Paige received two 10 mL samples of unknown liquids from her teacher. She combined the substances according to her instructions and recorded the following observations:

To determine which observations support Paige's conclusion that a chemical reaction took place, we can evaluate each observation in the context of evidence for a chemical change.

1. **Change in color (Observation 1)**: The color change from colorless to purple suggests that a new substance was formed, which is a strong indicator of a chemical reaction.

2. **Volume change (Observation 2)**: The volume of the liquid doubled from 10 mL to 20 mL. This by itself does not necessarily indicate a chemical reaction; it could simply be a physical mixing of two liquids.

3. **Temperature change (Observation 3)**: The rise of 10°C indicates that there was an exothermic reaction occurring. Temperature change associated with mixing two substances is often a sign of a chemical reaction.

4. **Density (Observation 4)**: The combined density of 1.7 g/mL does not provide direct evidence that a chemical reaction occurred. While the density of the mixture might change, it can also change due to physical mixing.

Based on the above analysis, the observations that indicate a chemical change are primarily the color change and the temperature change (observations 1 and 3).

Thus, the correct answer is **B. 1 and 3 only**.