Science Buddies: "Ask an Expert"

Hello.

I'm just starting the plan for my project does salt affect the freezing point of water. I will use 1 cup with no salt and the other 4 cups with different amounts of salt. Can you help me decide the amount of salt I should use for each cup?

Thank you.

JONATHAN

Hi Jonathan,

Your project sounds very interesting, and your question is a good one.

Since the earth is covered with so much sea water it would be most helpful to plan one of your samples the same as sea water. You can find info about the salinity of sea water vs. fresh water at http://www.gpc.edu/~pgore/Earth&Space/salinity.html. After you include that point, planning the other points might become a little easier.

If you have questions after you get there feel free to post them here in the forum.

Good luck!

Hello again.

My dad just helped me understand that if 35 pounds of salt is in 1000 pounds of seawater then 1/16 of a pound of salt is equal to 1.75 pounds of seawater. Do you think I can use this ratio?

Thank you.

JONATHAN

Terrific! It works out to about 1 ounce of salt to 28 ounce of water. Together with the no-salt solution you now have a second point at the sea-water level.

I would also like to test a sample above and below that amount.

I will have:
Cup #1 with no salt
Cup #2 with 1/2 ounce salt / 28 ounces of water
Cup #3 with 1 ounce salt / 28 ounces of water
Cup #4 with 1 1/2 ounce salt / 28 ounces of water

I will put a thermometer in each cup and place all the cups in the freezer. Do you think it's okay if the thermometer is just touching the water? Or do you think it should be placed entirely into the water?

I will observe each cup (frozen or not) and record the temperature in a table every 10 minutes.

Do you think this is a good plan?

Thank you again.

JONATHAN

Jonathan,

First off, you seem to have very interesting idea!

As to your question, the thermometer would adjust to the temperature of the solution much faster if it is completely submerged. However you should probably make sure that at each 10 minute interval you break up the ice a little bit so the thermometer doesn't become encased in the solution( if you find this to be ap problem). If your thermometer is just barely touching the solution it will probably adjust to the freezer temperature slower ( the conductivity of water is much higher than that of air).

Checking whether each cup is frozen or not might be better in slightly shorter intervals (perhaps 2-3 minutes) because you are quantifying them as frozen and not frozen and not actually measuring the quantity of ice formed. This way your results will be more accurate since if two cups do end up freezing in the same 10 minute interval you can't really tell exactly when.

Just as a side note the phenomena you are investigating is referred to as freezing-point depression. It is the reason why, in in snowy and icy areas, trucks spread calcium salts over roads which in essence causes the ice to melt (because to remain frozen the temperature must be lower). I have provided a link about it if you are interested:http://en.wikipedia.org/wiki/Freezing_point_depression

Good luck! feel free to post any further questions you may have.

Phi-unit

Okay, I will check the samples every 3 minutes instead of 10 minutes. I have to read the freezing point information again and again. I thought water was simple to understand but I see that it's not.

But now it makes sense why my dad uses salt in the winter on our icy walkway and driveway.

I have to hand in the plan to my teacher next week. I'm really excited about this experiment. Do you think 3 trials are enough?

Thank you.

JONATHAN

Jonathan,

Your plan sounds good; you might even want to mention why you shortened the time interval in your report -- it shows scientific reasoning.

Don't stress too much about the freezing-point depression information. Actually you probably won't encounter till about 11th grade when you take AP Chemistry(which I hope you do). I just provided it as a reference to give you a little bit of background on your project.

There is also really no set number of trials you should do. It's usually nice if you end up getting consistent results (relatively similar times between different trials for a certain type of solution.) I might suggest doing a pre-trial(time permitting) so you can work all the kinks in your procedure. However 3-5 trials is sufficient(more is better) but do what you can.

Please feel free to respond if you have any further questions about your project or formatting your report. (

Phi-unit

Hello

I have another question about my project. Do you think when i move the thermometer in the freezer do you think it will affect the results.

Thank you Jonathan

Hello.

I'm using a mercury thermometer and will be putting entirely in the liquid.

Do you see any problems with this?

