Chem-chemical eqaution(precipitate)

Firstly, you have to know what ionic compounds are soluble in water, and what are insoluble.

Each ionic compound usually consists of a cation and an anion. Only the following ions are important :
Cations : K+, Na+, NH4+, Ba2+, Ca2+, Mg2+, Al3+, Zn2+, Fe2+, Fe3+, Pb2+, Cu2+, Ag+
Anions : NO32‑, SO42-, CO32-, HCO3-, X- (Cl-, Br-, I-), OH-

Rules :
1. All potassium salts, sodium salts and ammonium salts are soluble in water.

2. All metal ntrates and metal hydrogencarbonates are soluble in water.

3. Most metal sulphates are soluble in water, but CaSO4 (slightly soluble), BaSO4 and PbSO4 are insoluble.

4. Most metal halides (chlorides, bromides and iodides) are soluble, but PbCl2 and AgCl are insoluble.

5. Most metal carbonates are insoluble in water, but K2CO3, Na2CO3 and (NH4)2CO3 are soluble.

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Secondly, when two solutions of ionic compounds are mixed, cations and anions will attract together.
If cations and anions attract together to form a soluble compound, the ions will then separate and no precipitate is formed.
If cations and anions attract together to form an insoluble compound, a precipitate is formed.

e.g. 1
AgNO3 and CaCl2 solutions are mixed.
The mixed solution contains Ag+, NO3-, Ca2+ and Cl- ions.
There are 4 combinations of ionic compounds : AgNO3, AgCl, Ca(NO3)2 and CaCl2. According to the rules above, only AgCl among them is insoluble in water. Therefore, the precipitate of AgCl is formed.
2AgNO3(aq) + CaCl2(aq) → 2AgCl(s) + Ca(NO3)2(aq)
or Ag+(aq) + Cl-(aq) → AgCl(s)

e.g. 2
Na2CO3 and CuSO4 solutions are mixed.
The mixed solution contains Na+, CO32-, Cu2+ and SO42- ions.
There are 4 combinations of ionic compounds : Na2CO3, Na2SO4, CuCO3 and CuSO4. According to the rules above, only CuCO3 among them is insoluble in water. Therefore, the precipitate of CuCO3 is formed.
Na2CO3(aq) + CuSO4(aq) → CuCO3(s) + Na2SO4(aq)
or CO32-(aq) + Cu2+(aq) → CuCO3(s)

e.g. 3
NaCl and CuSO4 solutions are mixed.
The mixed solution contains Na+, Cl-, Cu2+ and SO42- ions.
There are 4 combinations of ionic compounds : NaCl, Na2SO4, CuCl2 and CuSO4. According to the rules above, all of them are soluble in water. Therefore, no precipitate is formed.