N.B.Nota Bene

Home  /  Chemistry  /  MYP 4

Acids and Bases

From the vinegar in your kitchen to the antacid tablet that settles a stomach, acids and bases are everywhere. The pH scale measures them, and neutralisation is the reaction that cancels one with the other.

MYP 4ChemistryAcids & BasesCriteria A · C~11 min read

Written by Fayek Jawad Haider · MYP student

The pH scale

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. Below 7 is acidic, exactly 7 is neutral (like pure water), and above 7 is alkaline. The further from 7, the stronger the acid or alkali.

Universal indicator is a mixture of dyes that turns a range of colours to match the pH: red for a strong acid, through orange and yellow, green at neutral, then blue and purple for stronger alkalis. You compare the colour against a chart to read off the pH.

pHNatureIndicator colourExample
1 to 3Strong acidRedStomach acid
4 to 6Weak acidOrange to yellowVinegar
7NeutralGreenPure water
8 to 10Weak alkaliBlueBaking soda solution
11 to 14Strong alkaliPurpleOven cleaner
pH scale
A scale from 0 to 14 measuring acidity: below 7 is acidic, 7 is neutral, and above 7 is alkaline.

Acids and alkalis

An acid is a substance that releases hydrogen ions, H+, when dissolved in water. That is what makes a solution acidic. Common lab acids are hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3).

A base is a substance that neutralises an acid, usually a metal oxide, hydroxide or carbonate. An alkali is simply a base that dissolves in water, releasing hydroxide ions, OH-. Sodium hydroxide (NaOH) is a typical alkali.

Base vs alkali

Every alkali is a base, but not every base is an alkali. The dividing line is solubility: a base that dissolves in water is called an alkali. Copper oxide is a base but not an alkali, because it does not dissolve.

Neutralisation

When an acid reacts with a base, they cancel each other out and the products are less acidic or alkaline. This is neutralisation. At heart it is the hydrogen ions from the acid joining the hydroxide ions from the alkali to make water:

\[ \text{H}^{+}(aq) + \text{OH}^{-}(aq) \rightarrow \text{H}_2\text{O}(l) \]

The other product is always a salt, an ionic compound where the hydrogen of the acid has been replaced by a metal (or ammonium) ion. The salt's name comes from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates.

The three reaction types

Three combinations come up again and again. Learn the general word equation for each, then the symbols follow.

1. Acid + base (or alkali) to salt + water.

Worked example

Write the word and balanced symbol equation for hydrochloric acid reacting with sodium hydroxide.

1
Word equation: hydrochloric acid + sodium hydroxide → sodium chloride + water.
2
Write the formulae: HCl + NaOH → NaCl + H2O.
3
Count atoms. Each side has 1 Na, 1 Cl, 1 O and 2 H, so it is already balanced.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

2. Acid + metal to salt + hydrogen. A reactive metal displaces the hydrogen in the acid, and hydrogen gas bubbles off (it gives a squeaky pop with a lit splint).

Worked example

Write the word and balanced symbol equation for magnesium reacting with hydrochloric acid.

1
Word equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen.
2
Write the formulae: Mg + HCl → MgCl2 + H2.
3
The right has 2 Cl and 2 H, the left only 1 each, so put a 2 before HCl: Mg + 2HCl → MgCl2 + H2.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

3. Acid + carbonate to salt + water + carbon dioxide. This one fizzes as carbon dioxide is released (it turns limewater cloudy).

Worked example

Write the word and balanced symbol equation for hydrochloric acid reacting with calcium carbonate.

1
Word equation: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide.
2
Write the formulae: HCl + CaCO3 → CaCl2 + H2O + CO2.
3
The right needs 2 Cl and 2 H, so put a 2 before HCl: 2HCl + CaCO3 → CaCl2 + H2O + CO2. Now every element balances.
2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g)

Where this is assessed

Predicting the products from the reaction type is Criterion A. Using the gas tests (squeaky pop for hydrogen, cloudy limewater for carbon dioxide) as evidence to identify a product is exactly the observe-and-conclude skill of Criterion B and C in a practical.

Check yourself

1. A solution turns universal indicator orange. What can you say about its pH and nature? +

Orange sits between red and yellow, so the pH is about 4 to 6. The solution is a weak acid. It is acidic but not as strongly as something that turns the indicator red.

2. Write the balanced symbol equation for sulfuric acid reacting with magnesium. +

This is acid + metal to salt + hydrogen. Sulfuric acid makes sulfates, so: Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g). Each side has 1 Mg, 2 H, 1 S and 4 O, so it is balanced.

3. You add an acid to a white solid and it fizzes, giving a gas that turns limewater cloudy. What type of compound is the solid, and what is the gas? +

The fizzing and the cloudy limewater show carbon dioxide is being released, so the solid is a carbonate. The reaction is acid + carbonate to salt + water + carbon dioxide, and the gas is carbon dioxide.

Part of the Chemistry library. Spotted an error or want a topic added? That feedback makes the notes better.