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    Physical properties of seawater

    Polar nature of the water molecule -- hydrogen bonds -- dissolve salts into ion due to polar nature -- interaction between ions and water molecules by polarity

    pH of seawater

    • pH of seawater is slightly above 8.0
    • pH of seawater increases with phytoplankton production
    • pH of lake water is mostly slightly below 7

    Background Reminder:
    pH = –log[H+]
    H2O + H2O = H3O+ + OH-
    H2O = H+ + OH-
    The more H+ in the water, the more acidic it is. Neutral water (pH = 7) contains 10-7 moles H+ (or H3O+) and OH-

    Chemical properties of seawater

    • Six ions account for 99% of dissolved components in seawater: chloride (Cl-), sodium (Na+), sulfate (SO42-), magnesium (Mg2+), calcium (Ca2+), potassium (K+)
    • Biologically important elements such as oxygen, nitrogen, iron occur in variable concentrations depending on utilization by organisms (e.g. nitrate NO3- may range from 0 to 60 mM*
    • * Remark on units: 1 mM = 1 mmol l-1 = micromole per liter

    Where do the salts come from?

    • Ancient: Water vapor in Earth’s early atmosphere reacted with volcanic gases and dust; most important gases: chlorine, sulfur dioxide, carbon dioxide – formed acids (HCl, H2SO3, H2CO3) in rain water, which dissolved minerals from rocks 
    • Similar processes occur today, but slower, less volcanic activity

    • Weathering: interaction water – rocks, dissolves new minerals & loss by precipitation
    • Water / sediment interaction at seafloor – gain of dissolved material and loss of precipitates; both high-temperature reactions at mid-ocean ridges & low temperature reactions with volcanic rocks and sediments.