Indicators

Category : 10th Class

*       Introduction

 

The chemicals which we come across in our daily life can be categorized as acids, bases or salts. For example the common salt is sodium chloride and the detergent powder that is used for washing clothes is another salt of sodium i.e. sodium carbonate.

The vinegar used in pickles is an acid i.e. acetic acid, the citrus fruits also contains the acid.

The solution which is used for white washing contains calcium hydroxide which is a base. The soaps which we use is made from the base sodium chloride. Now in order to identify whether a given substance is an acid or a base, an indicators is used.

 

*          Indicators

Those chemical substance which change their colour when they are put in acid or base are known as indicators. The three common indicators are:

Litmus paper, methyl orange and phenolphthalein.

Litmus: It is very common indicator present in the labs in the form of litmus solution or litmus paper.

Acids turn blue litmus red and bases turn red litmus blue.

Methyl Orange: It is an orange coloured synthetic indicator, which gives pink colour with acids and yellow colours with base.

Phenolphthalein: It is a colour less indicator which, shows no change in acidic solution but turn pink in basic solution.

Indicator Acid Base
Litmus Paper Turns blue litmus red Turns red litmus blue
Methyl Orange Gives red colour Gives yellow color
Phenolphthalein Remains colorless Gives pink color

Let us now understand more about acids, bases and salts.

 

*            Acid

The word acid is derived from latin word sour. According to Arrhenius acids are those substances which give hydrogen ions in water. Since hydrogen ion does not exist, it combines with water to form hydronium ion.

\[{{H}^{+}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}\]

So acids can be defined as the substances which can furnish hydronium ion in aqueous solution. e.g.

\[HCl+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}+C{{l}^{-}}\]

\[{{H}_{2}}S{{O}_{4}}+2{{H}_{2}}O\to 2{{H}_{3}}{{O}^{+}}+SO_{4}^{2-}\]

 

 

*             Base

The substances which combines with acids to form salt and wat3r. Base are the compounds which can furnish hydroxide ions in aqueous solution e:g.

\[NaO{{H}_{(aq)}}\to N{{a}^{+}}+O{{H}^{-}}\]

\[Mg{{(OH)}_{2(aq)}}\to M{{g}^{2}}+2O{{H}^{-}}\]

The water soluble bases are called alkalis. All alkalies are base, but all bases are not alkalies.

 

*             Salt

It is a compound which when dissolved in water yields positive and negative ions other than hydrogen ion and hydroxide ion e.g.

\[NaC{{l}_{(aq)}}\to N{{a}^{+}}+C{{l}^{-}}\]

Salts are the compounds formed by neutralization of an acid with a base. e.g.                                                      

\[NaOH+HCl\to NaCl+{{H}_{2}}O\]



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