**Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration**

The experiment involves placing potato cylinders into solutions with different concentrations. The first step in the experiment is to create the solutions. One of the solutions will be pure water and the other solutions will contain progressively more of the solute (sugar). We measure concentration in mol/dm^{3}. I am going to go up in 0.2 mol/dm^{3} intervals starting from 0 mol/dm^{3} (which is pure water) to 1.0 mol/dm^{3}. I will be testing 6 solutions.

We create the potato cylinders using an apple corer. We need to ensure that the cylinders are the same size because surface area effects the rate of osmosis. Also, we try to use the same potato for all of the potato cylinders as the concentration of solutes will differ in different potatoes.

We then divide the potato cylinders into groups of 3 and use a mass balance to find the mass of each group.

After we have found the mass of the groups of cylinders, we place the groups into each of the 6 different solutions, like what is shown below.

We then leave the cylinders in the beakers for a set period of time, such as 24 hours. After the set period of time, we take the cylinders out of the solutions and dry them gently with a paper towel to remove excess water. We then use a mass balance to find the final mass for each of the groups.

The next step is to work out the percentage change in mass of the groups of cylinders. We find the percentage change in mass rather than actual change in mass because the mass of each of the groups of potato cylinders will be slightly different to start with, which means that percentage change will be better than actual change. We calculate percentage change by using the formula below:

**Example**

These are the values for the 0.6 mol/dm^{3} solution. The mass of the group of potato cylinders before they were placed into the 0.6 mol/dm^{3} solution was 10.5 g, and the mass of the group of potato cylinders after being in the solution for 24 hours is 10.1 g. Find the percentage change in mass. Give your answer to 1 decimal place.

The initial mass is the mass before the potato cylinders were placed into the solution, which is 10.5 g. The new mass (or final mass) is the mass of the potato cylinders after they have been in the beaker for 24 hours, which is 10.1 g. We have everything we need for percentage change; the initial mass is 10.5 g and the new mass is 10.1 g.

A positive percentage change would mean that the mass of the potato cylinders would have increased; the mass would have been greater after being in the solution compared to before being in the solution.

**The Results**

After we have worked out the percentage change for all of the 6 solutions, we can plot our results on a graph and draw a smooth curve connecting all of the plotted points. My results are plotted on the graph below.

From looking at the above graph, we can see that the mass of potato cylinders increased when they were placed in lower concentrated solutions (e.g. pure water [0 mol/dm^{3}] to 0.2 mol/dm^{3} solution). A positive percentage change in mass means that the net movement of water is from outside the potato cylinders to inside the potato cylinders (from the solution into the potato cylinders). This happens because the water concentration outside the cylinders is higher than the water concentration inside the cylinders, and therefore water moves from outside the cylinders (high water concentration) to inside the cylinders (low water concentration). As water enters the potato cylinders in the lower concentrated solutions, the mass of the potato cylinders increased, which is a positive percentage change in mass. The lower the concentration of the solution, the greater the net movement of water to inside the cylinders because of a larger concentration gradient, which results in a higher percentage change in mass; the percentage change for pure water (0 mol/dm^{3}) was 4.4 %, and the percentage change for 0.2 mol/ dm^{3} was only 0.6 %.

Also, from the graph, we can see that when the potato cylinders are placed in more concentrated solutions (e.g. 0.4 mol/dm^{3} to 1.0 mol/dm^{3}), the mass of the potato cylinders decreased. A negative percentage change in mass means that the net movement of water is from inside the potato cylinders to outside the potato cylinders (from the potato cylinders to the solution). This happens because there is a higher water concentration inside the cylinders compared to outside the cylinders, and therefore water moves from inside the cylinders (high water concentration) to outside the cylinders (low water concentration). As water is leaving the potato cylinders, the mass of the potato cylinders decreased, which is a negative percentage change. The more concentrated the solution is, the greater the net movement of water is from inside the potato cylinders to the solution, which results in a larger negative percentage change in mass; the percentage change for 1 mol/dm^{3} was -6 %, and the percentage change for 0.4 mol/ dm^{3} was only -2 %.

There will be one point on the curve on the graph where there would be no change in mass. For our graph this happens with a concentration of 2.4 mol/dm^{3}. At this concentration, the net movement of water molecules is 0 (the same number of water molecules enter the potato cylinders as the number of water molecules that leave the potato cylinders). This happens when the solution and the potato cylinders have the same water concentration; at this point, we say that the cylinders and the solution are isotonic. Therefore, an estimate for the sugar concentration of the potato cylinders is 2.4 mol/dm^{3}. The estimate for the concentration of the potato cylinders will always be where the curve crosses the *x* axis.

**Only Change One Variable**

The only variable that we are changing in this experiment is the concentration of the solutions (this is our independent variable). The dependent variable is the mass of the potato cylinders. Everything else in the experiment should be kept the same to ensure that the test is reliable; examples of other variables that should be kept the same are the volume of the solution, temperature, time for the cylinders in the solution etc.