In wheat-growing areas, such as the plains of the central United States and Canada, harvested wheat is stored in tall grain elevators that are visible for miles in the flat prairie.

The wheat is dumped from trucks into a lower part of the elevator and then moved up the elevators into storage areas. The combustible dust produced as the wheat grains rub together has caused grain elevator explosions. Use the cube analogy in Exercise 11.37 to explain why wheat dust dispersed in the air in a grain elevator can react explosively if sparked, whereas whole wheat grains do not.

Data from Exercise 11.37

In a heterogeneous system such as wood burning in oxygen, the surface area of the solid can be a factor in the rate of the reaction. Increased surface area of the wood means more collisions with oxygen molecules. To understand how surface area increases when dividing a solid, do the following:

Draw a cube that is 2 cm on a side. What is its total surface area?

Divide each face of the cube into four equal sections.

If you were able to take the cube apart on the lines you have just drawn, how many cubes would you have?

What are the dimensions of the new cubes?

What is the total surface area of all the new cubes? What will happen if you divide the cubes again?

Now consider burning a log or burning all the toothpicks that could be made from the log. What effect would surface area have on the rate that each burns? Would you rather try to start a campfire with a log or with toothpicks?