Acids, bases, salts — pH scale, indicators, neutralization, salt preparation

Question

The pH of a solution is 3. Is it acidic, basic, or neutral? What happens when this solution is mixed with a solution of pH 11? Identify the type of salt formed if the acid is HCl and the base is NaOH.

(CBSE Class 10 — Acids, Bases and Salts)

Acid-Base Identification

flowchart TD
    A["Unknown Solution"] --> B{Test pH}
    B -->|"pH < 7"| C["Acidic"]
    B -->|"pH = 7"| D["Neutral"]
    B -->|"pH > 7"| E["Basic/Alkaline"]
    C --> F{Indicator tests}
    E --> F
    F -->|"Litmus: Red"| G["Confirms acid"]
    F -->|"Litmus: Blue"| H["Confirms base"]
    F -->|"Phenolphthalein: Pink"| I["Base present"]
    C --> J["Strong acid: pH 1-3"]
    C --> K["Weak acid: pH 4-6"]
    E --> L["Weak base: pH 8-10"]
    E --> M["Strong base: pH 11-14"]

Solution — Step by Step

The pH scale runs from 0 to 14. A pH of 3 is strongly acidic (below 7). The lower the pH, the higher the concentration of H $^+$ ions.

pH 3 means $[\text{H}^+] = 10^{-3}$ mol/L.

For comparison: lemon juice has pH about 2, vinegar about 3, pure water is 7.

A solution of pH 11 is strongly basic. When an acid and a base mix, they undergo neutralisation:

\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}

The H $^+$ ions from the acid combine with OH $^-$ ions from the base to form water. The resulting solution’s pH depends on the relative strengths and quantities. If mixed in the right proportions, the result is a neutral salt solution (pH 7).

\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)

HCl is a strong acid and NaOH is a strong base. Their salt, NaCl (common salt), is neutral — its aqueous solution has pH 7.

\boxed{\text{NaCl is a neutral salt with pH} = 7}

Types of salts based on parent acid and base:

Acid	Base	Salt Nature	pH
Strong	Strong	Neutral	7
Strong	Weak	Acidic	< 7
Weak	Strong	Basic	> 7
Weak	Weak	Depends on relative strength	Around 7

Why This Works

The pH scale is logarithmic — each unit change represents a tenfold change in H $^+$ concentration. pH 3 has 10 times more H $^+$ ions than pH 4. Neutralisation works because H $^+$ and OH $^-$ have a very strong tendency to combine and form water, which is stable and neutral.

The nature of the salt depends on which parent (acid or base) is stronger. The stronger parent “wins,” making the salt solution slightly acidic or basic.

Alternative Method — Indicator-Based Identification

Instead of pH meters, we can use indicators:

Indicator	In Acid	In Base	In Neutral
Litmus	Red	Blue	Purple
Phenolphthalein	Colourless	Pink	Colourless
Methyl orange	Red	Yellow	Orange
Universal indicator	Red/Orange	Blue/Violet	Green

CBSE loves asking: “Why does the colour of pH paper change when dipped in a solution?” The answer is that the dye molecules in the indicator react with H $^+$ or OH $^-$ ions, changing their molecular structure and therefore their colour. Universal indicator gives a continuous colour range — red for strong acid, green for neutral, violet for strong base.

Common Mistake

Students confuse “strong acid” with “concentrated acid.” A strong acid (HCl, H $_2$ SO $_4$ , HNO $_3$ ) is one that fully dissociates in water — every molecule releases H $^+$ . A concentrated acid simply has a lot of acid dissolved per unit volume. You can have a dilute solution of a strong acid (like 0.001 M HCl) — it is still a strong acid, just dilute. Similarly, acetic acid (vinegar) is a weak acid even if you make it very concentrated.