When an acidic solution and a basic solution are mixed, hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base meet one to one and become water. This is called neutralization.
Add sodium hydroxide solution little by little to dilute hydrochloric acid and the H⁺ ions turn into water one at a time; the moment the numbers of H⁺ and OH⁻ become equal is the neutralization point. Neutralization gives off heat, so the solution is warmest at that point.
The sodium ions (Na⁺) and chloride ions (Cl⁻) take no part in the reaction, and when the water is evaporated they are left behind as sodium chloride, ordinary salt.
The beaker below holds hydrochloric acid with BTB indicator added, so the ion counts, the colour and the titration curve all change with the amount of sodium hydroxide solution you add.
Watching neutralization happen
⚗️ H⁺ and OH⁻ meet and become water
①In water an acid releases H⁺ and a base releases OH⁻
②When the two solutions are mixed, H⁺ and OH⁻ join one to one to form water (H₂O)
③Heat is given off, so neutralization is an exothermic reaction
④The beaker in the picture starts with 10 H⁺; each 1 mL of sodium hydroxide solution adds one OH⁻ and removes one H⁺, so the neutralization point comes at 10 mL
⑤Na⁺ and Cl⁻ do not react and simply stay in the solution
seegongsik.com
A beaker of hydrochloric acid with BTB indicator, showing how the numbers of H⁺, OH⁻ and water molecules and the colour change as sodium hydroxide solution is added
10 mL
pH change on a titration curve
📈 The pH leaps near the neutralization point
①The hydrochloric acid starts at about pH 1, strongly acidic
②As long as some H⁺ is left, adding sodium hydroxide solution raises the pH only very slowly
③Within a drop or two either side of the neutralization point the pH jumps from about 3 to about 11
④Past the neutralization point OH⁻ is left over and the solution is basic, and BTB changes from yellow through green to blue
seegongsik.com
The pH change (titration curve) when sodium hydroxide solution of the same concentration is added to 10 mL of 0.1 M hydrochloric acid, with the neutralization point
Neutralization in everyday life
🌿 Matching acids and bases
①Antacids, taken for heartburn caused by too much stomach acid, contain a basic substance that neutralizes the acid
②Acidified soil and lakes are treated with lime, which is basic
③The substances behind the smell of fish are basic, so a squeeze of lemon juice or a splash of vinegar reduces the smell
④Brushing with toothpaste, which is basic, neutralizes the acid that bacteria make from food left in the mouth
Summary
The heart of neutralization
H⁺ + OH⁻ → H₂O
Hydrogen ions from the acid and hydroxide ions from the base react one to one to form water
General equation for neutralization
acid + base → water + salt
Example: HCl + NaOH → H₂O + NaCl
Heat of neutralization
neutralization → heat is released (exothermic)
The temperature is highest at the neutralization point, where the most H⁺ and OH⁻ have reacted
🎯 Points that often come up
①The net ionic equation for neutralization is H⁺ + OH⁻ → H₂O
②At the neutralization point the number of H⁺ from the acid equals the number of OH⁻ from the base
③Neutralization is exothermic, and the temperature is highest at the neutralization point
④The pH changes sharply near the neutralization point, and BTB goes yellow → green → blue
⑤Na⁺ and Cl⁻ do not react and stay in the solution; evaporating the water leaves the salt NaCl
Worked examples and unit test
Example 1
What reaction takes place when sodium hydroxide solution (NaOH) is added to dilute hydrochloric acid (HCl)? Write the net ionic equation and the full equation.
1
H⁺ from the hydrochloric acid and OH⁻ from the sodium hydroxide solution meet and form water. The net ionic equation is H⁺ + OH⁻ → H₂O.
2
Including the Na⁺ and Cl⁻ that remain, the full equation is HCl + NaOH → H₂O + NaCl.
▸ H⁺ + OH⁻ → H₂O; HCl + NaOH → H₂O + NaCl
Use a different acid or a different base and the particles that actually react are still H⁺ and OH⁻.
Example 2
What condition holds at the neutralization point, and what happens to the temperature of the solution there?
1
At the neutralization point the number of H⁺ released by the acid equals the number of OH⁻ released by the base, so neither is left over.
2
Neutralization gives off heat, so the temperature is highest at the neutralization point, where the largest number of H⁺ and OH⁻ have reacted.
▸ The numbers of H⁺ and OH⁻ are equal, and the solution is at its highest temperature
Add more sodium hydroxide solution past this point and nothing more reacts; cooler solution is simply mixed in, so the temperature slowly falls.
Unit Test
Which statement about neutralization is correct?
①In neutralization, H⁺ and OH⁻ meet and form oxygen
②At the equivalence point the number of H⁺ equals the number of OH⁻
③Neutralization is an endothermic reaction that absorbs heat
④When an acid and a base react, another acid is formed
⑤At the equivalence point the temperature of the solution is lowest
▸ ② At the equivalence point the number of H⁺ equals the number of OH⁻
1
In neutralization H⁺ + OH⁻ → H₂O forms water (① wrong, not oxygen), and neutralization is exothermic (③ wrong).
2
Acid + base → water + salt is formed (④ wrong, not another acid), and being exothermic, the temperature peaks at the equivalence point (⑤ wrong); there the number of H⁺ equals the number of OH⁻ (② correct).