Science Buddies: "Ask an Expert"

My teacher wants me to explain why my hypothesis was correct in the conclusion portion of my experiment. I know what happened but I’m not sure how to explain why it happened. I could really use some help here. Are there any corrections I need to make?

Also, I am not sure if the stove and the pot are considered constants. Are they?

Thank you so much for your help!!

Erik



Here is an outline of my experiment:

1. Title:

What effect does table salt have on the boiling point of water? (Should it be effect or affect?)

2. Purpose:

To find out how table salt affects the boiling point of water.

3. Hypotheses:

If I add table salt to water in increasing amounts; 2 tablespoons, 4 tablespoons, and then 6 tablespoons, then the water will boil at a higher temperature with the most salt.

4. Independent Variable:

The amount of salt added to the water

5. Dependant Variable:

The boiling point

6. Constant:

Amount of water
Stove
Pot (are these constants?)

7. Materials and Equipment

Table Salt
Tap Water
Four 2 Quart Cooking Pots
Kitchen measuring cup
Kitchen Tablespoon
Kitchen Digital Thermometer
Stirring Spoon
Gas Stove Top

8. Procedure

1. I filled four 2 quart cooking pots with 4 cups of tap water.
2. I measured out the table salt using a kitchen tablespoon and leveled the spoonfuls; adding 2 tablespoons, 4 tablespoons, and 6 tablespoons. to three of the four pots.
3. I measured the highest temperature of all four pots while the water was vigorously boiling. The pot without table salt was my control that I compared the other data against.
4. I repeated each trial three times and recorded the data from each trial.

9. Repeated Trials/Data Table:

Test # 1 Test #2 Test #3 Test #4
Water 4 cups 4 cups 4 cups 4 cups
Trial I Salt 0 Tbls. 2 Tbls. 4 Tbls. 6 Tbls.
Boiling 211.5° F 211.5° 213.5° 218.5° F
Water 4 cups 4 cups 4 cups 4 cups
Trial II Salt 0 Tbls. 2 Tbls. 4 Tbls. 6 Tbls.
Boiling 211.5° F 213.0° F 214.5° F 217.5° F
Water 4 cups 4 cups 4 cups 4 cups
Trial III Salt 0 Tbls. 2 Tbls. 4 Tbls. 6 Tbls.
Boiling 212.0° F 213.5° F 218° F 220.5° F

10. Results

Test # 1 Test #2 Test #3 Test #4
Average 211.7° F 212.7° F 215.3° F 118.7° F

As I increased the amount of table salt in the water the boiling point increased.




11. Conclusion:
My hypothesis was correct. The water with the most salt had the highest boiling point. In my experiment, I observed that as I added more salt, there was less steam (vapor pressure). I also observed that as I added more salt the boiling points increased. I learned that water boils when its vapor pressure equals its surrounding pressure, so I knew that when I saw less bubbling and steam, the vapor pressure must have been decreasing. I knew the atmospheric pressure in my kitchen was constant (staying the same) and that’s why the temperature of the water had to increase for it to boil.

Science terms I learned in this experiment:
The boiling point* is the temperature at which a liquid can change its state from a liquid to a gas. It is the temperature at which the vapor pressure (i.e. evaporation) of a liquid equals the pressure (i.e. atmospheric pressure) of its surroundings.
The vapor pressure* of a liquid is the pressure exerted by its vapor. In my experiment the pressure was the atmospheric pressure in my kitchen.
A solution* is a mixture of one or more substances, known as solutes (salt), dissolved in another substance, known as a solvent (water).

*Definitions are from the Wikipedia Dictionary


If I were to conduct this experiment again I think it would be fun to repeat it up in the mountains. There is a lower atmospheric pressure at higher elevations and it would be interesting to see what my results would be. I would expect the water would boil at a lower temperature.

You're doing very well. You asked for a critique and didn’t say what grade you are in, so I'm going to be a real nit picker. Probably too critical for your grade level so don’t be discouraged...

Effect / Affect
You need to look this up in a 19th century dictionary (I use my Grandmother’s cira 1880) or an unabridged Cambridge dictionary one so you understand it. The difference in these two words is subtle and is being lost in “American�. The British would say we haven’t spoken “English� for years. When I’m trying to decide which to use, I typically go with “something or somebody can have an effect� but “people and things are affected�. In your case, it fits the former, so “Effect�.

If I add table salt to water in increasing amounts; 2 tablespoons, 4 tablespoons, and then 6 tablespoons, then the water will boil at a higher temperature with the most salt.

I can't read this sentence and understand it in 5 seconds! Judges and people walking by don't have much time, so you need to simplify! Do the specific amounts affect your hypothesis? I think not. You are proposing a relationship that increasing salt increases the boiling point. Do you know what “salt concentration� means well enough to explain it to a judge in 20 seconds? If you don’t that is fine, don’t attempt to say something using complex terminology that you don’t understand well enough to explain!

Constants
Why are you listing some of your "test apparatus and measurement equipment" and not all? Does the stove have anything to do with "measuring"? If you used different stoves would that change your result? I'm thinking not. Could using different thermometers or ways of measuring temperature affect your results? I'm thinking yes. So I'm thinking the critical constants here are things like the water source (did it start with a different concentration of minerals and salts) AND the amount of water AND the critical measurement equipment (thermometer) AND you used “constant� in the write-up for things that aren’t here that should be!

