What distinguishes a physical change from a chemical change?

Physical change alters only appearance; chemical change produces new substances with different properties.

Fill in the blank: A chemical change is generally ___ by merely changing temperature and pressure.

Irreversible

Describe the process and outcome of a decomposition reaction.

A single compound breaks down into simpler substances, requiring energy (heat, light, or electricity).

True or False: All displacement reactions involve formation of a precipitate.

False. Only some displacement reactions, such as double displacement, produce a precipitate.

Fill in the blank: Exothermic reactions are characterized by the ___ of heat during product formation.

Release

What role do oxidizing agents play in chemical reactions?

Oxidizing agents accept electrons, causing oxidation of another substance.

Define the term 'redox reaction' and provide an example.

Redox reactions involve simultaneous oxidation and reduction.
Example: Combustion of fuel.

Fill in the blank: The oxidation state of an element in its free state is always ___.

Zero

What is the significance of the oxidation state of hydrogen and how does it vary?

Hydrogen is +1 in most compounds, -1 in metal hydrides; indicates electron gain or loss.

True or False: All chemical reactions are reversible.

False. Some reactions are irreversible and cannot revert to original substances.

Describe the difference between endothermic and exothermic reactions.

Endothermic absorbs heat; exothermic releases heat; both affect the surroundings’ temperature.

What defines a combination reaction, and can you provide an example?

Two or more reactants combine to form a single product. Example: Calcium oxide + water → slaked lime.