Which substance has the lowest pH?

NaOH

Tap water

Lemon juice

Blood

Lemon juice has lowest pH.

Explanation:

pH is the negative log of hydrogen ion concentration. By increasing the Hydrogen ion concentration pH will decrease.

pH = - log [H⁺]

According to pH scale,

The pH of 7 is consider neutral. The pH below the 7 is consider acidic and above the 7 is basic.

The sodium hydroxide is strongest base because it fully dissociate into ions.

It dissociate to gives the sodium and hydroxyl ions and solution became more basic.

NaOH → Na⁺ (aq) + OH⁻ (aq)

Its pH is 12. It is also known as caustic soda.

The pH of blood is vary from 7.35 - 7.45. it is slightly alkaline.

The pH of lemon juice is ranging between 2-3.

The pH of tap water is ranging between 6-8.5.

From above given information it can be noticed that lemon juice have lowest pH i. e 2-3. It is acidic, because its pH is less than 7.