6 min readConcept sheet1-mark MCQ and assertion-reason; 2-mark question on why acids conduct or how to dilute them

Acids and bases in water

In water, acids give H⁺ ions (present as H₃O⁺) and bases give OH⁻ ions. Without water, these ions do not form.

Acid in waterHCl + H₂O → H₃O⁺ + Cl⁻

Base in waterNaOH(s) → Na⁺(aq) + OH⁻(aq)

AlkaliA base that dissolves in water, e.g. NaOH, KOH

Dilution ruleAdd acid to water slowly, never water to acid

Activity: do acids conduct electricity?

1
Fix two nails in a cork and place it in a beaker. Connect the nails to a 6 V battery through a bulb and a switch.
2
Pour dilute HCl into the beaker and switch on.
The bulb glows: the solution has ions
3
Repeat with glucose solution and alcohol solution.
The bulb does not glow: no ions form
4★
Conclusion: acids give H⁺(aq) ions in water, and these ions carry the current.

★ key step

What happens in water

HCl gives H⁺ and Cl⁻ ions. The H⁺ joins a water molecule: H⁺ + H₂O → H₃O⁺, the hydronium ion.
H⁺ cannot exist alone, so it is always written as H⁺(aq) or H₃O⁺.
Dry HCl gas does not change the colour of dry blue litmus. Moist litmus turns red, because water is needed to form H⁺ ions.
Bases give OH⁻ ions: NaOH(s) → Na⁺(aq) + OH⁻(aq). KOH(s) → K⁺(aq) + OH⁻(aq). Mg(OH)₂(s) → Mg²⁺(aq) + 2OH⁻(aq).
All alkalis are bases, but not all bases are alkalis. Copper oxide is a base, but it does not dissolve in water.

Diluting an acid safely

1
Take water in a beaker.
2
Add the concentrated acid slowly, a little at a time, while stirring.
3
Feel the beaker: it becomes warm.
Mixing an acid with water is exothermic
4★
Never add water to concentrated acid.
The heat can make the mixture splash out and burn you; the glass may also crack
5
After dilution, the concentration of H₃O⁺ ions per unit volume decreases.

★ key step

How to draw the conductivity set-up

Draw a beaker holding the acid solution.
Draw a cork in the beaker with two iron nails fixed in it, dipping into the solution.
Join the nails with wires to a 6 V battery, a bulb and a switch, all in one loop.
Show the bulb glowing for dilute HCl.
Label: beaker, dilute HCl, cork, nails, bulb, switch, 6 V battery.

In real life

Rain water conducts electricity a little, because gases dissolved in it form ions. That is one reason never to touch switches or plugs with wet hands.

Exam angle

1 marks: “Rahul passes dry HCl gas over dry blue litmus paper and sees no change. What must he do to see a colour change, and why?” Moisten the litmus paper. HCl gives H⁺ ions only in water, so it shows acidic behaviour only when water is present.

2 marks: “A bulb glows when two nails joined to a battery are dipped in dilute sulphuric acid. Explain why, naming the ions.” In water, H₂SO₄ gives H⁺(aq) ions (present as H₃O⁺) and SO₄²⁻ ions. These moving ions carry the electric current, so the bulb glows.

2 marks: “A label on a bottle of concentrated sulphuric acid says: add acid to water, never water to acid. Explain the reason.” The process is highly exothermic. The mixture may boil, splash acid out and cause burns, and the glass container may break.

Remember it

No water, no ions, no acid action. For dilution: "Do as you oughta, add acid to water."