Question

SEP Form a Hypothesis The lake water changes color rapidly on exposure to air. Suggest factors that may affect the speed of the reaction in Blood Falls.

Answer 1

Short, testable hypothesis
- The speed at which the Blood Falls water turns red on exposure to air increases with higher dissolved Fe2+ concentration, higher dissolved O2 (or better gas exchange), higher temperature, higher pH, and greater mechanical mixing; it decreases if strong Fe-complexing ligands or sulfide are present, or if microbial iron-oxidizers are absent.

Why (factors, predicted effect, brief rationale)
- Fe2+ concentration — higher [Fe2+] → faster formation of Fe3+ and iron oxides/hydroxides → faster color change (more reactant available).
- Oxygen availability / gas exchange — more O2 (or more agitation/air contact) → faster oxidation of Fe2+ → faster color development.
- Temperature — higher T speeds chemical reaction rates (and microbial activity) → faster oxidation/precipitation.
- pH — oxidation and hydrolysis/precipitation of Fe3+ are faster at neutral to slightly basic pH than at very low pH → faster visible color change as pH rises.
- Salinity / ionic strength — high salinity and specific ions (Cl–) alter Fe2+ speciation and precipitation kinetics; extreme ionic strength can slow nucleation/precipitation or change precipitate form → affects rate (often slows visible floc formation).
- Reducing species (sulfide, high organic reductants) — these consume O2 or reduce Fe3+ back to Fe2+ → slower or different-colored products (e.g., iron sulfides) → slower red color development.
- Organic ligands / chelators — strong complexation of Fe2+/Fe3+ keeps iron in solution and can slow precipitation and color formation.
- Microbial activity — iron-oxidizing microbes can catalyze oxidation (or conversely, sulfate-reducers can generate sulfide and inhibit red color); presence/absence of microbes therefore changes the rate.
- Mixing / surface area / flow rate — increased turbulence or finer droplets increases contact with air → faster oxidation.
- Light / photochemistry — in some systems UV can drive redox reactions that alter Fe speciation and rate; might accelerate or modify the reaction depending on conditions.

How to test (simple experimental design)
- Measure time until visible red color appears (or monitor Fe2+ concentration / dissolved O2 / turbidity / absorbance) after exposing aliquots of the brine to air while varying one factor at a time: temperature (e.g., 0°C, 10°C, 20°C), O2 supply (still vs stirred vs aerated), pH (buffered samples), presence/absence of added sulfide or organic ligand, and mixing intensity. Keep other variables constant.
- Predicted observations: faster color change with increased [Fe2+], O2, T, pH, and mixing; slower with added sulfide or strong chelators or very high salinity.

These factor predictions provide clear, falsifiable hypotheses you can test in the lab or through field measurements at Blood Falls (feasible proxies: measure dissolved Fe2+/Fe3+, DO, pH, salinity, and microbial community while exposing samples to controlled air contact).