To determine which chemical equation has both a liquid and a gas as products, let's analyze each given equation one by one:
1. Equation 1:
```
2 HgO ( s ) → 2 Hg ( l ) + O_2 ( g )
```
- The reactant [tex]\( HgO \)[/tex] is solid (s).
- The products are [tex]\( Hg \)[/tex], which is liquid (l), and [tex]\( O_2 \)[/tex], which is gas (g).
- This equation has both a liquid and a gas as products.
2. Equation 2:
```
2 Mg ( s ) + O_2 ( g ) → 2 MgO ( s )
```
- The reactants [tex]\( Mg \)[/tex] and [tex]\( O_2 \)[/tex] are solid (s) and gas (g), respectively.
- The product [tex]\( MgO \)[/tex] is solid (s).
- This equation does not have both a liquid and a gas as products.
3. Equation 3:
```
2 KClO_3 ( s ) → 2 KCl ( s ) + 3 O_2 ( g )
```
- The reactant [tex]\( KClO_3 \)[/tex] is solid (s).
- The products are [tex]\( KCl \)[/tex], which is solid (s), and [tex]\( O_2 \)[/tex], which is gas (g).
- This equation does not have both a liquid and a gas as products.
4. Equation 4:
```
Mg ( s ) + 2 HCl ( aq ) → H_2 ( g ) + MgCl_2 ( aq )
```
- The reactants [tex]\( Mg \)[/tex] and [tex]\( HCl \)[/tex] are solid (s) and aqueous (aq), respectively.
- The products are [tex]\( H_2 \)[/tex], which is gas (g), and [tex]\( MgCl_2 \)[/tex], which is aqueous (aq).
- This equation does not have both a liquid and a gas as products.
After analyzing all four equations, only the first equation has both a liquid and a gas as products. Therefore, the correct equation is:
```
[tex]$2 HgO ( s ) \rightarrow 2 Hg ( l ) + O_2 ( g )$[/tex]
```
Thus, the answer is the first equation, and its index (starting from 0) is:
```
0
```
