How do impurities in water affect the speed of melting of ice?�

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Name: Natalie Liu

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School: Rancho San Joaquin Middle School

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Teacher: Ms. Morris

Make Ice Melt Slower — Make Cold Beverages Last Longer

Abstract

This project experimented how different aspects affect the melting speed of ice. The two tested elements were the filtered/unfiltered tap water, and the added table salt in the water.

I have four samples, two glasses of unfiltered（tap) water, and two glasses of filtered water. For each glass of these two categories, 3g of salt was added. Therefore we can compare the melting speed of four glasses with different scenarios.

Through the experiment, I found that: first, unfiltered water melted faster than filtered water; second, the same type of water, no matter filtered or unfiltered, melt fast when extra salt was added to. Therefore, my hypothesis has been verified —— impurities in the water would make ice melt faster.

Background Research

• How ice is made – as water molecules cool, they slow down, and once they reach 0°C, or 32°F, they form solid crystals.
• Freezing point is the temperature at which the liquid becomes solid.
• Increasing pressure can raise the freezing point.
• Impurities in liquid can lower the freezing point, like some lakes do not freeze even if they are under 0°C.
• Melting point is the temperature at which the solid becomes liquid.
• The freezing point and the melting point may not be the same.
• Elements that can make ice melt faster are salt, sugar, and baking soda (also a type of salt, called sodium bicarbonate). They do not actually melt the ice like temperature or wind, but they disrupt the hydrogen molecule bondings which cause ice to melt faster.
• The factors that cause ice to melt are ambient temperature and the surface area of the ice.
• Clear ice melt slower
• First of all, if we want to make clear ice, we need to purify the water. The elements that make ice not clear are minerals like calcium, magnesium, and potassium, etc. When ice naturally forms, ice crystals push the minerals outside the ice, so the ice is clear, but when we make ice in molds, the minerals and air are pushed into the center of the ice, resulting in a “cloudy” look.
• Also, the direction the water freezes also determines if the water is clear. If the water freezes from all directions, the minerals will be pushed into the center causing it “cloudy”.
• Melting of an impure solid into an impure liquid has a larger change in entropy, which lowers the melting point.
• Climate change (especially global warming) had a significant and negative impact to the north pole and south pole, causing tons of ice to melt and a lot of animals like polar bears lost their homes.

Hypothesis

• I think that impurities in the water would make ice melt faster because melting of an impure solid causes a larger change in entropy, therefore lower the melting point.

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Materials

• 4 clean, transparent glasses
• 400ml of tap water (100ml for each glasses; room temperature)
• 6g table salt (3g for each of the two glasses that salt will be added)
• Brita water filter
• A measuring cup
• A digital scale
• A freezer
• A pair of gloves
• A tray
• A timer
• 4 stickers
• A marker
• 2 stirring sticks
• Plastic wrap

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Procedure

1. Prepare four clean, dry, transparent, identical glasses with stickers, mark each of them by Sample #1, #2, #3 and #4
2. Use a measuring cup to measure 100 mL of tap water(unfiltered), then pour it into Glass #1, and do the same to Glass #2
3. Get 100 mL of water filtered by the Brita Water Filter, pour it into Glass #3, and do the same to Glass #4
4. Put Glass #2 on a digital scale, click on to clear the weight of the glass and water. Add 3g of table salt to Glass #2, and do the same to Glass #4
5. Stir till the salt in Sample #2 and Sample #4 is fully dissolved
6. Put all four glasses onto a tray, then put the tray into the freezer
7. Wait for a night to make sure all samples are fully frozen
8. Put on gloves to ensure the temperature of my hand would not impact the time of ice melting.
9. Take the tray out from freezer onto a flat surface to let ice melt. Start a timer immediately.
10. Observe the melting process and record the time for each sample to fully melt

Results - Data/Observations

• After 30 minutes, Sample #1 and Sample #3 had significant changes, with an ice sphere in each glass surrounded by water. In contrast, there was still condensation on Sample #2 (tap water with salt), and Sample #4 (filtered water with salt).
• After 1 hour and a half, the condensation on Sample #2 had fully dissolved into water, while there was still a little condensation on Sample #4. After 2 hours, the condensation on Sample #4 had fully dissolved into water.
• At 2 hours and 20 minutes, it was obvious that Sample #2 and Sample #4 had less ice than Sample #1 and Sample #3.
• At the time between 3 hours 40 minutes till Sample #2 fully melted, there was only a thin layer floating on the surface of the water, and it melted super slow.
• Sample #4 was fully melten first at 3h15’59’’, and sample #2 melted secondly at 4h05’58”, then Sample #1 melted at 4h08’32”. Sample #3 is the last one melted at 4h17’02”.

