Grade by Grade
C. Chemistry: Chemical Reactions · Chemical Reactions

Acid-Base Neutralization

C3.6 C3.4 C2.3 C2.6 A1.5 A1.8 A1.10About 70 minutes
Learning goals
  • I can describe neutralization as an acid reacting with a base to form a salt and, often, water.
  • I can predict the salt formed and write balanced neutralization equations.
  • I can explain neutralization using H⁺ and OH⁻ ions and describe how pH changes during a titration.
  • I can carry out an inquiry to compare how much base is needed to neutralize an acid.

Words to know

neutralization
A reaction between an acid and a base that produces a salt and often water.
salt
An ionic compound made from the positive ion of a base and the negative ion of an acid.
antacid
A base taken to neutralize extra stomach acid.
titration
Adding one solution a little at a time to another until a reaction (such as neutralization) is just complete.
end point
The point in a titration when the indicator changes colour.

Cancelling each other out

Have you ever had heartburn after a big meal? An antacid tablet helps because it contains a base that reacts with extra stomach acid. A farmer whose soil is too acidic spreads lime (a base) on the field. A lab technician who spills an acid sprinkles on sodium hydrogen carbonate. In each case an acid and a base react and neutralize each other.

In this lesson you will describe the process of acid-base neutralization (expectation C3.6), write equations for it, and investigate it yourself.

Acid + base → salt + water

In a neutralization reaction, an acid reacts with a base to form a salt and, often, water:

acid + base → salt + water

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

hydrochloric acid + sodium hydroxide → sodium chloride + water

In chemistry, a salt is any ionic compound made from the positive ion of a base and the negative ion of an acid. Table salt (NaCl) is just one example; others are KNO₃, CaSO₄ and MgCl₂.

Look closely and you will see that neutralization is a double displacement reaction: H⁺ and Na⁺ swap partners. The product that makes it happen is water. The heart of every acid-hydroxide neutralization is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The hydrogen ions that made the solution acidic and the hydroxide ions that made it basic join to form water, so the solution moves toward pH 7. Neutralization is also exothermic: the solution warms up.

HCl(aq)H⁺Cl⁻+NaOH(aq)Na⁺OH⁻NaCl(aq)Na⁺Cl⁻+H₂O(l)OHHThe key step: H⁺ + OH⁻ → H₂ONa⁺ and Cl⁻ do not change; they are left in solution as the salt.
Neutralization at the level of ions. Hydrogen ions from the acid and hydroxide ions from the base combine into water. The other ions form the salt.

Writing neutralization equations

  1. Write the formulas of the acid and the base.
  2. The salt is made of the metal (or ammonium) ion from the base and the negative ion from the acid. Use the charges to write its formula.
  3. The other product is water.
  4. Balance. Tip: the number of H⁺ from the acid must equal the number of OH⁻ from the base, and that number is the number of water molecules.

Not always water. The curriculum says a salt and "often" water. Two common cases:

  • Ammonia is a base without OH⁻ in its formula: HCl(aq) + NH₃(aq) → NH₄Cl(aq). The only product is a salt, ammonium chloride.
  • Carbonates and hydrogen carbonates react with acids to give a salt, water and carbon dioxide gas: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). This is how many antacids (calcium carbonate) and baking soda work, and why they fizz.
Worked example

Example 1: Sulfuric acid and potassium hydroxide

Write the word equation and balanced equation for the neutralization of sulfuric acid by potassium hydroxide.

  1. Acid: H₂SO₄ (2 H⁺ and SO₄²⁻). Base: KOH (K⁺ and OH⁻).
  2. Salt: K⁺ and SO₄²⁻ → K₂SO₄ (potassium sulfate). Other product: water.
  3. Skeleton: H₂SO₄ + KOH → K₂SO₄ + H₂O.
  4. The acid gives 2 H⁺, so we need 2 OH⁻ → 2KOH. Two H⁺ + two OH⁻ make 2H₂O.
  5. Check: K 2 = 2; S 1 = 1; H 2 + 2 = 4 = 4; O 4 + 2 = 6 = 4 + 2. ✔

sulfuric acid + potassium hydroxide → potassium sulfate + water; H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

Worked example

Example 2: An antacid

Milk of magnesia contains magnesium hydroxide, Mg(OH)₂. Write the balanced equation for its reaction with stomach acid (HCl).

  1. Salt: Mg²⁺ and Cl⁻ → MgCl₂. Other product: water.
  2. Mg(OH)₂ has 2 OH⁻, so it needs 2 H⁺ → 2HCl, making 2H₂O.

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l). The extra stomach acid is used up, so the pH in the stomach rises.

Worked example

Example 3: Name the acid and base

Which acid and which base would you react to make the salt calcium nitrate, Ca(NO₃)₂? Write the balanced equation.

  1. The positive ion Ca²⁺ comes from the base: calcium hydroxide, Ca(OH)₂.
  2. The negative ion NO₃⁻ comes from the acid: nitric acid, HNO₃.
  3. Ca(OH)₂ has 2 OH⁻, so use 2HNO₃ and make 2H₂O.

2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

Watching neutralization happen: titration

Chemists use a technique called titration to find out how much base is needed to neutralize an acid (or the reverse). A base is added a little at a time to a measured amount of acid that contains an indicator. When the indicator changes colour, the acid has just been neutralized. That moment is called the end point.

