Sort the following situations into chemical change or physical change:
1. salt dissolves in water
2. hydrochloric acid reacts with magnesium to produce hydrogen gas
3. a piece of copper is cut in half
4. a sugar cube is ground up
5. water is heated and changed to steam
6. iron rusts
7. ethyl alcohol evaporates
8. ice melts
9. milk sours
10. sugar dissolves in water
11. sodium and potassium react violently with water
12. pancakes cook on a griddle
13. grass grows on a lawn
14. a tire is inflated with air
15. food is digested in the stomach
16. water is absorbed by a paper towel
17. ethyl alcohol boils at 79 egress celsius
18. paper burns
19. water freezes at 0 degrees celsius
20. fireworks explode
21. alka-seltzer gives off carbon dioxide when added to water
22. clouds form in the sky

explanation:

the largest database of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. the number of potential organic compounds has been estimated at 1060—an astronomically high number. the existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities. the simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. in addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. we use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. the familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. we can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. this leads to differences in geometries and in the hybridization of the carbon orbitals.