How do you explain the thread sticking to an ice-cube when salt is sprinkled over it?

Question

So this experiment goes like this :

1. You take an ice-cube
2. You put a thread on top face of it
3. You sprinkle some common salt on the ice-cube wherever its touching the thread
4. You wait for sometime (approx 1 min)
5. Voila! now you can lift the thread and ice-cube will stick to the end of the thread so you can lift the whole thing altogether

How do you explain this in chemistry?

Answer 1

I conjecture that there are several things happening.

• Step 3: Addition of salt will melt the ice at its immediate point of contact. The string will naturally be surrounded by the highly concentrated brine (salt water).

• Step 4: This is enough time for the brine to get diluted, noting that there is an infinite reservoir of potential water by way of the ice cube.

• Step 5: Once the salt gets sufficiently diluted, then the comparatively large ice cube provides the necessary cold for the (now) very dilute salt water to reform as ice.

Recalling that in Step 3 the salt water surrounded the string, then ice naturally forms around the string in Step 5, hence you can pick up the ice cube with the string.

Answer 2

@Eric Brown has the result right qualitatively. We can add some more quantitative and chemical information.

Step 3: When salt is added, the freezing point of the water is decreased at that point, so the ice melts to water. This is a colligative property; i.e. has only to do with amount of substance, and not the particular substance.

Step 4: As the melting water dilutes, its phase shifts back towards pure water, which has a higher freezing point.

Step 5: At some point, the phase shifts back to freezing at 0 °C, and the water re-freezes given the ambient ice at 0 °C.

This Wikipedia article shows you the phase diagram corresponding to my description:

NaCl phase diagram

As I said, my answer merely augments your already selected correct answer, but I thought this might be useful additional information.