What is the definition of "compressed air"?

1. Air maintained at a low temperature
2. Air maintained at a high temperature
3. Air that is free from all impurities
4. Air that can be condensed into a much smaller space than normally occupied ✓

Answer: D. Air that can be condensed into a much smaller space than normally occupied

Compressed air is air that can be condensed into a much smaller space than it normally occupies.

Why this is the answer

The handbook explains that air can be squeezed into a much smaller space than it would normally fill, and car tires are given as an example of this. Squeezing air into a smaller space raises its resistance, and that resistance becomes pressure, which can be turned into mechanical force to apply the brakes. So the defining idea is air packed into less space than usual, not temperature or purity.

Why the other answers are wrong

• A. Air maintained at a low temperature — Temperature is not part of the definition; the handbook talks about squeezing air into a smaller space, not chilling it.
• B. Air maintained at a high temperature — Heating air is not what the handbook describes; the point is reducing the space the air fills, not raising its temperature.
• C. Air that is free from all impurities — The handbook never says compressed air must be cleaned of impurities; it only describes air squeezed into less space.

Remember it

Picture a car tire: the same air, packed into far less room, and that squeeze is what makes pressure.