Why does a thermometer need to be submerged in a ice water slush instead of only ice?

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation:

Why is the melting of ice a physical change ?

Given the following at 25c calculate delta hf for hcn (g) at 25c. 2nh3 (g) +3o2 (g) + 2ch4 (g) > 2hcn (g) + 6h2o (g) delta h rxn= -870.8 kj. delta hf=-80.3 kj/mol for nh3 (g), -74.6 kj/mol for ch4, and -241.8 kj/mol for h2o (g)