Red litmus solution test

45.
The answer is : (A) a glass cleansercontaining ammonia

When cabbage leaves is boiled in water, the solution formed is an acid-alkaliindicator.
In an alkali solution (e.g. sodium hydroxide solution), the indicator shows a purplecolour.
In acidic solution (e.g. hydrochloric acid), the indicator shows a red colour.

The given red solution is an acidic solution.

(A) Ammonia is an alkali solution. When it is added to the acidic (red)solution, neutralization occurs. If ammonia is in excess, the final solution isalkaline and thus turns purple.

(B) Concentrated nitric acid is acidic. When it is added to the acidic (red)solution, the solution remains acidic. Hence, the solution remains red.

(C) Lemon juice is acidic. When it is added to the acidic (red) solution, thesolution remains acidic. Hence, the solution remains red.

(D) Vinegar is acidic. When it is added to the acidic (red) solution, thesolution remains acidic. Hence, the solution remains red.


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acids + acids, pHchange ?

In some cases, the pH is unchanged. Forexample, when 0.1 M of nitric acid is added to 0.1 M of nitric acid, the pH isunchanged.

In most other cases, the pH is changed. For example, when 0.1 M of nitric acidis added to 2 M of nitric acid, the pH is changed.


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acids + neutral substances, pHchange?

It depends on whether the volume of the acid solution is changed or not.

If the volume of the acid solution is increased, the pH increases. For example,10 cm³ of water is added to 10 cm³ of 1 M sulphuric acid. Since the volume of the solutionis increased, and thus the acid is diluted, the pH increases.

If the volume of the acid solution is unchanged, the pH is also unchanged. Forexample, some sodium chloride (neutral) is dissolves in 10 cm³ of 1 M sulphuric acid. The change of the volumeof the solution is negligible, and thus the volume of the acid solution isnearly constant. Since the amount of acid and the volume of the solution areunchanged, the pH is unchanged.

2014-08-08 15:39:33 補充：
但係red solution遇到alkali不是應該turn blue嗎？why purple?

litmus indicator 在酸中變紅色，在鹼中變藍色。
但本題中的 neutral indicator 不是 litmus。

不會的 indicators 在酸和鹼中的顏色都可能不同。

2014-08-08 15:44:49 補充：
甘如果H2SO4+HCL conc. the same, pHchange?

1 M H2SO4 dissociates to give 2 M of hydrogen ions.
1 M HCl dissociates to give 1 M of hydrogen ions.

2014-08-08 15:44:58 補充：
Although their concentrations are the same, H2SO4 would give a higher concentration of hydrogen ions. Therefore, the pH of H2SO4 is lower than that of HCl when their concentrations are the same.

pH changes because the two solutions have difference pH.

2014-08-08 15:49:54 補充：
這題裏面的 red solution，是一個 indicator in an acidic solution。可用來 test whether the solution is alkali or not。

2014-08-08 15:50:16 補充：
Mg does not dissolve in water. Mg reacts to form Mg²⁺ ions, which is neutral.

CaO reacts with water to formed Ca(OH)2 which is alkaline.

Sodium chloride is neutral, while sodium carbonate is alkaline.

但是這題裏面的red solution是不是用來test alkali呢？

2014-08-08 15:47:12 補充：
噢！即是mix埋的solution overallpH 黎講是低了！

2014-08-08 15:53:57 補充：
喔！即是一有colour change就代表是alkali了！
ok!明白了！