What range of $pH$ is considered perfect for a fertile land?

When soap is scrubbed on a stain of curry on a white cloth,why does it become reddish-brown,and why does it turn yellow again when the cloth is washed with plenty of water?

Whose acid-base theory can be applied to aqueous and non-aqueous solutions?

$(a)$ Study the following chemical equation:
$CaSO_4 \cdot 2H_2O \xrightarrow{373K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$
Name the reactant and the product. State one use of the product.

Which of the following gives the correct increasing order of acidic strength?

What is the concentration of $[H_3O^+]$ in distilled water at $298 \; K$?