Chemical Reactions

Question 1

Why must chemical equations be balanced?

Answer 1

Atoms cannot be destroyed or created in a reaction.

Question 2

Can a chemical reaction be reversed?

Answer 2

No

Question 3

What are the evidences of a Chemical Reaction?

Answer 3

Precipitate, color change, gas production, temperature change, and reversal.

Question 4

What is precipitate?

Answer 4

A solid formed when two liquids are mixed and then react

Question 5

What is color change?

Answer 5

An unexpected change in color.
White + brown = light brown (expected)
White + brown = blue (unexpected)
A chemical change occurred with the second one.

Question 6

What is gas production?

Answer 6

This is bubbling and fizzing. Two liquids = gas or Solid + liquid = gas

Question 7

What is temperature change?

Answer 7

Endothermic rxn - absorbs energy (feels colder)
Exothermic rxn - releases energy in the form of heat (feels warmer)
For example: The hand warmers release energy and become warmer.

Question 8

What is reversal?

Answer 8

Change is not easily reversible. Bonds are broken, new ones are formed which is now a new property.
Mg + O2 —-> MgO
MgO cannot go back to Mg + O2

Question 9

What happens to chemical bonds during a chemical reaction?

Answer 9

Original bonds are broken and new bonds are formed.