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Rain, Deserts, and Salt Lakes

Learn why air changes temperature, chemistry of ions in water, pH scale, gas reactions, and elements in lakes. Discover the unique composition of salt lakes like Mono Lake. Explore chemical equations and reactions in water systems.

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Rain, Deserts, and Salt Lakes

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  1. Rain, Deserts, and Salt Lakes By Allan Treiman For “Life at the Limits” Teacher Training Workshop 2005

  2. Why does air change temperature as it rises and falls? • Pressure - Volume work, like an air conditioner. • Air gets cooler as it decompresses (inside AC unit), and hotter as it is compressed (outside AC unit). • From physical chemistry, an ideal gas has: ( T2 /T1 ) = ( p2 / p1 ) ,  >1. • This curve of T vs. P for air (without heat added) is its ‘lapse rate.’

  3. Quick Review of Chemistry - I • In water solutions, nearly all elements are as charged ions. • Positive charge (Na+, H+, Ca2+, Al3+) = cations • Negative charge (Cl-, SO42-, OH-) = anions • Mass is conserved – atoms don’t vanish or appear from nowhere. • Charge is conserved – charge doesn’t vanish or appear from nowhere. • The sum of charges on all ions is zero.

  4. Quick Review of Chemistry. II • Ions react among themselves, gas, solids. • H+ + OH - H2O – water • As concentration of H+ goes up, OH - goes down: equilibrium [H+][OH -] = 10-14 (25°C). • In pure water, [H+]=[OH -], so [H+] = 10-7 . • This is ‘neutral’, pH=7. • pH is –logarithm(base10) of H+ concentration • High H+ (like [H+] = 10-3) is acid (pH = 3). • High OH -(like [H+] = 10-9) is alkaline (pH = 9).

  5. Quick Review of Chemistry III • Gas - Carbon dioxide • CO2 (gas)  CO2 (dissolved) • CO2 (gas) + H2O  H2CO3 (carbonic acid) • [H2CO3] / [CO2] = a constant (10-1.47) • H2CO3 H+ + HCO3- (now acidic!) and • [H+][HCO3 -] / [H2CO3] = a constant (10-6.35) • HCO3- H+ + CO32- . • Solids – • Ca++ + CO32-  CaCO3 (calcite, tufa)

  6. So, What about our lakes?

  7. Sierra Nevada Spring Water mg/kg = ppm SiO2 24.6 Ca2+ 10.4 Mg2+ 1.70 Na+ 5.95 K+ 1.57 HCO3- 54.6 SO42- 2.38 Cl- 1.06 pH 6.8 Where do elements come from? Atmosphere CO2 makes carbonic acid, helps dissolve solids Rock (granite) – Si, Ca, Mg, Na, K, SO42- Atmospheric dust – these plus Na, Cl, SO42- from sea spray. Salt Lakes: From Spring Water

  8. Gaylussite - at Mono Lake Na2Ca(CO3)•2.5H2O Natron Na2CO3•10H2O Aphthitalite (K,Na)3Na(SO4)2 Halite NaCl - at 4-Mile pH high - always very alkaline Calculated Evaporation of SN Water

  9. Evaporate this water! What’s left? • Ca, Mg, Si precipitate as minerals • Na+, Cl-, SO42- become concentrated • HCO3- & CO32- become concentrated too, • AND, they react with H+ in the water to make CO2, which leaves into the air. HCO3- +H+ H2O + CO2(gas). • This reaction consumes H+, making the water less acid == more alkaline! • The result – an alkaline brine rich in sodium, chloride and carbonate !!

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