Question

which best explains why sawdust burns more quickly than a block of wood of equal mass under the same conditions?
the molecules move more quickly in the sawdust than in the block of wood.
the pressure of oxygen is greater on the sawdust.
more molecules in the sawdust can collide with oxygen molecules.
oxygen is more concentrated near the sawdust than the block of wood.

Explanation:

• For the first option: The speed of molecules in sawdust and a wood block of equal mass under the same conditions (temperature) should be similar as molecular speed is related to temperature, so this is incorrect.
• For the second option: The pressure of oxygen is the same under the same conditions for both sawdust and the wood block, so this is wrong.
• For the third option: Sawdust has a larger surface area than a wood block. A larger surface area means more wood molecules are exposed to oxygen, so more collisions between wood molecules and oxygen molecules occur, leading to faster burning. This is correct.
• For the fourth option: Oxygen concentration is the same near sawdust and the wood block under the same conditions, so this is incorrect.

Answer:

More molecules in the sawdust can collide with oxygen molecules.