Evaporation experiment

Design an investigation on a factor that could affect to the evaporation rate of a substance

Research Question

How different amounts of dissolved table salt can affect the evaporation of water at the same temperature?

Introduction

Evaporation is the process by which different atoms or molecules in a liquid obtain enough energy to become a gas and change the substance of state. The molecules thermal motion must be enough to reach the liquid surface tension caused by IMF’s in order for the liquid (in this case water) to evaporate, in other words, the liquid kinetic energy must overtake the surface function of attachment. So evaporation happens quicker with liquids with a lower surface tension at a higher temperature. The rate of evaporation is limited because only a small amount of the molecules are placed near the liquid surface and furthermore, a smaller amount of them are moving in the correct way. In addition, the remaining molecules have lower average kinetic energy as the liquid fastest molecules had escape, and, because of this, the temperature of the liquid decreases.

When evaporation occurs in a closed container, the escaping molecules assemble together as a vapour above the liquid. Many of this escaping molecules then return to the liquid and they become as returning molecules more common as the density and pressure of the vapour substance increases. When this process of the molecules of escaping and returning from the liquid reaches its equilibrium, the vapour is saturated. (Chemguide.co.uk, 2016)(Projects.icbse.com, 2010)

Factors that influence the rate of evaporation

1.     Temperature of the substance. The evaporation will be faster if the substance is hotter.

2.      Intermolecular forces. The stronger the forces keeping the molecules together, more energy is needed in order to evaporate the liquid.

3.        Surface area and temperature.

Surface area and temperature

Molecules or atoms evaporate from the surface of the liquid, when the surface area is larger, it allows more molecules or atoms to go from the liquid to the air becoming vapour, and because of this, evaporation happens quicker.

The higher and hotter the temperature is, the higher the rate of evaporation is. When the temperature is hotter, the molecules and atoms move with more speed, and because they are quicker, more particles are able to leave the liquid's surface.

Intermolecular forces

Most liquids are made up of molecules. The attractions between the molecules increase because normally, the molecules have a region with a slight negative charge, and other one with a slight positive charge. These different regions of electric charges are created because some atoms of the molecules are more electronegative than others. For example, in water, the oxygen atom is more electronegative and the hydrogen is less electronegative, or more electropositive. Water molecules are attracted all together because the positively charged hydrogen atoms are attracted with the lone pairs of electrons of the negatively charged oxygen between different molecules.

The intermolecular forces attractions between molecules affect the evaporation rate of a liquid because strong intermolecular forces attractions need more energy to separate them as they are all hold together in the liquid. Because of this, liquids with stronger intermolecular forces between the molecules evaporate slower than liquids with weaker intermolecular forces between the molecules.

(Projects.icbse.com, 2010)

Salinity of water and it boiling point

When we add salt to water, this makes take longer for the water solution to boil. The table salt really increases water boiling point.

When salt is added to the water, the phenomenon of boiling point elevation does it effect. Boiling point elevation only happen when a non volatile solute (in this case table salt) is added to a pure solvent (in this case water) creating a solution (in this case salt water).

Saltwater needs more temperature in order to start evaporating and also in order to boil than pure water alone. So with this phenomenon, when we add table salt to water, the boiling and evaporating point increases and, furthermore, the time the solution takes to boil and evaporate increases. Concluding, the evaporation is slower because the sodium and chlorine ions take up space on the surface of the liquid so that less water molecules can turn into a gas.

(Mental Floss, 2015)

Hypothesis

In my experiment, I'll put 0,2 litres (200 ml) in 5 beakers, all with the same amount of water. Then, I will add to the beakers 5 different amounts of table salt: 1st 0 grams of table salt, 2nd 10 grams, 3rd 20 grams, 4th 30 grams and 5th 40 grams. They will be heated all at the same temperature, 90 degrees, and they will be the same time heated in the water bath (10 minutes). I think that more water solution will evaporate in the 1st beaker and less water solution will evaporate in the 5th beaker because water evaporates faster without salt as the boiling point of water increases when adding more salt. And this is because when we add salt, the sodium and chlorine ions take up space on the surface of the liquid and they block the water molecules to turn into a gas, so when more salt is added, more sodium and chlorine ions take up in the surface of the liquid blocking the water molecules to pass. In other words, the solution boiling point is directly proportional to the amount of salt added.

Variables

- Independent: The independent variable is table salt, I will manipulate it by changing the amount to table salt in each trial, this will affect the solution boiling point, the amount of water evaporated. I will measure the amounts of table salt with a balance.

- Dependent: The dependent variable is the amount of water evaporated at the end of the experiment. I will measure the amount of water evaporated in ml, I will do the different from the initial solution with the final solution, and then the difference is the amount of water that has been evaporated in ml. To do this, I will use a beaker to see and measure the solution at the start of the experiment and at the end of the experiment.

- Controlled: The controlled variable is the water that I put to the solution, the container where the solution will be in, that it will be a beaker, and the temperature at which the solution will be exposed, that is 90 degrees. I will use a beaker to measure the amount of water too. All of them are controlled variables because in the experiment they are always the same quantity and they don't affect the other variables (dependent and independent).

Materials

- 5 beakers of 500ml each.

- Water (H2O) 200 ml in each beaker, and also some water for the water bath.

- 100 grams of table salt (10+20+30+40 = 100 grams).

- A balance (to weight the exact amount of table salt).

- A water bath (to heat up water).

- A stopwatch (to control that the beaker is being heated the exact amount of time).

