Understanding Acid Formation from Oxides

Acids are substances that typically produce hydrogen ions ($\text{H}^+$) when dissolved in water. The question asks about the formation of acids based on different chemical reactions or dissolving processes.

Analyzing the Options for Acid Formation

Let's examine each option provided to understand how acids are formed.

• Option 1: Oxides of non-metals dissolve in water

  Non-metals like carbon, sulfur, nitrogen, phosphorus, etc., form oxides. Many of these non-metal oxides are acidic in nature. When these acidic oxides react with water, they form corresponding acids. For example:

  • Carbon dioxide ($\text{CO}_2$) is a non-metal oxide. When it dissolves in water (like in carbonated drinks), it forms carbonic acid ($\text{H}_2\text{CO}_3$). The reaction is: $\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{CO}_3\text{(aq)}$
  • Sulfur dioxide ($\text{SO}_2$) and sulfur trioxide ($\text{SO}_3$) are non-metal oxides formed from sulfur. $\text{SO}_2$ reacts with water to form sulfurous acid ($\text{H}_2\text{SO}_3$): $\text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_3\text{(aq)}$. $\text{SO}_3$ reacts with water to form sulfuric acid ($\text{H}_2\text{SO}_4$): $\text{SO}_3\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_4\text{(aq)}$
  • Nitrogen dioxide ($\text{NO}_2$) and dinitrogen pentoxide ($\text{N}_2\text{O}_5$) are non-metal oxides formed from nitrogen. $\text{NO}_2$ reacts with water to form nitric acid ($\text{HNO}_3$) and nitrous acid ($\text{HNO}_2$): $\text{2NO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{HNO}_3\text{(aq)} + \text{HNO}_2\text{(aq)}$. $\text{N}_2\text{O}_5$ reacts with water to form nitric acid ($\text{HNO}_3$): $\text{N}_2\text{O}_5\text{(s)} + \text{H}_2\text{O(l)} \rightarrow \text{2HNO}_3\text{(aq)}$

  These examples clearly show that dissolving oxides of non-metals in water results in the formation of acids.

• Option 2: Bases dissolve in water

  Bases are substances that can accept protons or donate hydroxide ions ($\text{OH}^-$). When some bases dissolve in water, they form alkaline solutions, not acidic ones. Bases that dissolve in water are called alkalis. For example, sodium hydroxide ($\text{NaOH}$) is a base that dissolves in water to form an alkaline solution: $\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$.

• Option 3: Metals react with water

  Reactive metals like sodium, potassium, and calcium react with water to form metal hydroxides (which are bases) and hydrogen gas. For example, sodium reacts vigorously with water: $\text{2Na(s)} + \text{2H}_2\text{O(l)} \rightarrow \text{2NaOH(aq)} + \text{H}_2\text{(g)}$. Calcium reacts with water: $\text{Ca(s)} + \text{2H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)}$. These reactions form bases, not acids.

• Option 4: Metals combine with oxygen

  Metals react with oxygen to form metal oxides. Metal oxides are generally basic in nature. For example, magnesium reacts with oxygen to form magnesium oxide: $\text{2Mg(s)} + \text{O}_2\text{(g)} \rightarrow \text{2MgO(s)}$. Magnesium oxide ($\text{MgO}$) is a basic oxide and reacts with water to form a base, magnesium hydroxide ($\text{Mg(OH)}_2$): $\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(aq)}$. Thus, metal oxides typically lead to the formation of bases, not acids, when reacting with water.

Conclusion on Acid Formation

Based on the analysis of each option, the formation of acids occurs when oxides of non-metals dissolve in water. The other options describe processes that lead to the formation of bases or alkaline solutions.

Therefore, acids are formed when oxides of non-metals dissolve in water.