How does temperature affect the growth of sugar crystals?
In rather complicated ways. To keep it simple, I'll assume you are starting with hot water solution saturated with sugar. As soon as the temperature of this solution decreases, even a tiny amount, there will a "super saturated" condition, i.e., there will be a driving force for the precipitation of sugar crystals. If the temperature is reduced further, there will be a larger driving force (a greater degree of supersaturation). This is an important factor based on the thermodynamics. Another important factor is the kinetics, specifically, the nucleation. If the container is very clean and smooth and the water is pure with respect to everything other than sugar and there are no dust particles settling onto the surface of the solution, there will be no easy sites for nucleation to start. This will allow the temperature to decrease further without any sugar crystals forming, much less growing. Once nucleation happens, then the process is all about the crystal growth mode (what you specifically asked about). During growth, the transformation from sugar in solution to solid sugar involves heat. The lower the temperature, the greater the driving force so the faster sugar will grow. However, if you were able to suddenly cool the solution to a very low temperature, you may get back to a kinetic limitation, where the solution is so viscous (does not flow easily) that the rate controlling step will be the motion of the sugar molecules in the solution. If this were to happen, the driving force would be greated due to the lower temperature but the actual growth rate would be slower because of the kinetics. Another factor worth mentioning. If in the process of making the sugar water, there was some undissolved sugar remaining, then those sugar crystals provide the perfect nucleation sites for the precipitation of new sugar crystals and there will be no kinetic "barrier" for the beginning of sugar crystal growth.
Can virus and bacteria survive in salt water?
The keyword is “halophile”. Like other types of extremophiles, most of these are in the domain Archaea, which are kind of like bacteria, but kind of different.The issue with salt water is that, a high concentration of sodium chloride in the surrounding water environment, might suck out the water that is inside the cell. That is called “osmosis”.A way to avoid this is to have a compensating solute inside the cell, which does not have to be sodium, and can be a range of other things. Those substances keep the water inside the cell from being sucked out. It takes energy and certain genetic information to do this.It isn’t exactly a matter of just “surviving”. They are adapted to their environment, and actually need it.If you took them out of their normal high-salt water environment, and put them into distilled water (theoretically zero salt) they would die. A similar issue exists for hyperthermophiles, which would freeze to death at room temperature. These are some reasons why culturing and studying extremophiles is a major hassle.As for viruses, they don’t “survive” anything, because they aren’t technically alive in the first place.A virus may have a protein coat (capsid), which might be disrupted by a high-salt environment. However, as with bacteria, some species may be quite happy with it.Some other viruses are “enveloped”, and are coated with part of the membrane of the host cell that they had infected and escaped from. Those are a lot more fragile, and may be deactivated just by sitting around exposed to the air for awhile (e.g. that cold/flu virus that sat on the elevator button for a couple of days). So I would expect those to be vulnerable to every threat, including high salt.Fun Fact #1: “Salt” doesn’t just mean sodium chloride table salt. It really relates to ionic compounds, like a metal and a non-metal.Fun Fact #2: The most abundant biological unit on Earth is bacteriophages. They are viruses that infect bacteria, and the oceans (salt-water) are full of them. I only really know a lot about one (1, n=1, a single, solitary one) virus, and it is one of these, and apparently quite durable in many environments.
How would you set up a simple experiment to test whether salt-crystal growth is affected by tempeture???
The first guy got it. But I would add that in preparing the saltwater solutions, make sure you have the same amounts of water and salt in both containers. :)