Thank you.

Jonathan

Hi Jonathan,

I don't believe moving around the thermometer in the freezer will adversely affect your results. In fact it will probably just cause thermometer to adjust to the temperature faster.

Putting a mercury thermometer in the liquid sound just fine. However I must advise to be very careful not to break the glass when your moving it around as mercury is toxic. It might be better to get an alcohol thermometer if you can(the liquid inside is red).

Phi-unit

Hello.

My mom bought me several alcohol thermometers. Yesterday, my brother and I did some pre-trial testing and froze both a water and salt water sample. After seven hours, the temperature reading of the salt water sample was -15 degree celsius. Can this reading be correct? My mom said our freezer does not go below 0 degrees in temperature.

Thank you.

JONATHAN

Jonathan wrote:Hello.

My mom bought me several alcohol thermometers. Yesterday, my brother and I did some pre-trial testing and froze both a water and salt water sample. After seven hours, the temperature reading of the salt water sample was -15 degree celsius. Can this reading be correct? My mom said our freezer does not go below 0 degrees in temperature.

Thank you.

JONATHAN

You should check if all your thermometers are accurate. Get your mom to help you- You should test by putting them in boiling water, they should read 100 C. Put them in ice/water- it should read 0. If these checks are okay, then your reading is accurate. If not, post back here with the details.

Highly salted water freezes much below 0, so -15 C is not unreasonable.

As for what your freezer is set at... generally they are set at or below 0 C. The recommended value I saw several times was 5 *F (-15 C), so I think it is possible your freezer could be that cold.

Good luck, and be sure to get adult help with the boiling water experiment. Both the water and the steam are very hot, and you need to hold the thermometer there to get a good reading.

Louise

Hello.

My mom bought student thermometers from an online school store. They only go upto 50 degrees C and 120 Degrees F. We boiled the water and tested the thermometer. It went upto the very top but then they cracked and the alcohol came out. We did it twice and the same thing happened. Should I have my mom buy better thermometers or should I continue with what we have?

You can see my thermometers by going to: classroomproductswarehouse.com Thermometer ID #CN5592111.

Thank you.

JONATHAN

Jonathan wrote:Hello.

My mom bought student thermometers from an online school store. They only go upto 50 degrees C and 120 Degrees F. We boiled the water and tested the thermometer. It went upto the very top but then they cracked and the alcohol came out. We did it twice and the same thing happened. Should I have my mom buy better thermometers or should I continue with what we have?

You can see my thermometers by going to: classroomproductswarehouse.com Thermometer ID #CN5592111.

Thank you.

JONATHAN

Well, the thermometer you bought won't work at 100 C (as you found out!), so you cannot calibrate it that way. Are there instructions that came with the thermometer that explains how to calibrate? If so, use these instructions.

It is better to check your thermometer accuracy with two points, than with one, but since your thermometers have a smaller range, I would try just one point. Check them with the ice water. If they don't read 0 C, then there is a problem. The second thing to do is to fill a big container with warm water from the sink. Put all your thermometers in at once. Do they all read the same value? If they don't, then there is also a problem. Lastly, look at each thermometer carefully. The red alcohol should be a solid line from the bulb to the temperature. If you see any breaks (clear spot or bubble), then that thermometer is no good. Alcohol thermometers are known to get bubbles, which can cause problems with the readings.

Basically, you are uncertain if your result of -15 C is correct. There are two possibilities: it is correct (and the thermometer is working) or it is incorrect (and the thermometer is not working). We are trying to verify the thermometer is working correctly. I think your value is probably correct (-15 C), but since your mom thinks the freezer is set at 0, you should double check the thermometer.

Does all this make sense to you?
Louise

HI Jonathan,
Here are detailed instructions for testing a thermometer in ice water
http://www.globe.gov/tctg/soil_fg_therm ... tionId=568

According to the sales info you provided, these thermometers are calibrated at -35 degrees C and should be OK for measuring a temperature at -15C. In fact, many manufacturers recommend frozen food storage at -15 to -20 C.

Hello!

