Chemistry, Third edition

up water into the flask so fast that it sprays like a fountain. If blue litmus is added to

the bucket, the water entering the flask turns red because of the hydrochloric acid

formed and a red fountain is observed. If the experiment is repeated using ammonia,

red litmus is used and the colour change is red to blue.

Osmosis

Example of osmosis

When red blood cells are put into water they burst. What is happening? The cell

membrane allows only water to pass through. The fluid in the red cells contains a

mixture of organic compounds and inorganic ions, and is therefore more concen-

trated than the water outside. Water enters through the cell walls in an attempt to

dilute the solution within the cell. Eventually, so much water enters that the cell wall

bursts apart.

If fresh red blood cells are placed in a 2% solution of salt, the cells shrink. In this

case, the water within the cells enters the more concentrated solution outside the

cell, and the cells dehydrate.

Key points about osmosis

1.Osmosis is the passage of a solvent(usually water) from a zone of low concentra-

tion to one of high concentration. The solution of higher concentration is said to

behypertonic(‘hyper-’ means more), whereas the solution of lower concentra-

tion is said to be hypotonic(‘hypo-’ means less). After osmosis is complete, the

solutions are equally concentrated and are said to be isotonic(‘iso-’ means the

same). An example of osmosis is shown in Fig. 11.10.

2.The membrane allows solvent to pass through but not solute; it is called a

semipermeable membrane.

3.The movement of solvent from low to high concentration is similar to the

spreading out of a gas (diffusion) from high gas pressure to low pressure. The

idea of pressure is also applicable to osmosis, and the osmotic pressuremay be

thought of as the force acting per unit area of membrane which drives solvent

molecules through the membrane.

Semipermeable membranes may be natural or artificial. No membrane is

semipermeable to all solutes. Some membranes keep out ions (such as Naand Cl)

11.6

OSMOSIS 187

Fig. 11.10Osmosis involves the movement of a solvent across a semipermeable membrane.