Which of the following reactions is spontaneous?

water freezing on a hot summer day

mixture being separated

lumps os sugar dissolving in water

sodium chloride forming sodium metal and chlorine gas

Answer is: lumps os sugar dissolving in water.

Sugar has very good solubility in water and it dissolves readily, which is example of a physical change.

Gibbs free energy (G) determines if reaction will proceed spontaneously, if ΔG is negative, reaction is spontaneous (ΔG = ΔH - T·ΔS).

In other examples, reactions are spontaneous in reverse way, for example spontaneous is forming sodium chlorine from sodium metal and chlorine gas, but not sodium chloride forming sodium metal and chlorine gas, because a lot of energy is needed for that reaction.