A chemical change is defined as a reaction in which the products are chemically different from the reactants. If you know a chemical reaction happened, the change is a chemical change. A physical change is defined as a change in which the products are still the same chemical just under went a change in phase, volume, and so on.
1. is a chemical change. the would is being burned which is a chemical reaction since the wood is being changed to carbon dioxide and water in the presence of oxygen. (combustion reaction is happening)
2. is a physical change. The salt is breaking up into ions but is still there. if you evaporate the water, the salt appears again (you can think of dissolving as a change in state in a way).
3. chemical change. The water is being broken up into oxygen and hydrogen indicating a chemical reaction. (this reaction is called electrolysis).
4. Chemical change. The iron in the presence of oxygen is turning into iron oxide. (this is a oxidation-reduction reaction sometimes referred to as a redox reaction).
5. Chemical reaction. I am not sure how to explain it other than color changes really only occur as a result of a chemical reaction. (the actual reaction that is going on is a somewhat complex redox reaction).
6, physical change. when ice melts, it is solid water turning into liquid water which is a physical change (the water is not being changed into a different chemical). The fact that the water gets colder is not a chemical reaction. In order to melt the ice, the ice has to absorb energy and the only source of energy is the water around it so it will take energy from the water around it making the water get colder.
7. physical change. The butter's chemical identity is not changing, it is just undergoing a physical change.
8. chemical change. The burning of natural gas is a combustion reaction in which the natural gas in being changed into water and carbon dioxide in the presence of oxygen.
9. physical change. The sugar got dissolved in the water and as the water cooled, the solubility of sugar decreased causing it to precipitate out. The sugar was always sugar, it was just in two different physical states.
10. physical change. The water is under going a phase change. the bubbles indicate that it is boiling which means it is turning into a gas. The chemical identity of the water is not changing, it is just going from liquid to gas phase. (bubbles often indicate a chemical change as well, you have to read the question well to distinguish a chemical change from physical change. In this question we know it is a physical change since it says heat was added and the water we can say that the water is changing phase. If it says two chemicals were added and bubbles formed, we can say it is a chemical change since its implied that the two chemicals reacted to form gas).
11. physical change. The water on the cloth is being evaporated which is basically the same thing as boiling in the sense that water is going from the liquid to gas phase. (the water's chemical identity is not changing)
12. I think it is a chemical change because the UV light from sun light break down the bonds in the molecules which cause the color to fade (color change also indicates a chemical reaction).
13. This is a chemical change. Vinegar and baking soda react together to form gas along with other products you will eventually learn (if you have not yet so) that this type of reaction is an acid base reaction. (This is an instance in which the presence of bubbles indicates a chemical change).
14. physical change. As the water evaporated, the salt that was dissolved in it precipitated out. Neither the salt or the water changed its chemical identity since they both just changed their physical state.
15. Chemical change. The gas is undergoing a combustion reaction showing with the oxygen which means the gas is being changed into other chemicals.
I hope this helps. Let me know in the comments if anything is unclear.
