Acid, Bases and Salts

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Acid : The chemical substance which are sour in taste , change the colour of blue litmus to red and gives H+ ion in aqueous solution is called acid.

There are two types of acids:

1.Strong acids : An acid which is completely (100%) ionised in water and produces (give rise) large amount (more number) of hydrogen ions (H+ ions) is called a strong acid.
Example : Hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃ ).

2.Weak acids : An acid which is partially (<100%) ionised (do not dissociate completely i.e incomplete ionised) in water and produces (give rise) small amount (number) of hydrogen ions (H+ ions) is called a weak acid.
Example : Acetic acid or Ethanoic acid (CH₃COOH), formic acid (HCOOH), Carbonic acid (H₂CO₃), sulphurous acid (H₂SO₃), citric acid, tartaric acid etc.

Base : The chemical substance which are bitter in taste , soapy to touch, change the colour of the red litmus to blue and gives OH- ion in aqueous solution is called base.

There are two types of base:

1.Strong bases : A base which is completely (100%) ionised in water and produces (give rise) large amount (more number) of hydroxide ions (OH– ions) is called a strong base.
Example : Sodium hydroxide (NaOH) and potassium hydroxide (KOH), Calcium hydroxide [Ca(OH)₂] etc.

2.Weak bases : A base which is partially (<100%) ionised (do not dissociate completely i.e incomplete ionised) in water and produces (give rise) small amount (less number) of hydroxide ions (OH– ions) is called a weak base.

Example : magnesium hydroxide [Mg(OH)₂], Aluminum hydroxide [Al(OH)₃], [Cu(OH)₂], Ammonium hydroxide (NH₄OH) etc.

Note : Strong acid and Strong bases are corrosive in nature.

Alkalis :
• All bases do not dissolve in water.

• An alkali is a base that dissolves in water. 

• Bases which are soluble in water are called alkalis.

• NaOH, KOH, Mg(OH)₂, Ca(OH)₂ and NH₄OH are alkalis because they dissolve (i.e soluble) in water.

• [Al(OH)₃], [Cu(OH)₂] are bases but not alkalis because they do not dissolve (i.e insolube) in water.

Indicators:
•Indicators are the substances which indicate (tells us) whether something (the given solution) is an acid or a base by changing its colour or smell (odour).

Types of indicators:

1. Natural Indicators

2. Synthetic Indicators

3. Olfactory indicators

4. Universal indicator

1. Natural Indicators:

• The indicators which are found in nature (plants) is called natural indicators.

For Example: Litmus, turmeric, red cabbage leaves, china rose, coloured petals of Hydrangea, Petunia and Geranim (flowers) etc.

Litmus:
• Litmus is a mixture of different dyes which is purple in colour and extracted (obtained/found) from different species of lichens such as Lecanora tartare, Roccella tinctoria etc.
• Lichen plants belong to the division of Thallophyta.

• Litmus dye obtained by the grinding and crushing of lichens.

Litmus Solution:
• A natural purple dye which is extracted from lichens is then dissolved in distilled water to obtain a litmus solution which is purple in colour i.e Litmus solution is made by dissolving a natural dye derived from lichens in distilled water.

• It turns red in acidic solutions and blue in basic solutions.

Litmus Paper:
• In laboratory litmus solution can be used in the form paper called litmus paper to distinguish between an acid and a base which is made by absorbing the litmus solution onto the filter paper.

Litmus paper is of two types : Blue litmus and Red litmus.
• An acid turns blue litmus to red and base turns red litmus to blue.

Red cabbage leaves:
• Red cabbage indicator is a purple-coloured solution which can be extracted by Chopping the red cabbage into small pieces and adding boiling water in a blender and blend it then filter out the soluition.
• The solution can be used to test whether substances are acid or base.

• It turns pink in acidic solution and bluish-green in basic solution.
Turmeric:
• Turmeric (haldi) contains a yellow dye.

• It turns reddish-brown in basic solutions.
• But remains same (i.e yellow) in acidic solutions.

• A yellow stain of curry on a white cloth (the spot of turmeric or gravy) turns reddish-brown when soap which is basic in nature is scrubbed on it.

Hydrangea:
• Hydrangea produces a blue-colored flower when grows in acidic soil and produces red and pink flowers in basic or neutral soil.

Petunia:
• Petunia produces reddish-purple flowers in acidic soil and blue flowers in basic soil.

Geranium:
• Geranium produces orange-red flowers in acidic soil and blue flowers in basic soil.

China Rose:
• China rose solution gives dark pink colour in acidic medium and green colour in basic medium.
2. Synthetic Indicators


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