Solid carbon dioxide is called dry ice.
Uses:
(i) It is used as a refrigerant
(ii) The gas is so cold that moisture in the air condenses on it, creating a dense fog used in stage shows for movie effects.

The increased green house effect is caused due to increase in the air pollutants and it results in the average increase of temperature of the atmosphere. This is Global warming.

Physical properties of Oxygen
(i) Oxygen is a colourless, odorless and tasteless gas
(ii) It is a poor conductor heat and electricity
(iii) Oxygen dissolves readily in cold water
(iv) It is denser than air
(v) It can made into liquid at high pressure and low temperature
(v) It supports combustion.

i) During night photosynthesis does not occur. So, tree will not release oxygen and take in oxygen.
ii) Trees respire and they take in oxygen and release carbon dioxide. Due to their huge size, they take up comparatively more Oxygen than a human being. So, we will not get sufficient oxygen under a tree during night. It may cause breathing problems and suffocation. So it is bad for health.

Effects of Acid rain
(i) It irritates eyes and skin of human beings
(ii) It inhibits germination and growth of seedlings
(iii) It changes the fertility of the soil, destroys plants and aquatic life.
(iv) It causes corrosion of many buildings, bridges etc.
Preventive measures
(i) Minimising the usage of fossil fuels such as petrol, diesel etc.
(ii) Using CNG (Compressed Natural Gas)
(iii) Using non-conventional source of energy
(iv) Proper disposal of the industrial wastes.

a) Potassium 
Lighter and more reactive metals like potassium combine with CO₂ to form corresponding carbonate - potassium carbonate (K₂CO₃).
\(4 \mathrm{~K}+3 \mathrm{CO}_{2} \rightarrow 2 \mathrm{~K}_{2} \mathrm{CO}_{3}+\mathrm{C}\)
b) Lime water
When a limited amount of CO₂ missed through lime water (calcium hydroxide) it turns milky due to the formation of insoluble calcium carbonate
\(\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{CO}_{2} \rightarrow \mathrm{CaCO}_{3}+\mathrm{H}_{2} \mathrm{O}\)
When an excess amount of CO₂ is passed through lime water, it first turns milky and the milkiness disappears due to formation of soluble calcium hydrogen carbonate Ca(HCO₃)₂.
c) Sodium hydroxide
Sodium hydroxide (base) is neutralized by carbon dioxide (acid) to form sodium carbonate and salt
\(2 \mathrm{NaOH}+\mathrm{CO}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}\)

a) Carbon - Carbondioxide CO₂
b) Sulphur - Sulphur dioxide SO₂
c) Phosphorous - Phosphorous trioxide P₂O₃
Phosphorous pentoxide P₂O₅
d) Magnesium - Magnesium Oxide MgO
e) Iron - Iron oxide Fe₃O₄
f) Sodium - Sodium Oxide Na₂O

When a limited amount of CO₂ is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
\(\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{CO}_{2} \rightarrow \mathrm{CaCO}_{3}+\mathrm{H}_{2} \mathrm{O}\)
When an excess amount of CO₂  is passed through lime water, it first turns milky and the milkiness disappears due to the formation of soluble calcium hydrogen carbonate Ca(HCO₃)₂.