solution produce .......... ions and alkalis in aqueous solution produce .......... ions.
Acids in aqueous solution produce H+ ions and alkalis in aqueous solution produce OH- ions.
True or False?
All reactions of acids with other substances are neutralisation reactions.
False.
Neutralisation requires both a salt and water to be produced. When a metal reacts with an acid, a salt and hydrogen gas are formed — but no water — so it is not a neutralisation reaction.
What is the net ionic equation for all acid-base neutralisation reactions?
H+ (aq) + OH- (aq) → H2O (l)
The H+ ions from the acid react with the OH- ions from the alkali to produce water.
In a neutralisation reaction, .......... ions from the acid react with .......... ions from the alkali to form .......... .
In a neutralisation reaction, H+ ions from the acid react with OH- ions from the alkali to form water.
Why are bases added to soil to treat acidity?
Bases such as limestone and quicklime neutralise excess acid in the soil, raising the pH to a level at which crops can grow.
True or False?
The pH of a neutral solution at room temperature is 7.
True.
A neutral solution has a pH of 7. This corresponds to equal concentrations of H+ and OH- ions in solution.
What are the products of the neutralisation of hydrochloric acid by sodium hydroxide?
Sodium chloride (NaCl) and water (H2O) are produced when hydrochloric acid reacts with sodium hydroxide solution.
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