7~ Chemical Changes

A chemical change is different than a physical change because in a chemical change

• Chemicals are used
• Molecules do not physically touch
• A new substance is formed and in a physical change no new substance is formed
• The change can be seen but in a physical change it cannot

In a chemical reaction

• The atoms of the reactants always stay together to form the products
• The atoms of the reactants unbond, rearrange, and then rebond to form the products
• Completely new atoms are formed which combine to make the products
• Some atoms disappear while others multiply to form the products

In a chemical reaction, mass is conserved. This means that

• The mass of the reactants stays the same during a chemical reaction
• The mass of the products stays the same during a chemical reaction
• The type and number of atoms in the reactants equals the type and number of atoms in the products
• The mass of the products is always twice the mass of the reactants

If the reactants on the left side of a chemical equation are C3H8 + 5O2, the products in a balanced equation could be

• 4CO2 + 3H2O
• 3CO2 + 4H2O
• 2CO2 + 3H2O
• 3CO + 4H2O

One clue of a chemical change is the formation of a precipitate. A precipitate is formed when

• Two liquids react and a gas is produced
• A solid dissolves in a liquid
• One liquid dissolves in another
• Two liquids react and a solid is formed

The PH Scale

• tells you the acidity or basicity of a liquid
• tells you how much of a chemical reaction occurred
• describes the atoms of a physical change

Which of the following about the PH Scale is true.

• Acids are from 0-7, Bases are from 7-14, and 0 is neutral
• Bases are from 0-7, Acids are from 7-14, and 7 is neutral
• Acids are from 0-7, Bases are from 7-14, and 7 is neutral

True or false? Dissolving salt is a chemical reaction.

• True
• False

How many carbon atoms are in this molecule? 
3Na + 2HO₃

• 0
• 1
• 2

How many oxygen atoms are in this molecule? 
3Na + 2HO₃

• 2
• 3
• 5
• 6