Question

questions, page 2
which of the following are chemical changes?
a) wood burning
b) milk going sour
c) kool - aid powder dissolving in water
d) sharpening a pencil
e) a glass cup falls from the table and shatters
f) freezing a cup of water
g) using a hot pack to keep warm
h) baking a cake
answer:
what happens in all chemical changes?
a) change in state of matter
b) bubbles are produced
c) a new substance is formed
d) an explosion
answer:

Explanation:

First Sub - Question (Which are chemical changes?)

A chemical change is a process where a new substance is formed.

• a) Wood burning: Combustion of wood produces carbon dioxide, ash, etc. (new substances).
• b) Milk going sour: Bacteria act on milk to form lactic acid (new substance).
• c) Kool - Aid powder dissolving in water: This is a physical change (no new substance, just mixing).
• d) Sharpening a pencil: Physical change (shape change, no new substance).
• e) A glass cup shattering: Physical change (shape change, no new substance).
• f) Freezing water: Physical change (state change, still $H_2O$).
• g) Using a hot pack: Chemical reaction in the hot pack produces heat (new substances formed).
• h) Baking a cake: Chemical reactions (like fermentation, protein denaturation) form new substances.

So the chemical changes are a, b, g, h.

• Option a: Change in state (like melting) can be physical, so not all chemical changes have state change.
• Option b: Bubbles (like boiling) can be physical, not all chemical changes have bubbles.
• Option c: By definition, a chemical change involves the formation of a new substance.
• Option d: Explosions are a type of chemical change, but not all chemical changes are explosions.

So the correct answer is c.

Answer:

a) Wood burning, b) Milk going sour, g) Using a hot pack to keep warm, h) Baking a cake

Second Sub - Question (What happens in all chemical changes?)