Properties of Acids and Bases

What makes an acid an acid. An acid is a substance that furnishes $\text{H}^+$ ions when dissolved in water. Because a bare proton cannot exist freely in solution, it immediately attaches to a water molecule to form the hydronium ion, so the more precise statement is:

where: $\text{HCl}$ is the acid, $\text{H}_2\text{O}$ is the solvent that accepts the proton, and $\text{H}_3\text{O}^+$ (hydronium) is the ion that actually does the acid chemistry — turning litmus red, reacting with metals, and so on.

What makes a base a base. A base is a substance that furnishes $\text{OH}^-$ ions in aqueous solution. A base that dissolves readily in water is further called an alkali — so sodium hydroxide and calcium hydroxide are both bases and alkalis, but insoluble bases such as copper hydroxide or iron hydroxide are bases without being alkalis. Every alkali is a base; not every base is an alkali.

Why the reactions matter. Once you accept these two ion definitions, five reaction families follow logically, and each one is used in the lab as a test to confirm whether a substance is acidic, basic, or which gas has been produced. The figure below maps how the two ions connect to the five reaction families you must know.

Every one of the five reactions is really the same neutralisation idea wearing different clothes, and seeing why makes the whole chapter fall into place rather than needing to be memorised line by line.

Step 1. Start from the acid + metal reaction: $\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)\uparrow$. The $\text{H}^+$ ions from the acid are more strongly attracted to the metal's electrons than to their own molecule, so the metal displaces hydrogen, forming a salt and releasing hydrogen gas. This only happens with metals reactive enough to lose electrons readily — copper and gold sit below hydrogen in reactivity and do not react.

Step 2. Acid + carbonate follows the same displacement logic, but the carbonate ion is unstable once protonated: $\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\uparrow$. The $\text{H}^+$ ions convert carbonate into carbonic acid, $\text{H}_2\text{CO}_3$, which is too unstable to exist and instantly decomposes into water and carbon dioxide.

Step 3. Acid + base (neutralisation) is the most direct case: $\text{H}^+$ from the acid and $\text{OH}^-$ from the base combine directly to form neutral water, $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$, while the leftover ions pair up as a dissolved salt. For example, $\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$.

Step 4. Metal oxides and non-metal oxides extend the same rule to oxides instead of hydroxides. Metal oxides are basic in nature (they behave like a base once wet), so acid + metal oxide gives salt + water: $\text{CuO}(s) + 2\text{HCl}(aq) \rightarrow \text{CuCl}_2(aq) + \text{H}_2\text{O}(l)$. Non-metal oxides are acidic in nature, so base + non-metal oxide also gives salt + water: $\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$. In every case, an H+-supplier meets an OH- (or effective OH-) supplier and the result is a salt plus water, or a salt plus water and a gas.

Step 5. None of this happens without water. Dry HCl gas does not turn dry blue litmus paper red and does not react with a dry metal strip, because ionisation into $\text{H}^+$/$\text{H}_3\text{O}^+$ and $\text{Cl}^-$ only occurs in the presence of water. Acidity is therefore not a property of the pure substance alone — it is a property of the ion released once water is present.

Question. Dilute hydrochloric acid is added to a test tube containing granules of marble (calcium carbonate). Write the balanced equation, describe two confirmatory tests for the gas produced, and state what you would observe if the same acid were instead poured onto a strip of dry (moisture-free) zinc kept in a sealed, water-free flask.

Step 1. Marble is calcium carbonate, $\text{CaCO}_3$. With dilute HCl it reacts as: $\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\uparrow$.

Step 2. Check the balance: 1 Ca and 2 Cl on each side; on the right, O totals $1 (\text{H}_2\text{O}) + 2 (\text{CO}_2) = 3$, matching the 3 O on the left; H totals 2 on each side. The equation is balanced.

Step 3. Confirm the gas is $\text{CO}_2$ two ways: (a) pass it through freshly prepared lime water — it turns milky because insoluble $\text{CaCO}_3$ forms; (b) hold a burning splint at the mouth of the tube — it is extinguished, since $\text{CO}_2$ does not support combustion (unlike $\text{H}_2$, which burns with a pop).

Step 4. For the second part, remember acidity requires water. If the HCl used is truly dry (anhydrous) hydrogen chloride gas and the zinc strip and flask are both moisture-free, no ionisation into $\text{H}^+$ and $\text{Cl}^-$ can occur. Answer: with aqueous HCl, marble fizzes releasing CO2 confirmed by milky lime water and by extinguishing a burning splint; with dry HCl and dry zinc, no reaction is observed at all, because acidic behaviour needs water to generate the H+/H3O+ ion.

• Assuming all metals liberate hydrogen from acids — unreactive metals like copper or gold do not react with dilute HCl or $\text{H}_2\text{SO}_4$.
• Swapping the confirmatory tests — the "pop" sound is for hydrogen; "turns lime water milky" is for carbon dioxide. Mixing these two up is one of the most common exam slips.
• Believing dry HCl gas is acidic on its own — acidity needs ionisation in water, so dry HCl does not turn dry litmus paper red.
• Treating "base" and "alkali" as synonyms — every alkali is a base, but insoluble bases such as $\text{Cu(OH)}_2$ and $\text{Fe(OH)}_3$ are bases that are not alkalis.

Ask the tutor: if you want more practice writing and balancing acid-base equations, or a deeper look at why metal oxides are basic while non-metal oxides are acidic, ask the AI tutor to walk through extra worked examples with you.