Question

Investigations were carried out in a science lab to explore the topic of chemical and physical changes. Investigation A Step 1. Add 5 tsp. salt to 100 ml warm water and stir until most or all of the salt is no longer visible. Step 2. Heat the salt solution on a burner until only a white solid remains. Investigation B Step 1. Mix 10 tsp. white sugar into 100 ml water and stir until most or all of the sugar is no longer visible. Step 2. Heat the sugar solution on a burner until the solution thickens and turns brown. In which step(s) did a chemical change most likely occur?

Answer 1

Answer : B) In step 2, there was a chemical change which was observed in sugar.

Explanation : In A step 2 there was a physical change that was seen. By just boiling the dissolved salt solution salt was obtained. Therefore, it is a physical change. In B step 2 there was a chemical change seen as sugar solution was thickened and turned brown. It was not obtained in its original form; there was a chemical reaction that took place during sugar evaporation. As chemical change is the one where the reaction is irreversible.

Therefore only in B step 2 there was a chemical change that was observed.

Answer 2

Answer is: B step 2,  Heat the sugar solution on a burner until the solution thickens and turns brown.

This is chemical change (chemical reaction), because new substance (caramel) is formed, the atoms are rearranged and the reaction is followed by an energy change.

In other examples physical change is occured.

The same substance is present before and after the psysical change, just with different form or state of matter. Making mixture is physical change (physical change is reversible).