This graph shows pH (1–12) plotted against solubility / mmol l-1 (0–11). The pH bracket from about 3 to 8.5 is labelled as 'natural waters'. Within this, several segments are highlighted in blue, measured approximately as follows: 3.5–4.5 is labelled ‘acid rain’, 5.1–6 is labelled ‘unpolluted rainwater’, 7.6–8.3 is labelled ‘seawater’. The solubility of SiO2 and Al2O3 are plotted on this graph. Aluminium oxide is highly soluble at low pH (4) and high pH (10) – its curve on the graph is roughly parabolic and it rises very steeply in solubility to 11 mmol l-1 at these pHs. Between pH 4.5–9, it is much less soluble, at less than 1 mmol l-1. Silica, on the other hand, is not very soluble at low pH, gradually rising from 1.7–2.3 mmol l-1 between pH 1–4. At this point, while silica's solubility curve continues rising, it intersects with the plummeting aluminium oxide curve, indicating that silica is more soluble between pH 4–10. At around pH 9.5, silica solubility starts to increase expontentially, and the curves cross again, as both materials are very soluble at pH 10.