Final answer:
The qualitative observation of the unknown substance's emission test is that it emitted a bright blue light. This description provides information about the substance's physical characteristics without numerical measurements.
Step-by-step explanation:
The correct answer is option a) The unknown substance emitted a bright blue light. This observation is qualitative because it provides a description without any numerical measurement. When substances are heated, their electrons get excited and jump to higher energy levels. Upon returning to their ground states, these electrons emit light at specific wavelengths that appear as colors to the human eye. Each element has a unique "fingerprint" in its emission spectra, which allows scientists to identify the elements present in an unknown substance by comparing its emission spectrum to those of known substances.
For instance, when a pure hydrogen gas is heated, it emits a characteristic pattern of colors different from that of sodium. In the case of an unknown substance emitting a bright blue light, one might suspect the presence of elements like copper or arsenic, which typically produce blue or bluish-green flames in emission spectroscopy. This contrast to a quantitative observation such as recording numerical values or precisely measuring the emission spectra of substances. The emission spectrum analysis is a crucial tool in understanding the elemental composition of substances in chemistry and astrophysics.