A student tested three colourless solutions, labelled A, B and C, using red litmus paper and blue litmus paper. Solution A was dilute hydrochloric acid, sol...
Question 1 Report
A student tested three colourless solutions, labelled A, B and C, using red litmus paper and blue litmus paper. Solution A was dilute hydrochloric acid, solution B was aqueous sodium hydroxide and solution C was distilled water. Fig. 1.1 shows the three test tubes in the rack before a fresh piece of each litmus paper was dipped into each portion.
| Solution | Colour of red litmus | Colour of blue litmus |
|---|---|---|
| A (hydrochloric acid) | red | ? |
| B (sodium hydroxide) | ? | blue |
| C (distilled water) | ? | ? |
(a) Complete Table 1.1 to show the colour of each piece of litmus paper after the test. [3]
(b) Name a suitable piece of apparatus for measuring a 25.0 cm3 portion of solution A. [1]
(c) State one safety precaution the student should take when handling the sodium hydroxide solution. [1]
Answer Details
Litmus is an indicator that only shows whether a solution is acidic, alkaline or neutral: it turns red in acid and blue in alkali, and does not change colour in a neutral solution. Each piece of litmus starts a fixed colour (red or blue) and you report the colour it becomes after dipping.
(a) Completing Table 1.1 [3]:
| Solution | Red litmus becomes | Blue litmus becomes |
|---|---|---|
| A (hydrochloric acid) | red (unchanged) | red |
| B (sodium hydroxide) | blue | blue (unchanged) |
| C (distilled water) | red (unchanged) | blue (unchanged) |
The acid turns the blue litmus red [1]; the alkali turns the red litmus blue [1]; the neutral water leaves both papers unchanged [1]. Notice that a colour that is already the correct one (red litmus in acid, blue litmus in alkali) simply stays that colour, so the key change to spot is the one paper that flips.
(b) To measure a fixed 25.0 cm3 portion accurately you use a pipette (a volumetric or graduated pipette; a burette is also accepted) [1]. A measuring cylinder is not accurate enough for a reading quoted to one decimal place.
(c) Sodium hydroxide is corrosive, so a suitable precaution is to wear safety goggles / eye protection (accept: wear gloves / avoid skin contact / wash off any splashes with plenty of water) [1]. The reason is that alkalis attack the eyes and skin.
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