Discussion

• For the pair without salt added, Sample #1 (unfiltered, no salt added) melted faster than Sample #3 (filtered, no salt added), which has verified my hypothesis. The reason is that the unfiltered water has higher impurities than the filtered water. This also prove that water filter is effective to take out some impurities from tap water.
• For the unfiltered pair, Sample #2 (unfiltered, salt added) melted faster than Sample #1 (unfiltered, no salt added). For the filtered pair, Sample #4 (filtered, salt added) melted much faster than Sample #3 (filtered, no salt added). Both has verified my hypothesis. Added salt increases impurities in the water, which speed up the melting speed.
• Impurities dissolve in the thin layer of liquid water on the ice surface, lowering its freezing point. This causes the ice to melt at a temperature below 0°C (32°F), making it melt faster at typical environmental temperatures.

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• Surprisingly, Sample #4 (filtered, salt added) melt faster than Sample #2 (unfiltered, salt added) which is different from my hypothesis. Supposedly, Sample #4 should be slower than Sample #2 because Sample #2 should have higher impurity without filtering. After carefully reviewing the whole experiment process, I find the reason. In my experiment design, both Sample #2 and #4 should be added 3g of salt and I did so. However, due to my digital scale is not that precise, the actual amount of salt added could be different even though both showed 3g on the scale. For example, if I have added 2.6g in #2 and 3.4g in #4, though both showed 3g in the scale, the latter is 30% more than the former. (3.4-2.6)/2.6=30%
• For Sample #1 and Sample #3, the melting direction was from outer area to inner area, so the ice became spheres. For Sample #2 and Sample #4, the melting direction is vertical, and the ice looks like cylinders, and the surface is rough.

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Conclusion

• The answer to my question is the impurities does make the melting speed faster, and added salt would speed up the melting speed. My hypothesis has been verified.
• Primarily, the impurities in the water would have a larger change in entropy when the ice is melting, and a larger change in entropy would accelerate the melting speed. In addition, the reason why salt would speed up the melting pace is because “it disrupts hydrogen bonds between water molecules, this is due to the ions that make up salt that are attracted to the oppositely charged oxygen and hydrogen atoms.”

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Reflection/Application

• This project helped me learn the scientific methods, including hypothesis generation, experiment design, background research, experiment observation and recording, data analysis and conclusion drawing, which will be useful in my future study.
• My background research gave me knowledge of how each element in the water would affect the speed of melting. In this project, I learned salt would make ice melt faster, impurities in water would accelerate ice’s melting speed. Long-lasting ice can be applied to many aspects in everyday life: they can make cold beverages last longer; they can help isolate temperature better and longer. More importantly, ice plays an vital role in the environment because it lowers the temperature of air and impacts the entire Earth. However, global warming causes the ice to melt much faster, and we lost an incredible amount of ice during these years. Helping ice melt slower may would partially solve this problem and help us better protect the earth.
• If I have another chance to do this project, I would definitely start earlier, so that I would have plenty of time to work on it. Also, I should take into consider of the accuracy of the experiment measures to avoid problem in this project. I would use more advanced filter to purify the water and use more accurate scale to weigh the salt to ensure the accuracy of my experiment.
• For my next steps of researching this project, I would research and test other elements which can impact the melting speed of ice.

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References Cited

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• Vasyl Kolodii (was not written). How to make ice last longer, and science behind freezing and melting.

https://bftech.pro/how-to-make-ice-last-longer/#:~:text=However%2C%20incorporating%20materials%20such%20as,when%20it's%20used%20in%20beverages.

• Block Ice (2024, February 8). What causes ice to look clear and cloudy, how to make clear ice that last long

(https://blockice.com/blog/whats-the-science-behind-clear-ice/)

• Britannica, T. Editors of Encyclopaedia (2023, August 23). freezing point. Encyclopedia Britannica. https://www.britannica.com/science/freezing-point
• Emily Dilandro (2023, November 21) Why does salt make ice melt faster

https://study.com/learn/lesson/why-does-salt-melt-ice.html#:~:text=Salt%20makes%20ice%20melt%20faster,charged%20oxygen%20and%20hydrogen%20atoms.

• Lisa Nichols, Butte College (was not written) the melting point theory; why do impurities make ice melt faster

https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_Lab_Techniques_(Nichols)/06%3A_Miscellaneous_Techniques/6.01%3A_Melting_Point/6.1C%3A__Melting_Point_Theory#:~:text=Melting%20of%20an%20impure%20solid%20into%20an%20impure%20liquid%20therefore,rationalized%20either%20mathematically%20or%20conceptually.

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