Main->Readings->5th Grade Readings->Chemistry->Part 6: Reactions
Vocabulary |
People always talk about "solid matter," but matter doesn't have to be solid.
| Air is matter. It is a mixture of nitrogen, oxygen, carbon dioxide, and other gases. It has mass, which is why it doesn't rush away from the Earth, and it takes up space, or you wouldn't be able to blow up a balloon. But because people can't see it and can't feel it, sometimes it is hard for them to believe air is matter. Think of a fish. A fish lives in water, and probably doesn't even notice the water is there. A fish would probably find it hard to believe that water is matter! |
It can be liquid like motor oil or water, it can be a gas like oxygen or nitrogen, and it can even be a plasma like the hydrogen and helium in our Sun. All of these things take up space and have mass, and all of them are made of atoms, so they are matter.
We cannot always believe our eyes, either. The best example of this is water and the way it behaves. You can freeze water to make ice, and you can boil water to make steam. Even though ice, water, and steam are all the same thing, it is tough for people to believe it because they can see how different they are.
Yet ice, water, and steam are all just phases, or states, of the same kind of matter, H2O. What has changed when you freeze, melt, or boil water is the way its molecules behave. The pictures below show what this means.
In a solid the molecules are close together and vibrate in place.
They do not have much energy because they are cool.
In a liquid, the molecules move more and slide around each other.
They have more energy because they have been heated.
In a gas, the molecules move a lot and bounce
all over the place
They have been heated so much they are very lively.
Any kind of matter can become a solid, a liquid, a gas, or a plasma. All you have to do is get it cold enough or hot enough. This is not always easy. For instance, nitrogen freezes at -210.1 °C, so you're not likely to see solid nitrogen any time soon. Iron melts at 1538 °C and copper at 1084.62 °C, so you need a very hot oven to make liquid metal.
When matter changes state like this, we call it a physical change. A physical change happens when matter changes but its molecules stay the same.
| The word physical comes from the Latin word physica, meaning nature. The word chemical comes from the old word alchemical, which came from the Arabic word alkimiya, meaning chemistry. |
Other easier examples of physical changes are breaking things or grinding them up. Also, when you dissolve sugar or salt in water, that is a physical change. You can tell the molecules are still the same because you can taste the sugar or salt in the water, and if you boil away the water into the air, the sugar or salt is left behind. Can you still see the sugar in the water? No, of course not. Molecules are too small to see, and in sugar water the particles of sugar are individual molecules!
What happens when molecules change? How can you tell the molecules are changing when they are too small to see? Chemists use logic and some very important clues to figure out when this has happened.
When chemists are trying to see if new molecules have been formed, they look for changes in the properties of the matter they are studying.
A change in matter where new kinds of molecules are formed is called a chemical change.
You can divide water (using electricity) into two substances: hydrogen and oxygen. Water is a liquid which puts out fire. Hydrogen and oxygen are gases which can burn. The properties have changed, so this is a chemical change.
Many chemical changes happen when you bring together two or more different substances. This kind of change, where two or more substances form new substances, is also called a chemical reaction.
When iron rusts, oxygen in the air is combining with the iron. A shiny, bendy gray metal turns into a reddish brown powdery substance which is crumbly. A new substance has formed and a chemical reaction has happened.
Another example of a reaction is fire, or combustion. Paper is tough, crisp, and solid. Light it. What is left behind when the flame goes out? Some crumbly gray-black ashes and some gas in the air which will not burn. The chemical reaction between the paper and the oxygen in the air produced two new substances with different properties.
There are five easy ways you can tell if a chemical reaction (otherwise known as a chemical change) is happening:
| When matter changes state, it can produce a gas or a solid from a liquid too, yet changing state is a physical change. What makes it different from a chemical change? In a change of state, the new state can easily be changed back into the old state with just a change of temperature. This does not happen with a chemical change. |
In this unit, you have learned some new things about what is right in front of you: matter. You have learned how to classify it, what is made from, how to describe it, and some of the things that you can do with it. You have even learned some of the chemist's code! Many people decide to become chemists because of the wonderful reactions they can cause, complete with color changes, glowing lights, explosions, and loud sounds. However, there are even better reasons. In the modern world, chemists are the people who make medicines, computers, car engines, artist's paints, and many more of the products we all use. Today's chemists, with the scientific method, have learned to do things about which the medieval alchemists could only dream.
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This page last modified August 15, 2002
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