Example 1
Find the silicate anion in Mg2SiO4, treating Mg as Mg2+.
Two Mg2+ give total +4, so the anion is SiO44−. The compound’s total charge is zero.
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Year 12 · SACE Stage 2
Soil nutrients, silicates and ion exchange
Apply nutrient, ion-charge and soil-exchange rules.
Method and worked examples
Plants take up mineral nutrients as dissolved species in soil water. Nitrogen, phosphorus and potassium are major fertiliser nutrients. Nitrogen can be replenished through biological fixation, lightning-related chemistry and decomposition. Excess nitrogen and phosphorus entering waterways can promote algal growth; decomposition of biomass consumes dissolved oxygen and can harm aquatic organisms.
Silicates and aluminosilicates provide negatively charged sites that retain nutrient cations in exchange equilibrium with soil water. Roots and changing solution concentrations can shift this exchange. Excess H+ or Na+ can displace useful cations and promote nutrient loss. Determine a silicate anion’s charge from the positive ions in its formula. Ion exchange conserves charge, so replacing one Ca2+ requires two monovalent cations.
Example 1
Example 2
How many moles H+ replace 0.0050 mol Ca2+ on exchange sites?
Each Ca2+ balances two negative sites. Replacing it requires 2×0.0050=0.010 mol H+, with calcium released to soil water.
Watch out: Exchangeable cations can move into soil water through equilibrium and become available for uptake.