Science 8th grade
UNIT 9: Material changes 9.1 Physical and chemical changes
Science 8th grade
UNIT 9: Material changes 9.1 Physical and chemical changes
Physical changes
When liquid water freezes it becomes a solid. When liquid water evaporates it forms a gas. These are changes of state. They are physical changes.
In a physical change, no new substances are formed.
The solid water can be changed back to a liquid by heating it. The gaseous water can be changed back to a liquid by cooling it. All of the time it is still water. It looks different, but it is still the same substance.
Chemical changes
In a chemical change, new substances are formed.
For example, when iron is heated with sulfur, a new substance-iron sulfide - is formed. You learnt about this on pages 92-93.
The iron and sulfur have reacted together to form a new substance. A chemical reaction has taken place. The iron atoms have bonded with the sulfur atoms.
We can show what happens in a reaction using a word equation:
iron + sulfur → iron sulfide
In the equation, the arrow represents the chemical reaction. The reactants are the substances that react together. The products are the new substances that are made in the reaction.
reactants → products
Questions
1) Imagine that you drop a glass beaker and it breaks.
a: Has a new substance been formed?
b: Is this a physical change, or a chemical change?
2) In the reaction between iron and sulfur, what are the reactants and the products?
Another chemical reaction
In the reaction between iron and sulfur, two reactants join together to form a product. We say that the iron and sulfur combine to form iron sulfide.
In some chemical reactions, a substance breaks apart to produce two or more new substances. On page 83, you saw how electricity can be used to make water split into hydrogen and oxygen.
Chemical reactions in living organisms
Chemical reactions happen everywhere. They happen in the garden inside plants when they grow and when leaves decay. They happen inside your body to keep you alive - for example, reactions to digest food (sce page 22).
Questions
3) Inside your digestive system, protein molecules are broken down to amino acid molecules. You can read about digestion on page 28.)
a: Has a new substance been formed?
b: Has a chemical reaction taken place?
4) Respiration is a chemical reaction. You can see the word equation for respiration on page 46.
a: What are the reactants in the respiration reaction?
b: What are the products in the respiration reaction?
5) Can you name a chemical reaction that happens in plants, but not in animals?