Question 143 out of 3 pointsIt is quite common for students to incorrectly use the terms "melt" and "dissolve" interchangeably. Select the appropriate term for the following statement:If salt crystals are placed on your tongue, they disappear because they have _____________.Selected Answer:dissolvedResponse Feedback:When a solvent is present, the solid is dissolving in that solvent to form a solution. In this case the solvent is water on your tongue. The MELTING point of NaCl (the point at which it turns into a liquid) is 801 °C!Question 156 out of 6 points
How should you properly dispose of used melting point tubes?Selected Answer:place them in the glass waste containerAnswer Feedback:Good idea! All glass waste, not just broken glassware, should beput in a special waste container so it will be appropriately handled with necessary precautions.Response Feedback:You should leave all instrumentation clean and ready for the next user.Never re-use a melting point tube! Once a sample has been melted, it is no longer a tightly packed sample of crystals so you cannot take an accurate mp reading. Melting point tubes are single-use and disposable.Remember, only non-hazardous, neutralized, aqueous waste can be put down the sink (with your instructor's approval).Question 166 out of 6 pointsCertain compounds can incorporate water molecules into their crystal structure to form HYDRATES. Which of the following might describe a possible melting point behavior of a hydrate?Selected Answer:the release of a gas at 100 °C, followed by a sharp melting pointat a higher temperature.Answer Feedback:This behavior would indicate that the water has evaporated (gasat 100 °C might be steam) and the pure compound, which is no longer a hydrate, continues to give a typical melting point.Response Feedback:1. Hydrates ARE pure compounds; the water molecules are incorporated in fixed ratios.3. Hydrates look like any other ordinary crystals. The water molecules are locked into the crystals structure so there is no reason the sample would look "wet".4. There's no way we can make any broad assumption about melting points. That's like saying "all alcohols should melt at X °C".5. Hydrates are just like any other compound; they can have higher or lower melting points, depending on their structures. The only compounds that have melting points that are always "extremely high" are salts (ionic compounds).Question 176 out of 6 pointsYou look up the melting point of serine (an amino acid) in the CRC Handbook and it reads, "228d". What does that reported melting point mean?Selected Answer:"d" stands for decomposition
Answer Feedback:A "d" indicates that the sample decomposes (turns a darker color) at the given temperature. Some organic compounds are not stable and will break down before reaching their melting point.Response Feedback:All temperatures given in the CRC are in degrees Celcius (just a"228" would imply 228 °C). (Degrees Farenheit are not typicallyused in scientific literature.)STP (standard temperature and pressure) of the surroundings have no significant effect on the mp measurement.