#### Answer

- No, it's not possible to conclude that the solution is acidic.
- It's necessary to know the temperature of the solution.
- Yes, it can be zero or negative.
- $HCl: 1.00 M --> pH = 0$
- $HCl: 10.00M --> pH = -1$

#### Work Step by Step

- The $K_w$ variates with the change of temperature, the value $10^{-14}$ is used for $25^{\circ}C$.
Therefore, if the temperature changes, the value of neutral pH also changes, so we don't know if $6.7$ is higher, lower or equal to this value, and we can't conclude that the solution is acidic.
- Let's use the formula: $[H^+] = 10^{-pH} $
If pH = 0:
$[H^+] = 10^{0} = 1M$
- Therefore, if we use a $1.00M$ strong acid solution, its pH will be 0.
If pH < 0
Let's use (-1) as an example.
$[H^+] = 10^{-(-1)} = 10^1 = 10M$
- If we use a 10.00M strong acid solution, the pH will be less than 0.