By Acid base and salts
Acids:- these are the substances that
- Are sour in taste.
- Turn blue litmus red.
- Give H+ ions.
- Can accept a pair of electrons.
Base :- These are the substances that
- Are bitter in taste.
- Turn red litmus blue.
- Soapy in touch.
- Give OH– ions.
How to recognise the acid and bases?
To answer this question lets remind an activity:
What happens when we dip a cloth with curry stains in soap solution?
It turns RED
If we see chemical aspect of this, the curry had turmeric in it that turned red on reacting with soap solution(a base) . so from this observation we can easily conclude that the turmeric turns its colour from yellow to red on reacting with base.
So finally we come to know that there are certain substances that change their properties on reacting with acid or base.
THE INDICATORS
These are the substances that change their properties , colour or smell when react with an acid or base.
OLFACTORY INDICATORS
These indicators change their smell when they react with an acid or a base. For example :- clove oil, vanilla essence and the smell of onion juice.
The cloth dipped with onion juice looses its smell on treating with base.
TYPES OF INDICATORS
The indicators are of following types
- Natural indicators
- Synthetic indicators
Natural indicators:- these are obtained from the nature and are used directly without any laboratory treatment. For example;- turmeric and litmus.
Synthetic indicators:- These are the indicators that are synthesised in the laboratory. For example Phenolphthalein and Methyl orange.
Here are some of the observations in the tabular form:-
| Name of indicator | Turmeric (yellow) | Litmus (purple) | Phenolphthalein (colourless) |
| Colour in acid | No change | Red | Clourless |
| Colour in base | Red | Blue | Pink |
The Unconditional Guru 