I hope you used the same water source and thermometer.

I knew the atmospheric pressure in my kitchen was constant (staying the same) and that’s why the temperature of the water had to increase for it to boil.

The atmospheric pressure in MY kitchen is NOT constant. How do you control it in yours? I’ll give you that at any point in time, the atmospheric pressure at all four burners is very close to the same, especially if the temperatures are close. If you had four different concentrations boiling at the same time, the atmospheric pressure would be constant (enough) for the four concentrations in that trial. You need to simplify and reword your statement to be concise and accurate.

Because you can’t know (unless you had a barometer which you didn’t list), you can’t say anything about the atmospheric pressure between the trials. This invalidates the use of averages between trials to prove your point. You are going to have to rely on each trial by itself. So, did the boiling points in each trial increase with salt concentration? I hope so, if not, averages won’t help you. In your case, all your individual data supports your conclusion, you lucked out!

If you boiled four pots with the same concentration at the same time, you wouldn’t be able to make any claims! Again, you lucked out or carefully designed your experiment. Which was it? In complex research, that might go in a discussion section.

Average 211.7° F 212.7° F 215.3° F 118.7° F

I hope 118.7 is a typo or you have an error in your calculations!

My teacher wants me to explain why my hypothesis was correct in the conclusion portion of my experiment. I know what happened but I’m not sure how to explain why it happened.

Once you clean up the “constants� and figure out how to say that atmospheric pressure was constant for all each individual trial (but may have varied between trials), the rest should be simple. The first part of this belongs in the constant section, further explanation typically goes in a “discussion� section somewhere between the procedure and the conclusion sections.
Results
This section is problematic. I’ve jumped around In my critique and covered why the averages shouldn’t be used in this case. Based on your control, the last trial appears to have been done at a higher atmospheric pressure, no proof, but the data supports my guess. IMO this invalidates your claims of constant atmospheric pressure. Your control is telling you that you had a difference! That is why scientists use controls!

What you need to put here is something that calls attention to your data that shows for each of the three trials that the boiling points increased with the amount of salt added. If you put that here, what do you put in the conclusion? Maybe you want to do a “Results and Conclusion� section.

Discussion / Data Analysis Sections
When there is the potential for some controversy (and I’ve pointed out some in your experiment and data analysis), using a separate discussion section to explain and justify your methods and cite your data that supports your interpretations allows you to sort out any controversy outside the conclusion section.

That leads to a cleanly worded and simplified conclusion section. You have some “discussing to do�! Because you didn’t directly measure the atmospheric pressure, you should cite some reference that deals with the boiling point of water being affected by the atmospheric pressure if you are going to mention it in your conclusion section. You didn’t do the work (no problem), you are relying on somebody else’s work (no problem), you need to cite their work (if you don’t, big problem).

In your case, you probably don’t want a data analysis section because the only thing you tried (averages) wasn’t appropriate. If you had some higher level math, you could compare the changes between concentrations between trials using the control as a barometric pressure. I’m guessing you don’t have that knowledge yet.

Conclusion
This should be simple, short, and direct. In your case, each of your trials is consistent with your hypothesis, so there really isn’t much to say. I vote for combining “Results and Conclusions� for your project.

Craig_Bridge wrote:

I knew the atmospheric pressure in my kitchen was constant (staying the same) and that’s why the temperature of the water had to increase for it to boil.

The atmospheric pressure in MY kitchen is NOT constant. How do you control it in yours? I’ll give you that at any point in time, the atmospheric pressure at all four burners is very close to the same, especially if the temperatures are close. If you had four different concentrations boiling at the same time, the atmospheric pressure would be constant (enough) for the four concentrations in that trial. You need to simplify and reword your statement to be concise and accurate.

Because you can’t know (unless you had a barometer which you didn’t list), you can’t say anything about the atmospheric pressure between the trials. This invalidates the use of averages between trials to prove your point. You are going to have to rely on each trial by itself. So, did the boiling points in each trial increase with salt concentration? I hope so, if not, averages won’t help you. In your case, all your individual data supports your conclusion, you lucked out!

If you did all your trials on the same day, within a few hours, I think you are okay. What you should do is go find the barometric pressure for your area. There are places/people who log this data hourly, so you should be able to find this by googling. If the data you find indicates that you had a nice day with no storms/no changes in air pressure, then I think averaging is fine. If you see there was a storm front moving through, or the barometric pressure was changing, then you have a problem!

If you did this on different days, then it becomes more difficult to justify averaging the data, but again, if the weather was nice and the barometric pressure constant, then you are okay.

Here is a site with a ton of weather detail.
http://www.wunderground.com/
Put in your zip code and you get the weather. You want to find the link on that page that says "Personal Weather Stations (PWS)" for your city. Hopefully, there will be several choices. Here is an example for the city of Philadelphia:
http://www.wunderground.com/cgi-bin/fin ... =19143#PWS

And then the first weather station give me this when I click on historic data:
http://www.wunderground.com/weatherstat ... =KPADREXE2

You can see that today, March 22, the barometric pressure has fallen from 30.5 to 30.25 inches of mercury in only a few hours. I could have guessed this (I'm in Philly)... it looks like it will rain, and the barometric pressue changes with a storm.

Louise