Molecular Motion and Diffusion Lecture Notes



All molecules are in constant random motion. 

The higher the temperature, the faster molecules are moving.

The faster molecules move, the harder they collide. Two possible results when molecules collide:

        1. Molecules usually bounce off each other and move in a new direction.

 2. Bonds between atoms may break and a chemical reaction may occur. More likely at higher temp due to faster molecular motion.


Matter can be in any of three states:  Solid, liquid, or gas.

Molecules are packed together more closely in solid state.  Molecular motion is limited to tiny distances due to tight packing.

More space between molecules in a liquid state, so they can "slide" past each other after collisions.

Lots of space between molecules in a gas state.  Molecules can travel some distance before running into each other. 


Constant molecular motion causes DIFFUSION.

Diffusion = molecules spread themselves out evenly wherever they can move.

Or: There is a net movement of molecules from areas of higher molecule concentration to areas of lower molecule concentration.


    Evaporation = Diffusion of water or other chemicals from liquid to air


IMPERMEABLE BARRIERS block the diffusion of all molecules. Example: Glass

SEMIPERMEABLE BARRIERS block diffusion of some, but not all, molecules. Example: Cell membranes or human skin.

Water diffuses rapidly across the cell membrane. Most other molecules diffuse through much more slowly, or not at all.


OSMOSIS = diffusion of water across a semipermeable cell membrane.

Water flows across a cell membrane when the water concentration is NOT the same on both sides of the membrane.

The side with the LOWER water concentration is called HYPERTONIC (= more salt/less water).

The side with the HIGHER water concentration is called HYPOTONIC (= less salt/more water).

 ISOTONIC fluid has the same water concentration as fluid inside a cell.

Water always flows from hypotonic side to hypertonic side across a cell membrane.

If the fluid outside a cell is hypertonic, cell loses water and dehydrates (shrinks)

If fluid outside cell is hypotonic, cell gains water and may lyse (burst)

If fluid outside cell is isotonic with cytosol (cell fluid), cell size won’t change


SOLVENT: The molecule present in the highest amount in a liquid. WATER is the main solvent inside all living organisms.

SOLUTE: Any molecule that is dissolved in the solvent. Salts, sugars, proteins, DNA, etc. are all solute molecules.

DISSOLVE: When solute molecules become completely surrounded by solvent molecules we say the solute has dissolved in the solvent. For example: Sugar seems to disappear in water because water molecules eventually get between all the sugar molecules and separate them.

Note:  Substances can dissolve in a solvent without undergoing a chemical reaction. 


1.  Crackers usually become soft if they are left out in the air too long.  Which of the following ideas explains this observation?

a. Water slowly evaporates from crackers into the air

b. Water slowly diffuses from the air into the crackers

c.  Salt slowly evaporates from the crackers into the air

d.  Salt slowly diffuses from the air into the crackers

e.  Water leaves the crackers by osmosis 

2.  You place blood cells in a fluid and the cells rapidly shrink.  This observation indicates that the fluid outside the cells was ___________ compared to the liquid inside the cell:

a.  Hypertonic      b.  Isotonic      c.  Hypotonic      d.  Osmotic