SCH4C: Chemistry & The Environment Water SCH4C: Chemistry & The Environment

The importance of water Water is necessary for life – making up a large percentage of the bodies of most organisms. Water is required for life’s processes. Water affects climate and weather, helping to keep Earth’s temperatures stable. Water provides habitats for aquatic organisms

Distribution of Earth’s Water 97% of Earth’s water is found in the oceans, while the other 3% is fresh water, found at the Earth’s surface and underground. Nearly 70% of Earth’s water is unavailable for use as it is frozen in the icecaps and glaciers The remaining 30% is found as groundwater or lakes/rivers and streams and is available for drinking

Canada’s Fresh Water Canada has more lake area than any other country in the world. The Great Lakes make up 18% of the Earth’s water. Glaciers cover 200 000 km2 of Canada. The melt water from these glaciers is an important source of water. Groundwater is the water found in spaces between soil particles and cracks in rocks underground. ¼ of Canadians get their drinking water from ground water.

Water & Dissolved Materials Pure water doesn’t exist naturally – all water naturally contains dissolve substances. Example: Rainwater is naturally acidic, containing dissolved carbonic acid (H2CO3) As water filters through soil and rock, it tends to dissolve (leach) certain ions (Ca2+, Mg2+, Fe2+, Fe3+, SO42-)

Water & Dissolved Materials

Hard and Soft water Water with high concentrations of these ions is called hard water (it is ‘harder’ to lather with soap) Water with low concentrations of these ions is called soft water. Groundwater is usually harder than surface water in the same region. The hardness depends on the types of rocks through which the water flows, and the time that water is in contact with the rocks.

CaCO3(s) + H2O(l)  Ca2+(aq) + HCO3-(aq) + OH-(aq) The most common type of rock to form hard water is limestone (CaCO3), which is usually insoluble. CaCO3(s) + H2O(l)  Ca2+(aq) + HCO3-(aq) + OH-(aq) When the water contains dissolved acids, the H+(aq) ion increases the concentrations of calcium and bicarbonate ions by reacting with calcium carbonate. CaCO3(s) + H2+(aq)  Ca2+(aq) + HCO3-(aq)

2HCO3-(aq)  H2O(l) + CO2(g) + CO32-(aq) Bicarbonate ions can be costly to homeowners, as they decompose when heated, leaving carbonate ions. 2HCO3-(aq)  H2O(l) + CO2(g) + CO32-(aq) The carbonate ions recombine with calcium ions to form calcium carbonate deposits (boiler scale), which can damage kettles and build up inside hot water pipes. Adding acetic acid, CH3COOH (vinegar), removes boiler scale by reacting with either calcium or magnesium. CaCO3(s) + CH3COOH (aq)  Ca2+(aq) + 2CH3COO-(aq) + H2O(l) + CO2(g)

Acid Precipitation Rainwater is naturally acidic with a pH of about 5.6-5.8 This is because of CO2 dissolving in and reacting with water in the atmosphere to form carbonic acid (H2CO3). Lightning strikes can produce nitrogen compounds that form nitric acid (HNO3) Volcanoes add sulfur compounds that form sulfuric acid (H2SO4) The presence of other sulfur and nitrogen oxides can increase the acidity of precipitation. This can lead to environmental damage.

When SO2 is released into the atmosphere, it reacts with oxygen to form sulfur trioxide, SO3 Sulfur trioxide reacts with water to produce sulfuric acid This is similar to the reaction between water and NO2 2SO2 + O2  SO3 SO3 + H2O  H2SO4 NO2 + H2O  HNO3