what is Carbohydrates ?

Carbohydrates : 

Carbohydrates are also known as saccharides.

There are two concepts related to definition of carbohydrates -

1.classical concept 

and 

2.modern concept.

Classical concept of carbohydrate:

√ Carbohydrates are hydrates of carbon that fit in the formula of Cn(H2O)y.

ex :1.

 Glucose (C6H12O6) can be written as C6(H2O)6

ex:2.

Sucrose (C12H22O11) can be written as C12(H2O)11

Failure of classical concept of Carbohydrates:

√ The compound of molecular formula C2H4O2(i.e. acetic acid ch3cooh) is Fit in the formula as C2(H2O)2. 

But the physical and chemical properties of this compound is different from carbohydrates.

And also

A Organic compounds of molecular formula C6H12O5 (Rhamnose) have similar properties to the carbohydrates but not fit in that formula.

as C6(H2O)5.H2

This is the reason behind the failure of classical concept of carbohydrates definition.

Modern concept of carbohydrates:

Carbohydrates are optically active ,  polyhydroxy , aldehyde or ketones.

[ Here 

optically active compound means the organic compounds that have chiral carbon and that rotate the plane polarised light towards clockwise or anticlockwise.

(chiral carbon : SP3 hybridised carbon whose valencies completed with four different groups or atoms 

ex : Ch3C*(OH)(Br)(I). ]

✓  some carbohydrates are sweet in test  is Known as Sugar.

° domestic used sugar is known as sucrose that is also called table sugar.

° sugar present in milk is known as lactose.