As the particle size of a solid fuel decreases, the surface area per mass of fuel ______.

• A. Decreases
• B. Increases
• C. Remains the same
• D. Varies with color

Answer: (B)

When you look at how much surface is available for reactions relative to how much fuel you have, smaller particles expose more surface area per unit mass. For a solid particle, the surface area to mass ratio grows as the size (radius) decreases. If you think of a simple sphere, mass scales with volume (r^3) and surface area scales with area (r^2), so SA per mass ≈ 3/(ρ r). As the radius gets smaller, that ratio gets larger.

So, decreasing particle size increases the surface area per mass. This is important in solid-fuel combustion because more surface area gives oxygen more contact points, speeding up heat transfer and the reaction, which makes burning faster. (Be mindful that very fine powders also bring hazards like dust explosions.)