Quantitative observation is measured (contains the word quantity) which means you use an instrument of some sort (ruler, scale, graduated cylinder) in your observation.
Examples would be:
observing the snow fall for February 14, 2018 to be 2.625 cm.
Or how about ...
observing the weight of a sample to be 3.15 grams
I do not know the specific "statement" that illustrates the difference between a chemical reaction and a nuclear reaction but i can explain it my way: A nuclear reaction is relitively larger than a chemical reaction. A nuclear reaction can cause far more and more severe damage than as a chemical can.
Density=mass/volume=100g/20cm^3=5g/cm^3. The brick will fall in the lake, because its density is higher than that of water (1g/cm^3).