Class = 10th (Chemistry) Science Acids,Bases and Salts

2. Phenolphthalein: Phenolphthalien is a colourless substance. It is a synthetic indicator which is prepared in the laboratory or manufactured in the industry. In acidic solution, it remains colourless. However, adding a drop of phenolphthalein to a basic solution, turns the solution pink. 

3. Methyl orange : it is also a synthetic indicator manufactured in industry or prepared in the laboratory. Methyl orange is an orange colour dye. When a drop of methyl orange is added to an acidic solution, it turns the solution red. On adding a drop of methyl orange to a basic solution, the solution turns yellow.  

The colour changes of phenolphthalein and methyl orange in the acidic and basic (alkaline) solutions are given in the table 2.4 : 

Let as discussed an experiment to understand the role of indicators in the identification of acid and basic solutions.

EXPERIMENT :  To test acids and bases in the laboratory using change acid-base indicators. Collect the following samples from the science laboratory-hydrochloric acid (HCl), sulphuric acid (H/2 SO/4), potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)/2] and ammonium hydroxide (NH/4OH). 

Collect also the indicator solutions-red litmus solution, blue litmus solution, phenolphthalein solution  and methyl orange solution. Now, put a drop of each indicator solution in each of the acidic and alkaline solutions and observe the changes in the colour of the solutions. 

  The changes observed are          tabulated in table 2.5. 

Olfactory Indicators 

There are certain substances whose odour (smell) changes in an acidic or a basic (alkaline) solution. These substances are known as olfactory indicators. The term olfactory means relating to the sense of smell.

Olfactory indicators have a characteristic odour (smell). This  characteristic odour in olfactory indicators is changed in an acidic or a basic solution.  Thus, olfactory indicators can be used to test whether a given sample solution is acidic is acidic or basic in nature. A few some of olfactory indicators are given below :  

i) Onions have a characteristic odour (smell). When chopped onions are kept in a dilute solution of sodium hydroxide. They lose their characteristic odour. However, on keeping chopped onions in a delete acidic solution (such as hard hydrochloric acid) that characteristic odour of onions is not lost. This experiment can be used to identify acidic or basic solution.

ii) Vanilla extract has a sweet pleasant smell. This smell is due to a compound vanillin present in Vanilla extract. When Vanilla extract is added to a dilute sodium hydroxide solution, for the sweet pleasant smell vanishes. However, Vanilla extract when added to a dilute solution of hydrochloric acid still retains the sweet pleasant smell.

iii) Clove oil also has a characteristic odour but it can not be used as an olfactory indicator. The characteristic odour in clove oil is retained in both dilute acid and alkaline solutions.

TO BE Continued..........