Investigate the properties of acids and bases and the historical development of the Arrhenius model of acids and bases

What this dot point is asking

NESA wants you to describe the physical and chemical properties shared by acids and bases, recall the Arrhenius model and the reasoning behind it, and explain why later models (Bronsted-Lowry, Lewis) had to extend it. This is the entry point to Module 6 and the foundation for every later calculation, including reactions of acids, strong vs weak ionisation, and Bronsted-Lowry conjugate pairs.

The answer

Observed properties of acids

• Taste sour (citric acid in lemons, ethanoic acid in vinegar). Never taste laboratory chemicals.
• Turn blue litmus red.
• React with active metals (Mg, Zn, Fe) to produce hydrogen gas.
• React with metal carbonates and hydrogencarbonates to produce $CO_2$.
• React with bases to form a salt and water (neutralisation).
• Aqueous solutions conduct electricity (they are electrolytes).
• Have pH less than 7 at 25 degrees C.

Observed properties of bases

• Taste bitter and feel soapy or slippery (do not test by taste or touch).
• Turn red litmus blue.
• React with acids to form a salt and water.
• React with ammonium salts to release ammonia gas.
• Aqueous solutions conduct electricity.
• Have pH greater than 7 at 25 degrees C.

Indicators

An indicator is a weak acid or weak base whose protonated and deprotonated forms have different colours. The colour change occurs across a narrow pH range, usually about 2 pH units wide.

Universal indicator is a mixture of indicators that gives a continuous colour scale from red (very acidic) to violet (very basic).

The Arrhenius model

Svante Arrhenius (1887) proposed that acids and bases are substances that ionise in water.

• Arrhenius acid: a substance that releases $H^+$ in aqueous solution.
• Arrhenius base: a substance that releases $OH^-$ in aqueous solution.
• Neutralisation: $H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$.

The model elegantly explained why all aqueous acids share the same chemistry (because they all produce the same ion, $H^+$) and why all aqueous bases share the same chemistry (because they all produce $OH^-$).

Historical development

The Arrhenius model built on earlier ideas:

• Lavoisier (1780s). Believed all acids contained oxygen (the name "oxygen" means "acid former"). Disproved when Humphry Davy showed that hydrochloric acid contains no oxygen.
• Davy (1810). Proposed that hydrogen is the essential element in acids.
• Liebig (1838). Refined Davy's idea: an acid is a hydrogen-containing compound whose hydrogen can be replaced by a metal.
• Arrhenius (1887). Provided the ionic explanation: acids dissociate in water to give $H^+$.

This progression is a classic example of how a scientific model is refined as new evidence (electrolysis, conductivity, ionic theory) becomes available.

Limitations of the Arrhenius model

Arrhenius works well for simple aqueous acid-base reactions, but it cannot explain:

1. Basic species without hydroxide. Ammonia ($NH_3$) turns litmus blue and reacts with acids, yet contains no $OH^-$. Arrhenius cannot account for its basicity.
2. Non-aqueous acid-base chemistry. $HCl$ reacts with $NH_3$ in the gas phase to form $NH_4Cl$ with no water involved.
3. The hydrated proton. The bare $H^+$ ion never exists in water; it always attaches to a water molecule to form $H_3O^+$. Arrhenius treats $H^+$ as a free species.
4. Acid-base behaviour of salts. Solutions of $NH_4Cl$ are slightly acidic and solutions of $CH_3COONa$ are slightly basic, yet Arrhenius offers no mechanism for these effects.

These limitations motivated the Bronsted-Lowry model (1923), which defines acids as proton donors and bases as proton acceptors.

Examples in context

Example 1. Acid sulfate soils in the Tweed-Richmond floodplain. Coastal floodplains in northern NSW contain pyrite ($FeS_2$) buried in waterlogged sediment. When agricultural drainage exposes the pyrite to oxygen it forms sulfuric acid: $FeS_2 + \frac{7}{2}O_2 + H_2O \rightarrow Fe^{2+} + 2SO_4^{2-} + 2H^+$. The released acid drops creek pH below 3, killing fish at the Yamba prawn fishery and corroding concrete drains. NSW DPI monitors with blue litmus paper for quick screening, with pH probes for quantitative work. The properties students learn (acids release $H^+$, lower pH, conduct, react with carbonates) explain every observation in the field, including the white limestone treatment beds installed to neutralise drainage downstream.

Example 2. Drain cleaner manufactured at Bunnings supplier near Smithfield. Domestic drain cleaners sold across NSW contain 30 to 50 percent sodium hydroxide pellets. When dissolved the pellets give $Na^+_{(aq)} + OH^-_{(aq)}$, a textbook Arrhenius base. The solution turns red litmus blue, conducts strongly, feels slippery on skin (saponification of skin fats), and neutralises acidic blockages by hydrolysing fats and proteins. Safety labelling references the corrosive nature explicitly. Arrhenius theory explains the bulk behaviour cleanly but cannot explain why aqueous ammonia bottles on the same shelf are also basic, which is precisely the reason Bronsted-Lowry was developed to extend the framework.

Try this

Q1. List four observable properties of acids and four of bases, and state how the Arrhenius model accounts for each. [4 marks]

• Cue. Acids: sour, conduct, redden blue litmus, react with metals/carbonates; release $H^+$. Bases: bitter, slippery, turn red litmus blue, react with acids; release $OH^-$.

Q2. Calculate the pH of a 0.0250 mol L$^{-1}$ solution of $Ca(OH)_2$, a strong diprotic base. [3 marks]

• Cue. $[OH^-] = 2 \times 0.0250 = 0.0500$, $pOH = 1.30$, $pH = 12.70$.

Q3. Discuss the limitations of the Arrhenius model. (a) State one species that is basic but contains no hydroxide. (b) Identify one solvent in which Arrhenius theory does not apply. (c) Explain why Bronsted-Lowry theory was a necessary extension. [1+1+2 marks]

• Cue. (a) Ammonia. (b) Liquid ammonia or any non-aqueous solvent. (c) Bronsted-Lowry defines acids and bases by proton transfer, accommodating non-aqueous solvents and species without hydroxide.