Change

Chemical or Physical

Reason

Steel wool turned the blackened pot silver

 Physical

 Steel wool removed burnt food from pot.No new substances formed in the process

Lightning flashed across the sky

 Physical

 This one is a bit ambiguous. Even though energy is released no new substances are formed. For more details: http://science.howstuffworks.com/lightning.htm

Acid caused the limestone to fizz

 Chemical

 The fizzing is carbon dioxide gas that forms when the acid reacts with the calcium carbonate that makes up the limestone

The  wooden steps rotted

 Chemical

 Microorganisms, insects and worms are digesting the wood breaking cellulose into simpler compounds

Lemon juice turned the tea cloudy

 Chemical

 The acid in the lemon juice reacts with compounds called tannins in tea to produce an insoluble solid (precipitate) that makes the teas cloudy.

See:http://blog.planetpreterist.com/index.php?itemid=818&catid=62

A puddle of gasoline dried, but left a bad odor

 Physical

 State changes like evaporation are physical changes

Gravy in the refrigerator gelled

Chemical

The strands of protein reacted with one another forming cross-linking that holds them together to make a solid. If you heat the gravy, the bonds break and the gravy become a liquid

See:

http://www.sciam.com/article.cfm?id=what-is-jell-o-how-does-i

A blue-green lobster turned red as it cooked

 Chemical

 The pigments in the shell that make the lobster blue-green (or brown) are chemically changed during cooking. The result in the change in chemical structure resulted in a color change to red.

See:

http://www.guardian.co.uk/uk/2002/jul/30/highereducation.science

A cavity formed in a tooth

 Chemical

The rotted part of the tooth is a new substance. Acid produced by bacteria digesting sugar results in a chemical reaction between acid and tooth enamel

Flash cameras flashed after the wedding

 Physical

 This is similar to the discharge of lightning. See:

http://electronics.howstuffworks.com/camera-flash1.htm

Ice cubes shrink in the freezer

 Physical

 This is an example of sublimation. Such changes in state are physical changes

A raw egg became a cooked egg

 Chemical

 Cooking denatures, that is changes structure of the protein in the egg. The denatured protein is a new substances

A piece of paper became ash

 Chemical

 Burning results in new substances being formed such as carbon, dioxide, water and ash

Mothballs gradually disappeared

 Physical

 This is an example of sublimation. Such changes in state are physical changes

The steel railing rusted

 Chemical

 Iron in the steel reacted with oxygen in the air to produce a new substance, an iron oxide

A silver spoon tarnished

Chemical

The silver reacted with sulfur in the air to produce a new substance, silver sulfide

The copper pot turned green

 Chemical

 The copper reacted with oxygen in the air to produce the green substance called verdigris (just like the Statue of Liberty!)

A rubber band is stretched

 Physical

 A change in shape of the rubber band does not result in the formation of new substances

The baseball smashed the window

 Physical

 The broken glass is chemically the same as the unbroken glass. No new substance has formed

The cake batter  became cake after being baked

 Chemical

Baking the batter changes it chemically. The cake is a new substance has formed.