Acids, Alkalis and Titrations

5 marks · Easy

Two solutions, X and Y, are tested:

Test results with universal indicator and methyl orange.
Solution	Universal indicator colour	Methyl orange colour
X	Orange	Red
Y	Blue	Yellow

(a)

Estimate the pH value for each solution from the universal indicator colour. Show your reasoning.

[2 marks]

(b)

Classify each solution as strongly acidic, weakly acidic, neutral, weakly alkaline, or strongly alkaline based on your estimated pH values.

[2 marks]

(c)

Referring to the methyl orange results, explain whether your classifications are supported. Give reasons for your answer.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) X: Orange universal indicator means pH about 4–5. Y: Blue universal indicator means pH about 9–10.

(b) X is weakly acidic; Y is weakly alkaline.

(c) X gives red methyl orange (acidic), supporting weak acid classification. Y gives yellow methyl orange (alkaline or neutral), supporting weak alkali classification.

Common mistakes: incorrect pH ranges for universal colours; reversing acidic/alkaline for methyl orange

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · Estimates pH of solution X as 4–5 (orange universal indicator).

M2 (a) · 1 mark · Estimates pH of solution Y as 9–10 (blue universal indicator).

A1 (b) · 1 mark · Correctly classifies X as weakly acidic.

A2 (b) · 1 mark · Correctly classifies Y as weakly alkaline.

M3 (c) · 1 mark · Explains methyl orange results for both solutions and matches them to earlier classification.

Worked solution

a. Universal indicator orange means pH is about 4–5 for X (weakly acidic). Blue means pH is about 9–10 for Y (weakly alkaline).

b. X (pH 4–5) is classified as weakly acidic (pH 4–6). Y (pH 9–10) is classified as weakly alkaline (pH 8–10).

c. Methyl orange turns red in acid and yellow in alkali or neutral. X’s red methyl orange supports it being acidic, matching previous answer. Y’s yellow methyl orange supports it being alkaline, matching weakly alkaline classification.