# Properties of Acids and Bases

> What acids and bases do, indicators, the reactions of acids, neutralisation and salts, the enthalpy of neutralisation, and how the ideas of Lavoisier, Davy and Arrhenius changed over time.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/properties-of-acids-and-bases/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The red cabbage test
Zara's science teacher boils a red cabbage, pours the purple juice into six glasses, and adds a few drops of something from the kitchen to each. Lemon juice turns one bright pink. Vinegar turns another red. Baking soda turns a third blue-green. A drop of drain cleaner turns one yellow.

The cabbage juice is an **indicator**. It changes colour depending on whether a liquid is an acid or a base. Chemists have argued for more than 200 years about what an acid really is. This chapter follows that argument, and shows what acids and bases do.

**In short:** Acids have a pH below 7 and turn litmus red, while bases have a pH above 7 and turn litmus blue. Acids react with metals to give a salt and hydrogen, with carbonates to give a salt, water and carbon dioxide, and with bases to give a salt and water (neutralisation).

## Properties of acids and bases

| | Acids | Bases |
|---|---|---|
| Taste (never test!) | Sour | Bitter |
| Feel | Sting | Slippery, soapy |
| Litmus | Turns **red** | Turns **blue** |
| pH | Below 7 | Above 7 |
| Conduct electricity in water | Yes | Yes (if soluble) |
| Examples | $\mathrm{HCl}$, vinegar, citric acid | $\mathrm{NaOH}$, ammonia, baking soda |

A base that **dissolves** in water is called an **alkali**.

Both acids and bases are **corrosive** in concentrated form. Wear safety glasses.

## Indicators

An indicator changes colour over a certain pH range.

| Indicator | Colour in acid | Colour change range (pH) | Colour in base |
|---|---|---|---|
| Methyl orange | Red | 3.1 to 4.4 | Yellow |
| Bromothymol blue | Yellow | 6.0 to 7.6 | Blue |
| Phenolphthalein | Colourless | 8.3 to 10.0 | Pink |
| Litmus | Red | 4.5 to 8.3 | Blue |

**Universal indicator** is a mixture that shows a range of colours, from red (pH 1) through green (pH 7) to purple (pH 14). Natural indicators include red cabbage, beetroot and some flower petals.

## Reactions of acids

Learn these three general equations:

- acid + **metal** → salt + hydrogen gas
  $\mathrm{Mg(s)} + \mathrm{2HCl(aq)} \rightarrow \mathrm{MgCl_2(aq)} + \mathrm{H_2(g)}$
- acid + **carbonate** → salt + water + carbon dioxide
  $\mathrm{CaCO_3(s)} + \mathrm{2HCl(aq)} \rightarrow \mathrm{CaCl_2(aq)} + \mathrm{H_2O(l)} + \mathrm{CO_2(g)}$
- acid + **base** → salt + water (**neutralisation**)
  $\mathrm{HCl(aq)} + \mathrm{NaOH(aq)} \rightarrow \mathrm{NaCl(aq)} + \mathrm{H_2O(l)}$

Tests for the gases:

- **Hydrogen** makes a "pop" with a lit splint.
- **Carbon dioxide** turns limewater milky.

The net ionic equation for every strong acid + strong base neutralisation is the same:

$$\mathrm{H^+(aq)} + \mathrm{OH^-(aq)} \rightarrow \mathrm{H_2O(l)}$$

## Salts are not always neutral

A **salt** is the ionic compound made in a neutralisation. Its solution can be neutral, acidic or basic:

| Made from | Example | Solution |
|---|---|---|
| Strong acid + strong base | $\mathrm{NaCl}$, $\mathrm{KNO_3}$ | Neutral (pH 7) |
| Strong acid + weak base | $\mathrm{NH_4Cl}$ | Acidic |
| Weak acid + strong base | $\mathrm{CH_3COONa}$, $\mathrm{Na_2CO_3}$ | Basic |

So **neutralisation does not always give a neutral solution**. The reason (the ions react with water) is explained in the next chapter.

## Enthalpy of neutralisation

Neutralisation is **exothermic**. The **molar enthalpy of neutralisation** is the heat released per mole of water formed. For a strong acid and a strong base it is about **−57 kJ/mol**.

Measure it with a **calorimeter** (a polystyrene cup with a lid and thermometer):

$$q = mc\Delta T \qquad \Delta H = -\frac{q}{n}$$

- $m$ = mass of the **whole mixture** (1 mL of solution ≈ 1 g)
- $c$ = 4.18 J g⁻¹ K⁻¹ (or $4.18 \times 10^3$ J kg⁻¹ K⁻¹)
- $n$ = moles of water formed (moles of the limiting reactant)

### Worked example: A coffee-cup calorimeter
50 mL of 1.0 mol/L HCl is mixed with 50 mL of 1.0 mol/L NaOH. The temperature rises by 6.7 °C.

$q = 100 \times 4.18 \times 6.7 = 2801$ J

$n = 1.0 \times 0.050 = 0.050$ mol of water

$\Delta H = -\dfrac{2801}{0.050} = -56\,000$ J/mol $= -56$ kJ/mol

The measured value is usually a little **smaller** than the accepted value because some heat escapes to the cup, thermometer and air.

