When heated air will expands or contract?

Answer 1

Air is essentially a mix of nitrogen and oxygen (they amount to about 99% of the air) at a pressure of one atmosphere, so that its behaviour is essentially that of a gas.

The gas fundamental law is that

P V =nR T

where

P is pressure

V is volume

n is related to the considered quantity (number of moles of the substance)

R is a constant (the so called gas constant)

T is temperature.

if you provide heat (you heat air) the second member increases since the temperature increases. In natural conditions the pressure remains constant at one atmosphere. Thus, since also the first member has to increase, the volume has to increase of a quantity

nRT/P

When you heat air at constant pressure it always expand!

Considering equal to Q the heat you give to your quantity of air and c the specific heat of air (that is the heat needed to rise a unit quantity of air of one degree Celtius), we have that the temperature increases is

T=c Q

and the expansion (increase in volume) of air turns to be

n R c Q /P

Just to make an example, let us increase of 3 degrees one cubic meter of air passing from 25 °C to 28°C, using parameters for air the increase of volume at pressure of one atmosphere turns to be 21 cm3