Good News - we followed the instructions. We tested the thermometer in a bath of ice water. The readings were all between -5 degrees Celsius and +5 Degree Celsius.

I think I'm ready to conduct my 1st trial of the experiment.

JONATHAN

Jonathan wrote:Hello!

Good News - we followed the instructions. We tested the thermometer in a bath of ice water. The readings were all between -5 degrees Celsius and +5 Degree Celsius.

I think I'm ready to conduct my 1st trial of the experiment.

JONATHAN

Great! So this tells you about the error of your experiment. Two samples could be 10 degrees apart by thermometer, and still be reading the same temperature. [Note: I hope your measurement shows MUCH smaller error- I would expect the range to be much smaller, like -1 to 1 degree. You didn't mean 0.5 did you?] This data you collected is important- this is part of your experiment. If you did not do it carefully, you should redo it, and record the data in your notebook.

If ther errors is really as large as 5 or 10 degrees you can (should!) even use a sticker and "name" each thermometer (therm1, therm2, etc.). Then you can record the value for each one. For example, therm1 may read +5 C and therm2 may read -1 C when they should both read 0 C. (to be very accurate, you should do this several times to make sure they always give the same result (or that you are always reading them the same)) Then, you know every measurement with therm1 is 5 degrees too high and can fix this. If you don't understand this, please ask your mom for help or post back here.

You are making excellent progress.

Louise

Hello.

I'm doing my experiment. After 35 minutes, I noticed the fresh water had tiny needles forming on top of the water. The temperature was 4 degrees celsius. Nothing happened with the salt water samples yet. I also noticed as the temperature was dropping it would then go back up a little. Are my thermometers working correctly? Why would this happen?

After 135 minutes, the fresh water froze. The temperature was 0 degrees celsius. The salt water looks like slush is forming.

I also have a thermometer in the freezer. The temperature is varying from -4 degrees to -9 degrees.

The data I'm collecting is like this:

Temperature of each Cup

Time No Salt Cup #1 1/2 oz Salt Cup #2 1 oz Salt Cup #3 1 1/2 oz Salt cup #4 Observation
0 min. 20 degrees 20 20 20 degrees
5 min. 13 degrees 15 15 15
10 min.
15 min.

Right now, I'm at 205 minutes. Cup # 2 looks like it's starting to freeze. Cups #3 and #4 are also starting to freeze but I can still see some liquid.

Am I doing this correctly. Please let me know.

JONATHAN

Jonathan wrote:Hello.

I'm doing my experiment. After 35 minutes, I noticed the fresh water had tiny needles forming on top of the water. The temperature was 4 degrees celsius. Nothing happened with the salt water samples yet. I also noticed as the temperature was dropping it would then go back up a little. Are my thermometers working correctly? Why would this happen?

After 135 minutes, the fresh water froze. The temperature was 0 degrees celsius. The salt water looks like slush is forming.

I also have a thermometer in the freezer. The temperature is varying from -4 degrees to -9 degrees.

The data I'm collecting is like this:

Temperature of each Cup

Time No Salt Cup #1 1/2 oz Salt Cup #2 1 oz Salt Cup #3 1 1/2 oz Salt cup #4 Observation
0 min. 20 degrees 20 20 20 degrees
5 min. 13 degrees 15 15 15
10 min.
15 min.

Right now, I'm at 205 minutes. Cup # 2 looks like it's starting to freeze. Cups #3 and #4 are also starting to freeze but I can still see some liquid.

Am I doing this correctly. Please let me know.

JONATHAN

This sounds reasonable to me and your observations seem very good/detailed. I don't know how much your thermometer is going up and down, but it could be that when you have the door open to observe the thermometers the freezer warms up and then cools. If this were the case, the regular water/ the freezer thermometer should show this too. You should watch this carefully to check this, and also record the range (say 15-17 degrees). The other thing to consider is different parts of the freezer may cool differently, so if you do another run, you may want to rotate your cups so each one is in a different place from the first run. It may be one location will show these fluctuations more than the others.

