Changes Around Us: Physical and Chemical

A candle's solid wax melts into liquid and then cools back into solid. Deepika says melting is a chemical change, because the wax's form changed completely. Her brother says it is only a physical change. Who is right?

Adding a few drops of food colouring to a glass of water turns the water pink. Kabir says this is a chemical change, because the colour changed. His sister says it is a physical change. Who is right?

An iron rod is cut with a saw, and an identical rod is left out in the rain until it rusts. Karan says both changes are the same kind, since in both cases the iron's shape or colour changed. Is Karan right?

Salt dissolves in water and disappears completely. Tanya says this is a chemical change, because the salt no longer looks like salt at all. Her friend says it is still a physical change. Who is right?

Milk turns into curd, and curd does not turn back into milk. Gaurav says this must mean curd-forming cannot be a physical change — physical changes can always be reversed. Is Gaurav's reasoning right?

Tanya says fast changes are chemical and slow ones are physical — so iron rusting (slow) is physical, and wood burning (fast) is chemical. Her friend says speed does not decide this. Who is right?