Why does a sponge soak up water?

 Why does a sponge soak up water?

Spill some water and an instant remedy is

to mop it up with a cloth or a sponge. But 

why don't we reach for, say, a piece or

metal or wood? We know from experience

that highly porous materials, such as a

Sponge, a towel Or blotting paper, Will

Soak up water quickly and that solid sub-stances won' t.

Among water's bag of tricks is capillary

action, its ability to climb. You can see this

for yourself by inserting the tip of a fine

glass tube into a tumbler of water. The

level of water in the tube will be higher

than that in the tumbler. If you use sev-

eral tubes of different diameters, you will

see that the finer the tube, the higher the

water climbs.

Water behaves in this way, say scientists,

because its molecules will bind them-

selves to many other substances, particu-

larly those containing oxygen. The oxygen

draws the hydrogen in the surface molecules of the water, and these climb

higher, drawing a rope of molecules be-

hind them. The ascent halts only when

the weight of the raised water balances

the capillary forces.A sponge or a piece of blotting

paper is a mass of tiny tubes, and

water will invade them, unassisted

If you squeeze the sponge, you

employ an additional force. As the

Sponge expands to regain its

shape, it draws water into its

tubes, almost as though it were

sucking through a bunch of

straws. Then, when the sponge is

fully expanded, capillary action

draws in even more water

If water lacked its talent for

climbing, plants and trees would

probably die. Their root hairs take

up water through a process known as

Osmosis, but capillary action helps to

circulate it and dissolved nutrients t0

the branches, stems and leaves. In trees,

water can rise to a height of 30 or 40 m

(100 or 130 ft) by capillary action