Salt water also freezes differently than plain water. Basically, the water freezes as pure water and the remaing water gets saltier. So the slush is pure water crystals + extra salty water. This may also explain your temp variation. Pure freezes at 0, but the salt water can get much colder(like -15), so if you get more ice around the thermometer it would be warmer (0) than if you have more salty water (-15). So, I would expect to see a temp variation IF you had ice in the liquid. Not sure why you would see this above the freezing point, unless, as I said above, the freezer is cycling on and off.

Do these thoughts help?
Good luck!


Louise

Hi Louise.

Thank you for helping me. The temperature is only going up and down 1 to two degrees. I put the samples all on the same shelf in the freezer. I'm rotating them now. I think the back is colder.

Every 5 minutes, I check the samples and record the temperature and observations. I've been doing this since 11 am this morning. It's almost 10 pm and time for bed. The salt water samples have not all frozen yet.

I'm using a clear container so that I can see inside ( 28 ounces of water ) - The temperature is now at -9 degrees for all four samples. THIS IS WHAT I SEE:

Sample 1: Fresh Water. The frozen ice is shiny and clear. In the middle, I had seen a water bubble that moved before. Now it's not moving and all frozen.

Sample 2: 1/2 oz Salt / Water: Cloudy. Lots of ice needles. The slush is freezing. I put my finger thru the ice and made a hole. There's still liquid in the middle and is still not frozen. The top is frozen.

Sample 3: 1 oz Salt / Water: Cloudy. Lots of ice needles. The slush is freezing. I also put my finger thru it and there's liquid in the middle also.

Sample 4: 1 1/2 oz Salt / Water: Cloudy. Lots of needles. The slush is freezing but I still see some liquid water on top. I also put my finger thru and there's liquid in the middle.

This is taking a long time to freeze. I've recorded data every 5 minutes. I have 10 pages. I have to go to bed. My mom will continue recording for me. I hope it freezes soon. Please let me know what I should do.

JONATHAN

Jonathan wrote:Hi Louise.

Thank you for helping me. The temperature is only going up and down 1 to two degrees. I put the samples all on the same shelf in the freezer. I'm rotating them now. I think the back is colder.

Every 5 minutes, I check the samples and record the temperature and observations. I've been doing this since 11 am this morning. It's almost 10 pm and time for bed. The salt water samples have not all frozen yet.

I'm using a clear container so that I can see inside ( 28 ounces of water ) - The temperature is now at -9 degrees for all four samples. THIS IS WHAT I SEE:

Sample 1: Fresh Water. The frozen ice is shiny and clear. In the middle, I had seen a water bubble that moved before. Now it's not moving and all frozen.

Sample 2: 1/2 oz Salt / Water: Cloudy. Lots of ice needles. The slush is freezing. I put my finger thru the ice and made a hole. There's still liquid in the middle and is still not frozen. The top is frozen.

Sample 3: 1 oz Salt / Water: Cloudy. Lots of ice needles. The slush is freezing. I also put my finger thru it and there's liquid in the middle also.

Sample 4: 1 1/2 oz Salt / Water: Cloudy. Lots of needles. The slush is freezing but I still see some liquid water on top. I also put my finger thru and there's liquid in the middle.

This is taking a long time to freeze. I've recorded data every 5 minutes. I have 10 pages. I have to go to bed. My mom will continue recording for me. I hope it freezes soon. Please let me know what I should do.

JONATHAN

It is possible that the salty water won't ever freeze- your freezer may not be cold enough. For tonight, maybe you want to increase the spacing between the measurements to half hour or hour long. This is why Phi-unit suggested- "I might suggest doing a pre-trial(time permitting) so you can work all the kinks in your procedure. However 3-5 trials is sufficient(more is better) but do what you can." If you do another test, use smaller containers/volumes of water so the observations don't take so long. (That is, next time, keep short intervals but use smaller volumes.)

In my freezer the back is colder, because that is where the cool air comes in, so your hypothesis is reasonable.

Louise

Hello.

Last night, the salt water samples never completely froze. I went to bed. This morning (after 18 hours) the samples seemed all frozen. It was cloudy looking and my dad showed me the clumps of salt. Temperature reading was -21 celsius for all 3 salt samples.

