Which of the following statements is INCORRECT?
Understanding the Properties of Acids and Bases
The question asks to identify the INCORRECT statement among the given options related to acids and bases, common substances, and indicators.
Let's analyze each statement carefully:
- Statement 1: Lemon contains citric acid
This statement is correct. Lemons and other citrus fruits are well-known sources of citric acid. Citric acid is an organic acid responsible for their sour taste. - Statement 2: The basic nature of Sodium Hydroxide (NaOH) is due to the presence of sodium ions.
This statement is incorrect. Sodium Hydroxide ($\text{NaOH}$) is a strong base. When dissolved in water, it dissociates into sodium ions ($\text{Na}^+$) and hydroxide ions ($\text{OH}^-$). The basic nature of $\text{NaOH}$ solution is due to the presence of hydroxide ions ($\text{OH}^-$), not sodium ions ($\text{Na}^+$). Bases are substances that produce hydroxide ions ($\text{OH}^-$) in aqueous solutions. - Statement 3: Nonmetallic oxides are acidic in nature
This statement is generally correct. Oxides of nonmetals, such as carbon dioxide ($\text{CO}_2$), sulfur dioxide ($\text{SO}_2$), and nitrogen dioxide ($\text{NO}_2$), react with water to form acids. For example, carbon dioxide reacts with water to form carbonic acid ($\text{H}_2\text{CO}_3$). This is why they are considered acidic oxides. - Statement 4: Phenolphthalein turns pink in soap solution.
This statement is correct. Phenolphthalein is a common acid-base indicator. It is colorless in acidic and neutral solutions but turns pink in basic solutions. Soap solutions are basic in nature (due to hydrolysis of salts present in soap), so phenolphthalein will indeed turn pink in soap solution.
Based on the analysis, the statement that is INCORRECT is the second one, regarding the basic nature of Sodium Hydroxide being due to sodium ions.
Detailed Analysis of Basic Nature of NaOH
Sodium Hydroxide ($\text{NaOH}$) is an ionic compound. When it dissolves in water, it undergoes dissociation:
\(\text{NaOH(s)} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}\)
This equation shows that solid $\text{NaOH}$ dissolves to form aqueous sodium ions ($\text{Na}^+$) and aqueous hydroxide ions ($\text{OH}^-$). According to the Arrhenius definition of a base, a base is a substance that produces hydroxide ions ($\text{OH}^-$) in water. The presence of a high concentration of hydroxide ions ($\text{OH}^-$) in the solution is what gives $\text{NaOH}$ its strong basic properties. Sodium ions ($\text{Na}^+$) are spectator ions in this process; they are present in the solution but do not contribute to the basicity.
Understanding Acid-Base Indicators
Acid-base indicators are substances that change color depending on the pH of the solution. Phenolphthalein is a classic example. Its color change occurs within a specific pH range. For phenolphthalein, the color change from colorless to pink typically happens in the pH range of 8.2 to 10.0. Solutions with a pH below 8.2 are colorless with phenolphthalein, while solutions with a pH above 10.0 are distinctly pink or magenta. Soap solutions are generally alkaline, meaning they have a pH greater than 7, often in the range where phenolphthalein turns pink.
Summary of Statement Correctness
| Statement | Correct/Incorrect | Reason |
|---|---|---|
| Lemon contains citric acid | Correct | Lemons are a source of citric acid. |
| The basic nature of Sodium Hydroxide (NaOH) is due to the presence of sodium ions. | Incorrect | Basic nature is due to hydroxide ions ($\text{OH}^-$). |
| Nonmetallic oxides are acidic in nature | Generally Correct | They form acids when dissolved in water (e.g., $\text{CO}_2$ forms $\text{H}_2\text{CO}_3$). |
| Phenolphthalein turns pink in soap solution. | Correct | Soap solutions are basic, and phenolphthalein is pink in basic solutions. |
Therefore, the incorrect statement is that the basic nature of Sodium Hydroxide is due to sodium ions.
Revision Table: Key Concepts in Chemistry
| Term | Definition/Property |
|---|---|
| Acid | Substance producing $\text{H}^+$ ions in water (Arrhenius); proton donor (Brønsted-Lowry). |
| Base | Substance producing $\text{OH}^-$ ions in water (Arrhenius); proton acceptor (Brønsted-Lowry). |
| Indicator | Substance changing color with pH, used to detect acidity or basicity. |
| Nonmetallic Oxide | Oxides of nonmetals; often react with water to form acids. |
| Citric Acid | A common organic acid found in citrus fruits. |
| Sodium Hydroxide (NaOH) | A strong inorganic base. |
Additional Information: Acids, Bases, and Ions
Acids and bases are fundamental concepts in chemistry. Their properties are often explained in terms of the ions they produce in water. Acids increase the concentration of hydrogen ions ($\text{H}^+$) (often represented as hydronium ions, $\text{H}_3\text{O}^+$), while bases increase the concentration of hydroxide ions ($\text{OH}^-$). The strength of an acid or base depends on the extent to which it dissociates in water to produce these ions. Strong acids and bases dissociate almost completely, whereas weak ones dissociate only partially.
Sodium Hydroxide ($\text{NaOH}$) is a strong electrolyte, meaning it dissociates completely into its constituent ions, $\text{Na}^+$ and $\text{OH}^-$, when dissolved in water. This complete dissociation leads to a high concentration of $\text{OH}^-$ ions, making it a strong base.
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