Which of the following observations is qualitative?

• A. The temperature increased by 5 °C.
• B. The solution turned blue after adding copper(II) sulfate.
• C. The pH of the solution was measured as 4.2.
• D. The mass of the precipitate was 2.3 g.

Answer: (B)

Verified step by step guidance

1. Understand the difference between qualitative and quantitative observations: qualitative observations describe qualities or characteristics without using numbers, while quantitative observations involve measurements and numerical data.
2. Review each observation given in the problem: 'The temperature increased by 5 °C' involves a numerical value, so it is quantitative.
3. Consider 'The solution turned blue after adding copper(II) sulfate': this describes a color change, which is a characteristic and does not involve numbers, so it is qualitative.
4. Look at 'The pH of the solution was measured as 4.2': this is a numerical measurement, making it quantitative.
5. Examine 'The mass of the precipitate was 2.3 g': since it involves a numerical mass measurement, it is quantitative.