4. Discussion

4. Discussion

4.1 Key Findings, Comparison and explanations

From the data collected, we found that the melting point of water decreased up to -21 degrees Celsius, where it started to increase to 20.78 degrees Celsius as shown in table 2. The melting points of the solutions with 27g and 30g respectively were rather similar. The freezing point continuously decreases to around -21 degrees Celsius when the salinity is around 290g/kg. After this point, the freezing point begins to increase as the salinity increases. We can hence infer that saltwater has a relatively lower freezing point than freshwater, which freezes at 0 degrees Celsius. And for saltwater, its freezing point decreases by 0.28 degrees Celsius for every 5 ppt increase in salinity (NSIDC, 2017).

However, in the first reading, as shown in Table 2, the reading of the melting point of the solution with the 27g of salt was anomalous. The general trend was a decrease until -21 degrees Celsius, but in the first reading, the recorded melting point for the solution with 17g of salt was -17 degrees Celsius. This meant that the graph started to increase after -19 degrees Celsius. There was also a slight error with the second reading of the melting point of the solution with 5g of salt. This was probably due to the temperature probe being frozen at the side of the solution in the metal cup, thus it was not in contact with the frozen solution at all times during the process of melting. Hence, the results were inaccurate and anomalous. As a result, in our error bar graph, as shown in fig 4, the data graph is shown to have more straight lines than the hypothetical graph due to the errors in the readings.

Salt molecules block water molecules from packing together when the temperature is lowered. It then prevents them from becoming ice. Hence, the temperature of the water molecules has to be lowered further until it is enough for the water molecules to pack together and form ice (Da Vinci Science Center, 2022). But, as shown in our findings, the freezing point of the salt solution eventually stabilizes at -21 degrees Celsius before increasing again. The reason for this finding is that no more salt can be dissolved into the solution. When the solution has reached that freezing point, the solution will have reached the saturation point (Britannica, 2022).

4.2 Limitations and Recommendations

In this experiment, there were 4 main limitations that we dealt with.

The first limitation was that we used metal cups in our experiment, which is not ideal in our experiment. As metal is a good conductor of heat, it was not an ideal material to use for the container. This is because the metal cup will gain more heat from the surroundings, and since it has a higher thermal conductivity, the heat flows into the inner part of the cup faster. Hence, this will affect the results as certain cups with the frozen solution may have melted earlier than the rest. The recommendation for this limitation is to put a layer of styrofoam around the cup. As styrofoam traps air in small pockets in the material, it blocks the flow of heat energy. Hence, by putting a layer of the material around the metal cup, the heat in the surrounding area will not interfere with the recording of data of the melting point of the solution.

The second limitation was that the placement of the temperature probes was not consistent. Specifically, the probes were not in the middle of the frozen solution in the cup. The probes were frozen slanted or at the edge of the frozen solution. To get an accurate melting point, we were supposed to record the temperature until the point in which the middle of the frozen solution has melted completely. But, since the temperature probes were not consistently in the middle of the solution, the data we recorded was not as accurate as it could have been. The recommendation for this limitation would be to place a sturdy lid on top of the metal cup to hold the temperature probes in place in the middle. We could do this by cutting a hole in the lid, which will be made of styrofoam or plastic. The temperature probe could then be placed in the hole to hold it in place. This will allow the temperature probes to be consistent in the middle of the solution and hence, allow a more consistent and accurate result.

The third limitation of our experiment is that the water level in the cup was not filled to the height in which it the temperature probes were completely submerged in the solution. As we do not know the placement of the temperature sensor in the temperature probe, not completely submerging the probes in the solution during the freezing process might have affected the results. The recommendation for this limitation would be to fill the metal cup with more water until the temperature probes are completely submerged in the solution. By doing so, we can reduce the number of errors in our results.

The final limitation is that we did not do the setup in which the melting point goes back to 0 degrees celsius. This would affect our explanation of whether we verified our hypothesis. In the hypothesis, the melting point increased back to 0 degrees celsius after the salt solution reached the saturation point. However, in our results, the melting point only reached the saturation point and did not increase. Hence, our results did not completely verify our hypothesis. The recommendation for this limitation would be to do an experimental setup for another salt solution with a salt concentration of 72g of salt. This would allow us to get the melting point of 0 degrees celsius which would show the increase in melting point.

In conclusion, by ensuring that there are no careless mistakes that may affect the results in the experiment, we will be able to get more accurate from our experiment.

4.3 Evaluation of Hypothesis

From the results obtained above, we can generally conclude that the hypothesis that “the freezing point of water will decrease as the salt concentration increases.“ is proven partially correct. Specifically, we hypothesize that the freezing point of

the water does decrease as the salt concentration increases, but only until a certain point. Where the freezing point will start increasing back to 0 degrees Celsius.