Rain Deserts and salt lakes

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Rain, Deserts, and Salt Lakes : 

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

Why does air change temperature as it rises and falls? : 

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.’

Quick Review of Chemistry - I : 

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.

Quick Review of Chemistry. II : 

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).

Quick Review of Chemistry III : 

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)

So, What about our lakes? : 

So, What about our lakes?

Salt Lakes: From Spring Water : 

Salt Lakes: From Spring Water 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.

Calculated Evaporation of SN Water : 

Calculated Evaporation of SN Water 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

Evaporate this water! What’s left? : 

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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