Illustrated Guide to Home Chemistry Experiments

156 DIY Science: Illustrated Guide to Home Chemistry Experiments

LABORATORY 8 .3:

oBSERvE THE EffECTS of oSmoTIC pRESSURE

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance

£ beaker, 150 mL (1)

£ marking pen

£ sharp knife

£ paper towels

£ solutions A through f from Laboratory 8.1

£ celery stalk

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute any container of similar size or
  larger for the 150 mL beaker.

CUTIOA nS

Wear splash goggles, gloves, and protective clothing.

Be careful with the knife.

z

dISoALp S : dispose of the solutions by flushing them

down the drain.

Osmotic pressure is the hydrostatic pressure

that exists when two solutions of differing

concentrations are in contact with the two

sides of a semipermeable membrane. The

semipermeable membrane allows small

molecules (such as water) to pass through

it freely, but physically prevents larger

molecules (such as a solute) from passing

through the membrane. In such a system,

water passes from the side that contains the

less-concentrated solution to the side with

the more-concentrated solution until the

concentrations of the solutions are equal, at

which point the system is in equilibrium.

If a biological cell is placed in a hypotonic environment (one in
which the solution inside the cell is more concentrated than the
solution surrounding the cell), water flows into the cell from the
outside solution through the semipermeable membrane that
surrounds the cell, causing the cell to expand and gain mass.
Conversely, in a hypertonic environment (one in which the
outside solution is more concentrated), water flows outward
from the cell, shrinking it and reducing its mass.

Children sometimes unwittingly illustrate the principle of
osmotic pressure by pouring table salt on a garden slug. This
creates an extreme hypertonic environment, sucks water out of
the slug, and thereby kills it by quick and extreme dehydration.
My editor told me to make sure that no slugs were harmed in
running this experiment, so I decided to use celery instead.
If that offends People for the Ethical Treatment of Vegetables
(http://www.petv.org), I’m sorry, but I had to use something.

In this laboratory, we’ll use the sodium chloride and sucrose
solutions left over from the first laboratory session in this
chapter to observe the effects of osmotic pressure on the
semipermeable membrane that surrounds celery cells.

POCEDURER

1.ou have not already done so, put on your splash If y

goggles, gloves, and protective clothing.

2. Cut the celery stalk into seven pieces of approximately
   equal mass.


3. Label the celery pieces A through G.


4. Weigh each piece of celery and record the mass of each in
   Table 8-3.


5. Drop each piece of celery, A through F, into the
   corresponding beaker, making sure that the celery is
   completely submerged.


6. Drop celery piece G into a container of pure water.


7. Allow the celery pieces to soak for at least one hour.


8. Remove each piece of celery from its beaker, rinse it
   briefly in running water, pat it dry with a paper towel, and
   reweigh it (Figure 8-3). Record the mass of each piece in
   Table 8-3.