Our weather is constantly changing and this can have an impact on our lives. Storms can be dangerous for fishing boats and coastguards study weather charts so they can warn against adverse conditions. Atmospheric pressure is measured in millibars and is represented on weather maps as isobars. When the air is cold, the molecules are packed tightly together; the air becomes denser and begins to sink. The air now presses on the Earth’s surface, creating high pressure. When the air warms, the molecules fly further apart; the air becomes lighter and rises, creating low pressure. High pressure often brings fine weather, but low pressure draws moisture from the ground creating clouds, rain and storms. Wind is caused by air moving from high atmospheric pressure to low atmospheric pressure.
Air consists of 78% nitrogen, 21% oxygen, <1% argon, 0.03% carbon dioxide, and 0-4% water vapour.
Air is a mixture. This is shown by the fact that its composition varies (e.g. there is less oxygen in the upper atmosphere).
Preparing Oxygen & Carbon Dioxide
Oxygen is prepared by the breakdown of hydrogen peroxide using manganese dioxide as a catalyst.
2H2O2 + MnO2 -> 2H2O + O2.
Carbon Dioxide is prepared by the reaction between hydrochloric acid and marble chips.
2HCl + CaCO3 -> CaCl2 + CO2 + H2O.
Carbon dioxide is tested for by passing it through limewater. It turns limewater milky.
Oxygen is tested for using a glowing splint - it relights a glowing splint.
Water is tested for using cobalt chloride paper. It turns from blue to pink in the presence of water.
Properties of Oxygen
Physical Properties: Colourless, odourless, tasteless, heavier than air, slightly soluble in water.
Chemical properties: Supports combustion, very reactive, neutral pH.
Properties of carbon Dioxide
Physical Properties: Colourless, odourless, tasteless, heavier than air, moderately soluble in water.
Chemical properties: Does not support combustion, turns limewater milky, forms an acid in water (called carbonic acid).
Uses of Oxygen
Uses of Carbon Dioxide