My dad said that I should tell you that we used 1 ounce of salt to 28 ounces of water which is the sea water level.

I'm testing again this morning using 6 ounces of water. I'm keeping the salt amounts the same (1/2 ounce, 1 ounce and 1 1/2 ounce). Is that okay?

Thank you.

JONATHAN

Jonathan wrote:Hello.

Last night, the salt water samples never completely froze. I went to bed. This morning (after 18 hours) the samples seemed all frozen. It was cloudy looking and my dad showed me the clumps of salt. Temperature reading was -21 celsius for all 3 salt samples.

My dad said that I should tell you that we used 1 ounce of salt to 28 ounces of water which is the sea water level.

I'm testing again this morning using 6 ounces of water. I'm keeping the salt amounts the same (1/2 ounce, 1 ounce and 1 1/2 ounce). Is that okay?

Thank you.

JONATHAN

I would keep the % of salt the same, so you can compare directly with your other trial. So, if you are using 1/4 the amount of water, use 1/8 oz, 1/4 oz, etc.

What did the freezer thermometer read? I'm surprised the salt got to -21 if the freezer is only at -4 to -9 degrees.

Louise

Here is a link to a website that might help you. Maybe your parents can look at it with you. It is written at the college level, but all I want you to do is look at the picture (called binary phase diagram). This picture shows what happens at different temperatures and amounts of salt
http://www.ucalgary.ca/~kmuldrew/cryo_c ... ap6_1.html

Here are what the words in the picture mean-
brine = salt water
ice= pure water ice
NaCl*2H20= a salt crystal


Each box has a description of what the sample will look like. Supposed you had 6 oz of water (. For the salt, I'm not sure if you are using fluid oz (a measuring cup) or oz (a scale). I'm going to assume you used a scale. So, 0.25 oz of salt in 6 oz water is about 4% salt by mass. Looking at the picture, we see for 4%, we will start at 20 *C (close to room temperature). In your experiment, you just change the temperature, so we will draw a line straight down...The chart says you have brine/salt water until about -5 *c, when you start forming brine + ice (slush!). It stays slushy until -21 *c, when you form ice and salt crystals, and now everything is frozen! This sounds very similar to what you observed! You have an additional variable- time, which is not shown on this chart.

One other note- my earlier post gave an example of how to reduce the concentrations by 1/4. Changing from 28 to 6 is not 1/4 (it is like .21 not 0.25), but for illustrating, dividing by 4 is easy to do. So, have your folks help you do the math. If you used 7 oz, that would be 1/4, and you could use the values I said.


Louise

Hello.

My parents helped me understand your suggestions. I will re-try my experiment on Thursday using 6 ounces of fresh water and 1/8, 1/4 and 1/2 ounces of salt. Please let me know if okay.

In my last trial, I observed the samples in the freezer every 5 minutes. I had alot of pages of data. My parents told me to ask you if it's okay to observe every 10 minutes. Also, should I have a time limit. In my last trial, the fresh water froze first, then the salt water sample with the least amount of salt. The other two salt samples with more salt never really froze. But I did not know when to stop.

Thank you.

JONATHAN

Jonathan wrote:Hello.

My parents helped me understand your suggestions. I will re-try my experiment on Thursday using 6 ounces of fresh water and 1/8, 1/4 and 1/2 ounces of salt. Please let me know if okay.

I think you want to use 7 oz of water, since your first trial used 28 oz, and you are dividing by 4.

In my last trial, I observed the samples in the freezer every 5 minutes. I had alot of pages of data. My parents told me to ask you if it's okay to observe every 10 minutes. Also, should I have a time limit. In my last trial, the fresh water froze first, then the salt water sample with the least amount of salt. The other two salt samples with more salt never really froze. But I did not know when to stop.

You should look at all the data you've collected. If you only looked every 10 minutes or every 20 minutes, would you still be able to tell what is going on? I see no problem lengthening the interval, but you should go through the data with your parents and see how long the gap can be.

