1. The lower, dense region of the atmosphere is known as   Stratosphere   Ionosphere   Troposphere   Hydrosphere   Hint Half-n-half Clue   2. Lime water turns milky after carbon dioxide is passed through it.   True   False   3. Carbon dioxide is lighter than air.   True   False   4. Nitrogen constitutes nearly __ % of the atmosphere.   57   21   79   42   Half-n-half Clue   5. Air is   An Element   An Alloy   A Mixture   A Compound   Half-n-half Clue   6. What gas is evolved when HCl is poured on marble chips?   Carbon Dioxide   Hydrogen Peroxide   Hydrogen   Calcium Carbonate   Half-n-half Clue   7. The atmosphere exerts pressure on the surface of the earth.   True   False