Chemical changes produce new substances. For this reason, they often cannot be undone. Some chemical changes can be reversed, sort of. To do so requires another chemical change to take place. For example, you can undo the tarnish on copper pennies by placing them in vinegar. The acid in the vinegar reacts with the tarnish. This is a chemical change that makes the pennies bright and shiny again. You can try this yourself at home to see how well it works. Other chemical changes cannot be reversed at all or may be difficult to do. Rusting is a chemical change. If metal rusts, the best you can do is to sand off the rust to get down to the shiny metal. Although the metal may now be shiny, the rust was removed. The rust was not changed back into the original metal. Some chemical reactions occur in only one direction. These reactions are called irreversible reactions. For example, you cannot change a fried egg back into a raw egg. Can you think of some other irreversible reactions related to cooking? Would you like a piece of cake?

What can you do to make pennies bright and shiny again?

The documentary portrays a behind-the-scenes look at the poor state of Argentina's civil aviation , and puts the blame on the Argentine Air Force . The Air Force has been in control of air traffic operations since the military takeover of General Juan Carlos Ongan a in 1966 . The producer/director , Enrique Pi eyro , claims Argentina and Nigeria are the only countries whose air force controls and regulates the airline industry . Pi eyro , a former Argentine airline pilot , makes his case in the documentary using diagrams , 3D animations , interviews , hidden cameras in the control tower , and a few props . For example , at one point he spills out a bag of plastic airplanes and equates it to the number of planes the air force has lost due to negligence . The film is heavily based on the Austral L neas A reas Flight 2553 plane crash . Enrique Pi eyro takes his camera , secretly , into the control tower of the Ministro Pistarini International Airport also-known-as Ezeiza , the international airport at Buenos Aires .

What is one type of prop the director uses to prove a point?

Is this an insect or an animal? A snail is an animal just like you and me. Thats right, you too are an animal. No, you don't look like a snail. You do have some things in common. Animals can be divided into many groups. These groups are decided based on their characteristics. All animals have some basic features in common. That does not mean they are the same. They also have many differences. For example, snails are mollusks and not insects. Mollusks have a unique set of features. Notice the large foot that allows it to move. Yes, it only has one foot. Did you notice the long antennas on its head? This is where the snail's eyes are. They are on the end of the antenna. They are not on its head like most animals. The foot and eyes are unique features. Scientists use these features to place animals into groups.
Where are the snails eyes?