Why is air a pure substance?

If two different theories describe experimental observations equally well, can one be said to be more valid than the other (assuming both use accepted rules of logic) ?

The factor being measured in an experiment is the a. Scientific law B. Variable C. Control D. Hypothesis

A rock is dropped into a well hits the water in 5 secs. if there is no air resistance, what would be the rocks final speed?

What is it's potential energy when it is 1000m above the ground

Orographic uplift is best described as A. Warm, moist air rising over cold, dense air B. Two airflows colliding head on C. Air being forced upward D. Airflow confronting a mountain barrier, causing the air to rise

How many turns are in the secondary coil of a step up transformer that increases voltage from 30 V to 150 V and has seven turns in the primary coil?

What is a nuclear bulge? a) The bulging corner of a spiral galaxy consisting of gas in the shape of a spiral sphere that circles a galaxy b) The bulging center of a spiral galaxy consisting of debris in the shape of a flattened sphere that circles a

Your friend of mass 50 kg can just barely float in fresh water. Calculate her approximate volume.

A flashlight beam strikes the surface of a pane of glass (n = 1.50) at a 61° angle to the normal. what is the angle of refraction?

Why is there controversy to the opinion that human contribution of carbon dioxide to the atmosphere is cause warming