Question

Collect the following solutions from the science laboratory: hydrochloric acid (HCl), sulphuric acid $\left( {{H}_{2}}S{{O}_{4}} \right)$, nitric acid $\left( HN{{O}_{3}} \right)$, acetic acid $\left( C{{H}_{3}}COOH \right)$, sodium hydroxide (NaOH), calcium hydroxide $\left[ Ca{{\left( OH \right)}_{2}} \right]$, potassium hydroxide (KOH), magnesium hydroxide $\left[ Mg{{\left( OH \right)}_{2}} \right]$ and ammonium hydroxide $\left( N{{H}_{4}}OH \right)$.
What change in colour did you observe with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each of the solutions given?

Answer 1

To solve this, you need to consider how each of the given indicators works in an acidic and basic medium. Remember than an acidic medium turns a blue litmus paper red a basic solution turns red litmus blue. Phenolphthalein is pink in basic medium and methyl orange is red in acidic medium and yellow in basic medium.

Complete step by step solution:
Here, a bunch of acidic and basic solutions are given to us along with some indicators. So firstly, let’s discuss the indicators and how they change in solutions of different pH then we can answer the change of colour in the given solutions.
Firstly, we have litmus paper.
A blue litmus paper will turn red in an acidic solution but remain blue in a basic solution.
A red litmus paper changes colour in a basic solution and becomes blue whereas remains unchanged in an acidic solution.
In a neutral solution, the paper is purple.
Then, we have phenolphthalein. Phenolphthalein is an organic compound with the formula ${{C}_{20}}{{H}_{14}}{{O}_{4}}$. It is a weak acid and below the pH of 8.5, it is colourless and addition of hydroxyl ions to the solution i.e. increasing the pH of the solution turns the colour to pink.
And lastly, we have methyl orange. It is an indicator and it is red or pink coloured in acidic medium and yellow in basic medium. Addition of sodium nitrile to sulfanilic acid solution produces a diazonium salt and thus methyl orange is prepared.
Now, let us differentiate the acids and the bases among the given solutions.
Acid - hydrochloric acid (HCl), sulphuric acid $\left( {{H}_{2}}S{{O}_{4}} \right)$, nitric acid $\left( HN{{O}_{3}} \right)$, acetic acid $\left( C{{H}_{3}}COOH \right)$
Base - sodium hydroxide (NaOH), calcium hydroxide $\left[ Ca{{\left( OH \right)}_{2}} \right]$, potassium hydroxide (KOH), magnesium hydroxide $\left[ Mg{{\left( OH \right)}_{2}} \right]$ and ammonium hydroxide $\left( N{{H}_{4}}OH \right)$.
Now, we can arrange the observation in a tabular form as-

| Red litmus| Blue litmus| Phenolphthalein| Methyl orange
---|---|---|---|---
HCl| No change| red| colourless| Pink/red
${{H}_{2}}S{{O}_{4}}$| No change| red| colourless| Pink/red
$HN{{O}_{3}}$| No change| red| colourless| Pink/red
$C{{H}_{3}}COOH$| No change| red| colourless| Pink/red
NaOH| blue| No change| pink| Yellow
$\left[ Ca{{\left( OH \right)}_{2}} \right]$| blue| No change| pink| yellow
KOH| blue| No change| pink| yellow
$\left[ Mg{{\left( OH \right)}_{2}} \right]$| blue| No change| pink| yellow
$\left( N{{H}_{4}}OH \right)$| blue| No change| pink| yellow

The above table is the required answer.
Note: We know that in chemistry, we use indicators during titration to mark the end point of a titration. We see that in acid-base titrations, generally the end is marked by the change in colour of the solution. Generally a litmus paper is used as an indicator for chemical reactions. However, it also causes a colour change even though the chemical reaction is not an acid-base reaction. Chlorine gas turns a filter paper white as it bleaches the paper due to presence of hypochlorite ions. In this case, litmus paper is not acting as an indicator.