B1 Osmosis (page 18) Flashcards
What is Osmosis?
It is a special case of Diffusion.
So you need to understand diffusion, and Osmosis will be a breeze, so go through page 17
In more detail, explain what is Osmosis?
it is the movement of water molecules across a partically permeable membrane from a region of high water concentration to a region of lower water concentration.
What is a Partially Permeable membrane?
it is just one with very small holes in it, so small in fact, only tiny molecules (like water) can pass through them, and bigger molecules (e.g. surcrose) can’t.
How do the water molecules pass through the membrane during osmosis? and explain why?
water molecules pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time.
There are more water molecules on one side than on the other, there’s a steady net flow of what?
there’s a steady net flow of water into the region with fewer water molecules. i.e. into the stronger sugar solution.
This means the strong sugar solution gets more dilute. The water acts like it’s trying to “even up” the concentration either side of the membrane.
Omosis is a type of diffusion, explain why?
due to passive movement of water particles from an area of higher water concentration to an area of lower water concentration.
There is a fairly dull experiment you can do to show osmosis at work, explain the process?
1) You cut up an innocent potato into identical cylinders, and get some beakers with different sugar solutions in them.. One should be pure water and another should be very concentrated sugar solution (e.g. 1 mol/dm³) Then you can have a few others with concentrations in between (e.g. 0.2 mol/dm³, 0.4 mol/dm³, 0.6 mol/dm³, etc).
2) You measure the mass of the cylinders, then leave one cylinder in each beaker to twenty four hours or so.
3) Then you take them out, dry them with a paper towel and measure their masses again.
4) If the cylinders have drawn in water by osmosis, they’ll have increased in mass. If water has been drawn out, they’ll have decreased in mass. You can calculate the percentage change in mass, then plot a few graphs and things. (see page 241 for calculating the percentage change).
You could also carry out this experiment using different salt solutions and see what effect they have on potato chip mass.
What is the dependent variable? and independant variable?
it is the chip mass, and the independant variable is the concentration of the sugar sulution.
All other variables (volume of solution, temperature, time ,type of sugar used etc, etc, must be kept the same in each case or the experiment wont be a fair test.
Like any experiment, what do you need to be aware of?
you need to be aware of how errors (see page 5) may arise. Sometime they may occur when carrying out the method, e.g. if some potato cylinders were not fully dried, the excess water would give a higher mass, or if water evaporated from the beakers, the concentration of the sugar solutions water evaporated from the beakers. the concentrations of the sugar solutions would change.
How can you reduce the effect of experiment errors?
by repeating the experiment and calculating a mean percentage change at each concentration.
Explain what will happen to the mass of a piece of potato added to a concentrated salt solution? (2 marks)
Water will move out of the piece of potato by osmosis (1 mark), so its mass will decrease (1 mark).
