User:LGreg/sandbox/Approaches to Knowledge (LG seminar 2020/21)/Seminar 18/Evidence/Evidence in Chemistry

Overview

In chemistry, evidence is usually gathered through repeated experimentation and the scientific method. This results in both quantitative and qualitative evidence, depending on the sort of experiments that are carried out. Quantitative evidence is usually used to determine the amount of a chemical, or lack of, in a specimen. On the other hand, qualitative evidence such as colour, texture and odour can be used to describe and identify substances.

Quantitative Evidence

There are many ways of gathering quantitative evidence in chemistry, such as acid-base titrations, whereby the concentration of an acid or base can be determined by neutralizing it with a known concentration of an acid or base. Due to chemistry being a natural science, it is often thought that quantitative evidence is the best source of knowledge in this discipline, but this has sometimes not been true. All chemical experiments are subject to different types of errors, including systematic and random error. While systematic errors are constant and reproducible, meaning that every data point taken will be affected in the same way, random error usually stems from human error, meaning that every data point will also be affected in a different way, resulting in anomalies in the quantitative evidence found. Percentage error can be an indicator of how reliable quantitative evidence is, and it is important to look at quantitative data with caution.

Qualitative Evidence

Qualitative evidence in chemistry can be defined as non-numerical information observed in a reaction or species. Though not always as precise as quantitative evidence, qualitative evidence is easier to produce and thus can be a convenient way of identifying what is happening in a reaction. For example, different metal ions can be identified simply by observing the colour of their flames. A lithium ion would produce a red flame, whereas a barium ion would produce a green flame. Wet chemistry is a form of analytical chemistry that relies on qualitative evidence such as observation, rather than quantitative evidence, to analyze materials.

Combining Quantitative and Qualitative Evidence

It is often more helpful to use a combination of qualitative and quantitative evidence in chemistry rather than to rely on one of the two. In the example of acid-base titrations, even though the evidence gathered is quantitative, the whole experiment is reliant on observing a colour change and using this observation to stop the reaction at the right moment. Thus, the quantitative data extracted from titrations couldn’t be reached without the use of qualitative evidence too.