Definition of an Arrhenius base
An Arrhenius base is a substance that, when placed in water, increases the concentration of hydroxide ions, \(\mathrm{OH^-}\), in the aqueous solution.
The Arrhenius criterion in water can be stated as: \[ \text{Arrhenius base} \;\Longrightarrow\; [\mathrm{OH^-}] \text{ increases in water.} \]
For comparison (same framework), an Arrhenius acid increases \([\mathrm{H_3O^+}]\) in water: \[ \text{Arrhenius acid} \;\Longrightarrow\; [\mathrm{H_3O^+}] \text{ increases in water.} \]
Question
Determine which are Arrhenius bases in water: \(\mathrm{NaOH}\), \(\mathrm{Ca(OH)_2}\), \(\mathrm{NH_3}\), and \(\mathrm{NaCl}\). Justify each choice by writing the aqueous dissociation or reaction that shows whether \(\mathrm{OH^-}\) is produced.
Step-by-step identification
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Check whether dissolving in water produces \(\mathrm{OH^-}\) directly by dissociation.
Sodium hydroxide dissociates completely in water: \[ \mathrm{NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)} \] Since \(\mathrm{OH^-}\) appears among the products, \(\mathrm{NaOH}\) is an Arrhenius base (and a strong base).
Calcium hydroxide dissociates to give two hydroxide ions per formula unit: \[ \mathrm{Ca(OH)_2(aq) \rightarrow Ca^{2+}(aq) + 2\,OH^-(aq)} \] This increases \([\mathrm{OH^-}]\), so \(\mathrm{Ca(OH)_2}\) is an Arrhenius base (strong, though its solubility limits how much dissolves).
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If \(\mathrm{OH^-}\) is not present in the formula, check whether the substance reacts with water to generate \(\mathrm{OH^-}\).
Ammonia reacts with water to form hydroxide ions: \[ \mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)} \] The equilibrium produces \(\mathrm{OH^-}\), so \(\mathrm{NH_3}\) increases \([\mathrm{OH^-}]\) in water and is classified as an Arrhenius base in the aqueous sense (it is a weak base).
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Exclude substances that dissolve without increasing \([\mathrm{OH^-}]\).
Sodium chloride dissociates into ions but does not produce \(\mathrm{OH^-}\): \[ \mathrm{NaCl(aq) \rightarrow Na^+(aq) + Cl^-(aq)} \] No \(\mathrm{OH^-}\) is produced, so \(\mathrm{NaCl}\) is not an Arrhenius base.
Summary table
| Substance | Key aqueous process | Does \([\mathrm{OH^-}]\) increase? | Arrhenius base? |
|---|---|---|---|
| \(\mathrm{NaOH}\) | \(\mathrm{NaOH \rightarrow Na^+ + OH^-}\) | Yes (direct \(\mathrm{OH^-}\) production) | Yes (strong) |
| \(\mathrm{Ca(OH)_2}\) | \(\mathrm{Ca(OH)_2 \rightarrow Ca^{2+} + 2\,OH^-}\) | Yes (direct \(\mathrm{OH^-}\) production) | Yes (strong; solubility-limited) |
| \(\mathrm{NH_3}\) | \(\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}\) | Yes (forms \(\mathrm{OH^-}\) by reaction) | Yes (weak) |
| \(\mathrm{NaCl}\) | \(\mathrm{NaCl \rightarrow Na^+ + Cl^-}\) | No | No |
Visualization: how an Arrhenius base raises \([\mathrm{OH^-}]\) in water
\(\mathrm{NaOH,\ Ca(OH)_2,\ NH_3,\ NaCl}\)
\(\mathrm{NaOH \rightarrow Na^+ + OH^-}\)
\(\mathrm{Ca(OH)_2 \rightarrow Ca^{2+} + 2\,OH^-}\)
\(\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}\)
\(\mathrm{pOH} \downarrow\)
\(\mathrm{pH} \uparrow\)
Final classification
\(\mathrm{NaOH}\), \(\mathrm{Ca(OH)_2}\), and \(\mathrm{NH_3}\) are Arrhenius bases in water because each increases \([\mathrm{OH^-}]\); \(\mathrm{NaCl}\) is not an Arrhenius base because it dissolves without producing hydroxide ions.
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