Observation and Experimentation: Determining the Insolubility of Calcium Phosphate

Introduction to the Laboratory Experiment

In a typical CHEM1901 laboratory session, a basic experiment is conducted to observe the behavior of calcium phosphate in different chemical reactions. The objective is to understand the concept of solubility, particularly focusing on the insolubility of certain compounds, in this case, calcium phosphate (Ca3(PO4)2).

Mixed Solutions and Observation

During the experiment, two clear and colorless aqueous reagents are prepared:

Calcium chloride (CaCl2), a white, odorless, and crystalline salt Sodium phosphate tribasis, also known as trisodium phosphate (Na3PO4), which is a water-soluble white powder

When these two solutions are mixed in equal volumes, a striking observation is made immediately: a white precipitate forms. This precipitate is calcium phosphate (Ca3(PO4)2).

Chemical Reaction and Precipitation

The chemical reaction that occurs during the mixing of the solutions can be represented by the following equation:

3CaCl2(aq) 2Na3PO4(aq) → Ca3(PO4)2(s) 6NaCl(aq)

This equation shows that three molecules of calcium chloride and two molecules of sodium phosphate react to form one molecule of calcium phosphate and six molecules of sodium chloride, which remains in solution as a spectator ion.

Distinguishing the Precipitate

One of the key questions may arise: how do we determine that the white precipitate is indeed calcium phosphate (Ca3(PO4)2)? To ensure accuracy, it's important to understand the precipitation chemistry involved, particularly the formation of insoluble compounds.

Given the following possible combinations of ions:

Ca^2 and Na – No reaction, both cations Cl^— and PO43- – No reaction, both anions Na^ and Cl^— → NaCl, a soluble salt – No precipitate, spectator ions Ca^2 and PO43- → Ca3(PO4)2, the only possible insoluble compound – Net ionic equation

The net ionic equation for the reaction is:

3Ca^2 (aq) 2PO43- (aq) → Ca3(PO4)2 (s)

This equation clearly indicates that the white precipitate is calcium phosphate, confirming its insolubility in water.

Conclusion

The laboratory experiment demonstrated the formation of a precipitate, indicating the insolubility of calcium phosphate (Ca3(PO4)2). By understanding the chemical reactions and precipitating ions involved, the experiment successfully showed a practical example of a poorly soluble compound.

Further studies on solubility and precipitation reactions are crucial in advancing the understanding of inorganic chemistry and its applications, making this a valuable educational tool in any chemistry laboratory setting.

References:

Chang, R. (2020). Chemistry. 13th edn. McGraw-Hill.