(a) The reaction is as follows:

Ca (OH)₂ + CO₂→ CaCO₃ + H₂O

This reaction is a self-balanced reaction as number of molecules on reactant side is same as that on the product side.

(b) The reaction is as follows:

PCl₅ + H₂O → H₃PO₄ + HCl

In this reaction there are five chlorine molecules on reactant side and only one molecule on the product side. So let us make the number of chlorine molecules 5 on product side also.

PCl₅ + H₂O → H₃PO₄ + 5HCl

Now there are eight molecules of hydrogen on product side and only two molecules on reactant side. So the total balanced reaction is as follows:

PCl₅ + 4H₂O → H₃PO₄ + HCl

(c) The reaction is as follows:

Zn + AgNO₃→ Zn (NO₃)₂ + Ag

In the above reaction there are two nitrate molecules on product side and only one nitrate molecule on reactant side. So let us first balance it.

Zn + 2AgNO₃→ Zn (NO₃)₂ + Ag

Now there two molecules of silver on reactant side and only one molecule on product side. So the total balanced equation is as follows:

Zn + 2AgNO₃→ Zn (NO₃)₂ + 2Ag

(d) The reaction is as follows:

Na + H₂O → NaOH + H₂

In the above reaction there three molecules of hydrogen on product side and only two molecules on reactant side. Let us first make the number molecules of hydrogen on product side even in number.

Na + H₂O → 2NaOH + H₂

Now there are two molecules of hydrogen on reactant side and four molecules on product side. So let us first balance it.

Na + 2H₂O → 2NaOH + H₂

Now there two molecules of sodium on product side and only one molecule on reactant side. So the total balanced equation is as follows:

2Na + 2H₂O → 2NaOH + H₂

(e) The reaction is as follows:

Al + CuCl₂→ AlCl₃ + Cu

In the above reaction there are two molecules of chlorine on reactant side and three molecules of chlorine on product side. Let us first make the number of chlorine molecules even on product side.

Al + CuCl₂→ 2AlCl₃ + Cu

Now there are six molecules of chlorine on product side. So let us make it six molecules of chlorine on reactant side also.

Al + 3CuCl₂→ 2AlCl₃ + Cu

After making the chlorine molecules same on both sides, the number of copper molecule son product side is only one and on reactant side it is three. So let us make the number of copper molecules equal on both sides.

Al + 3CuCl₂→ 2AlCl₃ + 3Cu

Finally there two molecules of aluminium on product side and only one molecule on reactant side. So the total balanced equation can be written as follows:

2Al + 3CuCl₂→ 2AlCl₃ + 3Cu