2HCO3 - + Ca2+ CaCO3 + CO2 + H2O Bicarbonate (HCO3-) combines with calcium ions in the water to make calcium carbonate (CaCO3, limestone). This process can occur both within organisms such as corals or as a simple chemical reaction in the water itself.
You could write about a fear you may have had as a child, eg. the dark, or underneath the bed, etc...
D = m/ V
8.9 = 3 / V
V = 3 / 8.9
V = 0.3 cm³
Answer A
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