In your last trial, you did 6 oz, but the same salt as the first one, right? (so 1 oz, 1/2 oz, 1/4 oz) More concentrated salt solutions may never freeze, so that is reasonable. This is shown in the picture I gave you the link to, but maybe it was too confusing for you?

In your first trial, all samples eventually froze, right? But it took 18 hours, or something long like that? For the plain water, how long did it take to freeze 28 oz vs. 7 oz?

I will check this forum again tonight, so please let me know the answers. I will write back- so don't start the experiment until I write you! We don't want you (or mom) checking the thermometer all night again!

Here are the questions again-
1) In your last trial, you did 6 oz, but the same salt as the first one, right? (so 1 oz, 1/2 oz, 1/4 oz)
2) In your first trial, all samples eventually froze, right? How long did it take?
3) For the plain water, how long did it take to freeze 28 oz vs. 6 oz?


Louise

Jonathan,
I am logging off- I will check for your reply tomorrow.

Louise

Hi, Louise.

I had a school play and just got home. I'm sorry I kept you waiting. My answers to your questions are:

1) In the last trial, I used 6 oz of water and salt amounts of: 1/2 oz, 1 oz. and 1 1/2 oz.
Fresh water froze after 160 minutes; temperature -5 celsius
1/2 oz. salt water froze after 245 minutes; temperature -13 celsius
1 oz. salt water froze after 260 minutes; temperature -12 celsius
1 1/2 oz. salt water never froze after 260 minutes remained slush; temperature -16 celsius

2) In the 1st trial, I used 28 oz. of water and same salt amounts of: 1/2 oz., 1 oz. and 1 1/2 oz.
Fresh water froze after 340 minutes; temperature 0 celsius
The salt water samples never really froze after 12 hours of testing. This is when I went to bed. In the morning (After 18 hours), it looked frozen. But when I touched it, I felt a little liquid on top of the frozen ice - does this make sense?

3) 6 oz. of fresh water froze at 160 minutes and 28 oz of fresh water froze after 340 minutes

If I continue to test with 6 oz. of water, I think I can check every 10 minutes. I saw the ice needles forming on top of the water approximately 15 to 20 minutes in the freezer. I just need to know if I can use 1/8 oz, 1/4 oz. and 1/2 oz. I will use the special measuring spoon for this and not the scale.

I will check back for your answers tomorrow.

Thank you.

JONATHAN

Jonathan wrote:Hi, Louise.

I had a school play and just got home. I'm sorry I kept you waiting.

Don't worry about it, I did not want to keep you waiting!

The idea is that you want to keep all the variables as close to the first trial as possible (except for the total volume- which you reduced so the experiment is faster). So, you don't want to change how you measure salt (scale to measuring spoon) or the concentration of salt. Then, you can compare this new data with the first trial you did, and have two sets of data.

In the first trial your concentrations were:
.5 oz/28 oz, 1 oz/28 oz, and 1.5 oz /28 oz.

The same concentration in a smaller volume would be:
1/8 oz/7 oz, 1/4 oz/7 oz and 3/4 oz in 7 oz.

In 6 oz of water, the amount of salt is not a 'nice number' so I don't know if you can measure this. It would be
0.10714 oz
0.21429 oz
0.32143 oz

You can use either the 6 or 7 oz values, depending on what you prefer. But, if you divide 0.5/28, (1/8)/7, or 0.10714/6, you will get the same number- this is what I mean by concentration.

From your trials, you know that the saltiest cup never really freezes. So, once the second cup freezes, I would check at 10 minutes for maybe 3 times, and then go to longer intervals (like 30 minutes or 1 hour) for a few times. Then, leave it over night, and make sure it doesn't freeze. You just want to verify that it doesn't freeze, so you don't need to check it every 10 minutes.

I'm concerned in your last trial that the pure water froze at -5 C. This doesn't seem right. You know it should freeze at 0 C. So, maybe check the thermometer again (or use a new one) and make sure all your containers are very clean and there is no salt stuck to the container.

Does this make sense to you? Good luck.


Louise