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

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?

• I'm fairly certain that's actually physics acting there. You may wish to ask it in the physics StackExchange. – Tanith Rosenbaum Jun 20 '13 at 15:19
• I see no relation to physics here :| – Peeyush Kushwaha Jun 20 '13 at 15:43
• @PeeyushKushwaha I'd at least hope that while studying chemistry, you try to see the bigger picture as it relates to physics... – jeremy Jun 20 '13 at 22:51
• I think "reduction to physics" is unnecessary for this problem. – Eric Brown Jun 21 '13 at 11:50

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.

@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