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Step 1: Answer question 11. Calcium (Ca) is in Group 2, forming $\text{Ca}^{2+}$ ions. Phosphorus (P) is in Group 15, forming phosphide ions ($\text{P}^{3-}$). To balance the charges, three $\text{Ca}^{2+}$ ions ($3 \times +2 = +6$) combine with two $\text{P}^{3-}$ ions ($2 \times -3 = -6$). The chemical formula is $\text{Ca}_3P_2$.

• Answer: $\boxed{Ca_3P_2}$

Step 2: Answer question 12. Table salt is commonly known as sodium chloride.

• Answer: $\boxed{Sodium chloride}$

Step 3: Answer question 13. Copper(II) sulfate pentahydrate ($\text{CuSO}_4 \cdot 5H_2\text{O}$) is a well-known compound that is blue in colour.

• Answer: $\boxed{Blue}$

Step 4: Answer question 14. Table salt is chemically known as sodium chloride, which has the formula $\text{NaCl}$.

• Answer: $\boxed{NaCl}$

Step 5: Answer question 15. The chemical formula for water is $\text{H}_2\text{O}$.

• Answer: $\boxed{H_2O}$

Step 6: Answer question 16. The chemical formula for sulphuric acid is $\text{H}_2SO_4$.

• Answer: $\boxed{H_2SO_4}$

Step 7: Answer question 17. The chemical formula for nitric acid is $\text{HNO}_3$.

• Answer: $\boxed{HNO_3}$

Step 8: Answer question 18. The chemical formula for hydrochloric acid is $\text{HCl}$.

• Answer: $\boxed{HCl}$

Step 9: Answer question 19. The chemical formula for ammonia is $\text{NH}_3$.

• Answer: $\boxed{NH_3}$

Step 10: Answer question 20. The chemical formula for methane is $\text{CH}_4$.

• Answer: $\boxed{CH_4}$