Question

Which onc of the following is not a postulate of Ostwald's theory of indicators? (1) The colour of an indicator in solution is duc to the ions furnished by it (2) The colour of an indicator is duc to the ions furnished by acidic substancc (3) \Lambdall indicators are weak acids or bases (4) \Lambdacidic indicators show decp colour in acidic solutions and vice versa

Short answer

Option (2) is not a postulate of Ostwald's theory of indicators.

Step-by-step solution

Understand the Question

Identify which statement is not a postulate of Ostwald's theory of indicators. An indicator is a substance that changes color to indicate the pH of a solution.

Analyze Each Option

Read each given statement carefully and compare it with the known postulates of Ostwald's theory of indicators.

Examine Known Postulates

List the known postulates of Ostwald's theory of indicators:1. The color of an indicator in solution is due to the ions furnished by it.2. All indicators are weak acids or bases.3. The color change occurs because the indicator ionizes to different extents in acidic and basic solutions.

Match Postulates with Options

Option (1) matches the first postulate.Option (3) matches the second postulate.Option (4) describes the behavior in acidic and basic solutions, which aligns with the third postulate.

Identify the Incorrect Statement

Option (2) states that the color of an indicator is due to the ions furnished by acidic substances. This does not match any of the known postulates.

Key concepts

postulates of Ostwald's theory

Ostwald's theory of indicators provides a fundamental explanation for how indicators work, especially pH indicators. According to Ostwald's theory, indicators are substances that change color at a certain pH level. Here are the key postulates of Ostwald's theory:

1. The color of an indicator in solution is due to the ions furnished by the indicator itself, not other substances.
2. All indicators are weak acids or bases. This means they do not completely dissociate in a solution.
3. The color change occurs because the indicator ionizes to different extents in acidic and basic solutions.

Understanding these postulates helps clarify why indicators behave the way they do in different pH environments. They are crucial for determining the accuracy of pH measurements.

color change in indicators

The color change in indicators is a fascinating phenomenon that lies at the heart of their functionality. Indicators change colors because of the different ions they produce in acidic or basic environments. Here’s the process in more detail:

1. In an acidic solution, an indicator (a weak acid) partly dissociates into hydrogen ions (H⁺) and its conjugate base.
2. In an alkaline or basic solution, the indicator (now a weak base) picks up a proton and becomes its conjugate acid.

This dissociation leads to different colorations for the indicator in each type of solution. For example, litmus turns red in acidic environments (more H⁺ ions) and blue in basic ones (more OH⁻ ions). The same indicator can show a sharp contrast between two pH levels, allowing us to easily spot the pH range of a solution.

weak acids and bases

Indicators are all weak acids or bases, a cornerstone of Ostwald's theory. Their weak nature affects the degree to which they dissociate in an aqueous solution. Here’s why this is important:

1. Weak acids only partially dissociate in solution, meaning they don’t release all their hydrogen ions.
2. Weak bases only partially accept hydrogen ions in solution.

Because of this partial ionization, indicators exhibit different colors based on the concentration of hydrogen ions (H⁺) or hydroxide ions (OH⁻) in the solution. This property makes them very effective for detecting fine differences in pH.

pH indicators

pH indicators are special types of weak acids or bases that change color at a specific pH range. Here’s what you need to know:

1. Each pH indicator has a unique pH range where it changes color, known as the transition range.
2. When the solution’s pH is within an indicator's transition range, the indicator will display a mix of colors, often seen as an intermediate hue.
3. Outside of this range, the indicator will show a distinct color corresponding to either an acidic or basic environment.

Common pH indicators include litmus, phenolphthalein, and bromothymol blue. These indicators are essential tools in chemistry for determining the acidity or basicity of a solution quickly and effectively.