Question

red cabbage juice can be used as a ph indicator. the indicator will change colour, depending on the ph of the colourless solution that the indicator is added to, as shown below:

red (ph 1), pink (ph 3), light purple (ph 5), dark purple (ph 7), blue (ph 9), green (ph 11)

in a test tube, a student mixes a neutral colourless solution with the indicator. then the student adds a strong base.

question 29
which of the following colour changes are expected to occur in the test tube?

red to green
dark purple to red
dark purple to green
green to red

Explanation:

<pre_analysis>
{
"quality": "clear",
"question_count": 1,
"skills_matched": [
"direct_cot",
"multiple_choice_formatter",
"explain_then_answer",
"tutor"
],
"needs_drawing": false,
"concepts_used": [],
"new_concepts": [
"pH Indicators",
"Acid-Base Neutralization"
],
"current_concepts": [
"pH Indicators",
"Acid-Base Neutralization"
]
}
</pre_analysis>

<reasoning>

Identify initial state

The student starts with a neutral colourless solution. A neutral solution has a pH of approximately 7. According to the provided red cabbage juice indicator scale, at pH 7, the colour of the solution is dark purple.

Identify final state

The student then adds a strong base to the test tube. Adding a strong base increases the pH of the solution, making it basic (alkaline). Looking at the indicator scale, basic pH values (such as pH 9 or pH 11) correspond to blue or green colours.

Determine colour change

Since the solution starts at pH 7 (dark purple) and becomes basic (moving towards pH 11, which is green), the expected colour change in the test tube is from dark purple to green.

Evaluate options

• "Red to green": Incorrect, as the starting solution is neutral (dark purple), not highly acidic (red).
• "Dark purple to red": Incorrect, this would occur if a strong acid were added instead of a base.
• "Dark purple to green": Correct, as the pH increases from neutral (7) to basic (11).
• "Green to red": Incorrect, this represents a change from basic to highly acidic.

</reasoning>

<answer>
<mcq-option>Red to green</mcq-option>
<mcq-option>Dark purple to red</mcq-option>
<mcq-correct>Dark purple to green</mcq-correct>
<mcq-option>Green to red</mcq-option>
</answer>

<post_analysis>
{
"subject": "Natural Science",
"question_type": "Multiple Choice",
"knowledge_point": [
"Natural Science",
"Chemistry",
"pH Indicators"
]
}
</post_analysis>

Answer:

<pre_analysis>
{
"quality": "clear",
"question_count": 1,
"skills_matched": [
"direct_cot",
"multiple_choice_formatter",
"explain_then_answer",
"tutor"
],
"needs_drawing": false,
"concepts_used": [],
"new_concepts": [
"pH Indicators",
"Acid-Base Neutralization"
],
"current_concepts": [
"pH Indicators",
"Acid-Base Neutralization"
]
}
</pre_analysis>

<reasoning>

Identify initial state

The student starts with a neutral colourless solution. A neutral solution has a pH of approximately 7. According to the provided red cabbage juice indicator scale, at pH 7, the colour of the solution is dark purple.

Identify final state

The student then adds a strong base to the test tube. Adding a strong base increases the pH of the solution, making it basic (alkaline). Looking at the indicator scale, basic pH values (such as pH 9 or pH 11) correspond to blue or green colours.

Determine colour change

Since the solution starts at pH 7 (dark purple) and becomes basic (moving towards pH 11, which is green), the expected colour change in the test tube is from dark purple to green.

Evaluate options

• "Red to green": Incorrect, as the starting solution is neutral (dark purple), not highly acidic (red).
• "Dark purple to red": Incorrect, this would occur if a strong acid were added instead of a base.
• "Dark purple to green": Correct, as the pH increases from neutral (7) to basic (11).
• "Green to red": Incorrect, this represents a change from basic to highly acidic.

</reasoning>

<answer>
<mcq-option>Red to green</mcq-option>
<mcq-option>Dark purple to red</mcq-option>
<mcq-correct>Dark purple to green</mcq-correct>
<mcq-option>Green to red</mcq-option>
</answer>

<post_analysis>
{
"subject": "Natural Science",
"question_type": "Multiple Choice",
"knowledge_point": [
"Natural Science",
"Chemistry",
"pH Indicators"
]
}
</post_analysis>