Method

1. We put 200 ml of water in each of the 5 beakers.
2. We add the exact amount of table salt, different each trial, to the water forming a solution (1st beaker 0 grams, 2nd 10 grams, 3rd 20 grams, 4th 30 grams and 5th 40 grams). I will stir the solution to make sure that the salt has dissolved totally.
3. We add water to the water bath and we wait until the water reaches the precise temperature and is at 90 degrees, we will measure it with a thermometer.
4. We put the beaker with the solution in the hot water that is in the water bath for 10 minutes.
5. We measure the solution dissolved using the beaker to collect the data.
6. We do step 4th and 5th with each beaker.
7. We do the experiment three trials to be more precise.
8. Once we’ve collected all the data, we make a table and a graph with the data to study the results of the experiment.
9. Finally, we do a conclusion and an evaluation of the method with the results explaining what we’ve studied and the conclusions we’ve got, and also we say the problems that happened while we were doing the experiment and how we solve them.

Results

Table with the difference of milliliters from the initial volume

	Difference of ml from the initial volume
Grams salt	trial 1	trial 2	trial 3	average	SD
0	5,0	6,0	4,0	5,0	0,8
10	3,0	4,0	3,0	3,5	0,5
20	2,0	3,0	2,0	2,3	0,5
30	1,0	1,0	2,0	1,3	0,5
40	0,0	1,0	1,0	0,6	0,5

Revision of the method

• The first error that we founded was that when we add the water inside the pipe we did not read correctly the measurements of the pipe so when we put after the water in the beakers there wasn't exactly 200 milliliters in each one. The concentration was not exactly as we planed and that could affect the final result. It is a systematic error as it arise from a problem in the experimental setup that results in the measured values always deviating from the “true” value in the same direction . This is an example of a parallax it is caused mainly by seeing the object at an oblique angle so we think that a way to solve this error is to observe the pipet with an angle of 0º. (Department of Natural Sciences, 2016)
• The second error that we founded was that the balance were we measured the mass of salt could be incorrectly calibrated and it always give us a slightly higher reading that it should. It is a calibration error.  It is a systematic error as it arise from a problem in the experimental setup. This error is important because it could affect the final result of the experiment as the concentration of salt was the independent variable and if affects directly the dependent variable.
• The third and final error that we founded was that the temperature of the water bath wasn't the same in all the trials because when we put the beakers inside the water bath the temperature was not exactly 90 degrees, it had a lower temperature because the beakers were cold and because the external temperature. This could made that the temperature varies so there wasn't the same temperature in all the trials, this could affect the final result. This is an example of  a systematic error as the temperature always is the same or lower than we expected.

Conclusion

Concluding, from the results we could say that the experiment went relatively good as the results were as we thought and wrote in the hypothesis, where the beaker with less amount of table salt dissolved (0 grams) was the beaker in which more water was evaporated and the beaker with more amount of table salt dissolved (40 grams) was the beaker in which less water was evaporated. From this, we could come to the conclusion that the amount of table salt dissolved in the water is inversely proportional to the amount of water solution evaporated, as expected. Also, we can see that the graph has a descending of the results and that the points of the results don’t vary so much from the trend line so this means that the results of the experiment are well represented in the graph, as it’s a constant descend. Although the results vary a bit because of the temperature and the time weren’t exactly the same and were some factors that could alter the results, they don’t vary so much and that can be seen in the standard deviation that it’s constant with all the beakers during the three trials.

Finally, even though we had to come through some difficulties during the experiment, as the change of the amount of table salt, temperature and water, the concept of the experiment was always the same and finally we passed this difficulties well and we could finish the experiment with the results and the conclusions we expected.

Photos of the experiment

Materials:

Salt

Water bath

Pipe, water, 3 beakers

Balance

1. Put 200 milliliters of water in each beaker:

2. Add the salt that we measured in the balance to the beakers and churn the solution:

3. Measure the temperature of the water bath until it is at 90º:

4. Put the beakers inside the water bath and wait 10 minutes, then put out the beakers and then measure the difference of volume from the initial volume:

References

ºº1º1Chemguide.co.uk,. (2015). Raoult's Law and non-volatile solutes. Retrieved 4 November 2015, from http://www.chemguide.co.uk/physical/phaseeqia/raoultnonvol.html

Google Books,. (2015). Applied Thermodynamics. Retrieved 4 November 2015, from https://books.google.com/books?id=SPAo9at6v-QC&pg=PA407&lpg=PA407&dq=Henery%27s+Law&source=bl&ots=O2hQAQbsd2&sig=yf0l8_zGn_vjL36hClaocw_HmBY&hl=es&sa=X&ved=0CCsQ6AEwAmoVChMIhIO3q_L0yAIVEPFjCh3C5AKZ#v=onepage&q=Henery's%20Law&f=false

Madsci.org,. (2015). Re: How does salt affect evaporation?. Retrieved 4 November 2015, from http://www.madsci.org/posts/archives/1999-11/941723614.Ch.r.html

Mental Floss,. (2015). Does Adding Salt to Water Make It Boil Sooner?. Retrieved 4 November 2015, from http://mentalfloss.com/article/60046/does-adding-salt-water-make-it-boil-sooner

Projects.icbse.com,. (2010). CBSE Projects Chemistry, C++, Physics, Maths, Biology, IP, Disaster Management. Retrieved 4 November 2015, from http://projects.icbse.com/chemistry-258