## How the idea of an acid changed

| Scientist | Year | Idea | Problem with it |
|---|---|---|---|
| **Lavoisier** | 1776 | Acids contain **oxygen** | $\mathrm{HCl}$ has no oxygen |
| **Davy** | 1810 | Acids contain **replaceable hydrogen** | Did not explain bases |
| **Arrhenius** | 1887 | Acids **ionise** in water to give $\mathrm{H^+}$; bases give $\mathrm{OH^-}$ | Only works in water; cannot explain $\mathrm{NH_3}$ as a base |
| **Brønsted and Lowry** | 1923 | Acids **donate protons**; bases **accept protons** | (next chapter) |

Each model kept what worked and fixed a problem with the one before. This is how science improves.

**Mind map**

```text
Acids and bases
  Properties
    Acid: sour, litmus red
    Base: slippery, litmus blue
    Alkali: soluble base
  Indicators
    Methyl orange
    Bromothymol blue
    Phenolphthalein
  Acid reactions
    Metal: salt + H2
    Carbonate: salt + water + CO2
    Base: salt + water
  Salts
    Neutral, acidic or basic
  Neutralisation
    Exothermic, about −57 kJ/mol
    q = mcΔT
  History
    Lavoisier: oxygen
    Davy: hydrogen
    Arrhenius: H+ and OH−
```

### Year 11: Gas volumes from acid reactions
At 25 °C and 100 kPa, one mole of any gas takes up **24.79 L** (from the data sheet).

**Example.** 1.22 g of magnesium reacts with excess hydrochloric acid. What volume of hydrogen forms?

$n(\mathrm{Mg}) = 1.22 \div 24.31 = 0.0502$ mol. The ratio $\mathrm{Mg} : \mathrm{H_2}$ is 1 : 1.

$V = 0.0502 \times 24.79 = 1.24$ L.

In a real experiment the volume is often less, because some gas escapes before the bung goes in, or the magnesium had an oxide coating.

**Purity from a gas volume.** 5.00 g of impure zinc gives 1.70 L of hydrogen. $n(\mathrm{H_2}) = 1.70 \div 24.79 = 0.0686$ mol, which is also the moles of zinc (1 : 1). Mass of zinc $= 0.0686 \times 65.38 = 4.48$ g, so the sample is $\dfrac{4.48}{5.00} \times 100 = 89.7\%$ zinc.

### Year 12: Harder neutralisation enthalpy problems
**Find the limiting reactant first.** 30.0 mL of 0.700 mol/L NaOH is mixed with 20.0 mL of 0.900 mol/L HCl, both at 21.5 °C. $\Delta H = -57.1$ kJ/mol.

NaOH: 0.0210 mol. HCl: 0.0180 mol. HCl is limiting, so 0.0180 mol of water forms.

$q = 0.0180 \times 57\,100 = 1028$ J. The mixture has a mass of 50.0 g.

$\Delta T = \dfrac{1028}{50.0 \times 4.18} = 4.92$ °C, so the final temperature is $21.5 + 4.9 = 26.4$ °C.

**Weak acids give a smaller value.** Neutralising a weak acid such as acetic acid releases less than 57 kJ/mol, because some energy is used to ionise the acid first.

**Diprotic acids.** One mole of $\mathrm{H_2SO_4}$ makes two moles of water with NaOH, so it releases about twice as much heat as one mole of HCl.

**Excess acid does not add heat.** 50.0 mL of 2.0 mol/L HCl with 50.0 mL of 1.0 mol/L NaOH makes only 0.050 mol of water, because NaOH runs out. Divide $q$ by 0.050, not 0.10.

**Two heats at once.** Solid NaOH gives out heat when it dissolves **and** when it is neutralised. 2.00 g of NaOH is 0.0500 mol, so $q = 0.0500 \times (44.5 + 57.1) = 5.08$ kJ. In 100 g of solution, $\Delta T = \dfrac{5080}{100 \times 4.18} = 12.2$ °C.

## Quick summary

- Acids turn litmus red and have pH below 7. Bases turn litmus blue and have pH above 7.
- Indicators change colour over set pH ranges.
- Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + carbon dioxide. Acid + base → salt + water.
- Salts can be neutral, acidic or basic.
- Neutralisation is exothermic, about −57 kJ/mol for strong acid + strong base. Use $q = mc\Delta T$ and $\Delta H = -q/n$.
- Lavoisier (oxygen), Davy (hydrogen), Arrhenius (ions in water), then Brønsted-Lowry (protons).

## Quiz

Try these questions from the chapter quiz.

1. An acid turns blue litmus:
   - A. Red
   - B. Blue
   - C. Green
   - D. Colourless
   - Answer: A. Red. Acids turn litmus red.

2. A base that dissolves in water is called:
   - A. A salt
   - B. An alkali
   - C. An indicator
   - D. An ester
   - Answer: B. An alkali. Soluble bases are alkalis.

3. Phenolphthalein in a strong base is:
   - A. Colourless
   - B. Pink
   - C. Yellow
   - D. Red
   - Answer: B. Pink. It is colourless in acid and pink in base.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/properties-of-acids-and-bases/#quiz