The graph below shows what happens to the pH as sodium hydroxide solution is slowly added to hydrochloric acid. At first the pH rises very slowly. Near the neutralization point, a single drop of base makes the pH jump by several units. After that, there is extra base and the pH levels off at a high value. You will do calculations with titrations in Grade 11 and 12 chemistry.

0246810121401020304050volume of sodium hydroxide solution added (mL)pHneutralneutralization point:acid and base used up exactlyacid in excessbase in excess
How pH changes as sodium hydroxide solution is added to a sample of hydrochloric acid (here the acid is neutralized by 25 mL of base). The steep jump is why one drop can change an indicator's colour.
Hands-on activity

Inquiry: how much base does it take?

Question: Which neutralizes more vinegar: a teaspoon of baking soda or one antacid tablet (calcium carbonate type)?

You need: a grown-up, eye protection, red cabbage indicator (from the pH lesson), white vinegar, baking soda, a calcium carbonate antacid tablet (check the label: the active ingredient should be calcium carbonate), 2 clear glasses, measuring spoons, a medicine dropper or 1/4-teaspoon measure, a spoon for stirring, paper and pencil.

Safety: eye protection on. The antacid is a medicine: the grown-up keeps the package and nobody swallows it. Do not drink anything you have tested.

  1. Plan: write a prediction and identify your independent, dependent and controlled variables.
  2. Make the base solutions: dissolve 1 teaspoon of baking soda in 1/2 cup of water in one glass; crush the antacid tablet and stir it into 1/2 cup of water in the other (it will be cloudy, because calcium carbonate does not dissolve well: stir before each addition).
  3. Put 2 tablespoons of vinegar and 2 tablespoons of cabbage indicator in a clean glass. It should be pink or red.
  4. Add the baking soda solution 1/4 teaspoon (or 10 drops) at a time, stirring after each addition. Count the additions until the colour changes to purple (about neutral). Record the number.
  5. Rinse the glass and repeat with fresh vinegar and indicator, using the antacid mixture.
  6. Repeat each test once more to check that your results are reliable.

What to look for: both mixtures fizz (carbon dioxide from a carbonate reacting with acid) and the colour moves from pink toward purple and then blue-green if you add too much. The solution that needs fewer additions to reach purple contains more base per spoonful. Results depend on the brand and on how well you stir. In your conclusion, write the word equations: acetic acid + sodium hydrogen carbonate → sodium acetate + water + carbon dioxide; acetic acid + calcium carbonate → calcium acetate + water + carbon dioxide. Clean-up: pour everything down the sink with lots of water.

Just for fun

An acid and a base walked into a café. Things got a little heated (neutralization is exothermic), but they worked out their differences and left together as salt water.

Your turn

1

Write the general word equation for neutralization.

2

What is a salt, in chemistry?

3

Complete and balance: HNO₃ + NaOH →

4

Complete and balance: HCl + KOH →

5

Write the ionic equation that is the key step in every acid-hydroxide neutralization.

6

What happens to the pH of an acid as a base is slowly added?

7

Why does an antacid tablet that contains calcium carbonate fizz in acid?

More practice: warm-up

1

What are the products of neutralizing hydrochloric acid with sodium hydroxide?

2

Is neutralization exothermic or endothermic?

3

What is the pH of a perfectly neutralized solution of a strong acid and a strong base?

4

Name a base used as an antacid.

5

In titration, what tells you the end point has been reached?

More practice: on your own

1

Complete and balance: H₂SO₄ + NaOH →

2

Complete and balance: HCl + Ca(OH)₂ →

3

Which acid and base would you use to make potassium nitrate? Write the equation.

4

Complete and balance: HCl + NaHCO₃ →

5

Explain why neutralization is a kind of double displacement reaction.

Challenge

1

Phosphoric acid (H₃PO₄) is neutralized by sodium hydroxide. Write the balanced equation.

2

A lab spill kit for acids contains sodium hydrogen carbonate rather than sodium hydroxide. Suggest two reasons why.

3

In the inquiry, Student A needed 6 additions of baking soda solution and Student B needed 9 additions for the same vinegar. Give two possible reasons other than the base itself.

Check your understanding

Quiz

1.

What are the products of a reaction between an acid and a hydroxide base?

2.

Which is the balanced equation for nitric acid reacting with potassium hydroxide?

3.

What is the 'key step' in an acid-hydroxide neutralization?

4.

Which salt forms when sulfuric acid is neutralized by sodium hydroxide?

5.

Which neutralization does NOT produce water?

6.

Why does the pH jump sharply near the end of a titration?

For parents and teachers

Big idea: C3.6 asks students to describe acid-base neutralization ("an acid reacts with a base to form a salt and often water"). Students should be able to predict the salt, write and balance neutralization equations, explain the ionic key step (H⁺ + OH⁻ → H₂O), and recognize everyday uses (antacids, liming soil and lakes, spill kits). Titration is introduced qualitatively; calculations come in Grade 11 and 12.

Common difficulties:

  • Thinking "salt" only means table salt.
  • Building the salt from the wrong ions. The salt is the base's positive ion + the acid's negative ion.
  • Balancing: the number of H⁺ from the acid must match the number of OH⁻ from the base.
  • Forgetting CO₂ when a carbonate or hydrogen carbonate is the base.

Safety at home: the inquiry uses vinegar, baking soda and a calcium carbonate antacid. Please keep the antacid package yourself, make sure eye protection is worn, and that nothing is swallowed. Do not use drain cleaner or other strong bases.