Video Transcript
In this video, we will learn about
the different types of chemical analysis used by chemists and other scientists.
Let’s define two key terms used in
chemical analysis. A sample is a small quantity of
material taken from a larger quantity which is representative of the original bulk
material. When we say a sample is
representative of the bulk material, we mean that it has the same or very similar
physical and chemical properties to the original source material. A substance in a sample which is
being investigated or analyzed is called an analyte. Now that we know these two
definitions, let’s have a look at some real-life examples of samples and analytes
that chemists and other scientists routinely analyze.
Chemical analysis is used in
medicine. For example, a sample of blood may
be analyzed for the following analytes: cholesterol, blood glucose, hormones, or
many other analytes. Doctors, chemists, pharmacists, and
laboratory healthcare workers perform these chemical analyses to see which
medication might be needed by a patient.
In farming, farmers and soil
scientists might analyze a sample of soil in terms of pH or elemental
composition. And this analysis helps farmers
determine whether a particular crop is suited to growing in a particular soil and
which fertilizer is best.
In the food industry, nutritionists
and food scientists study the nutrient proportions in different foods as well as the
compounds that give foods taste for the purpose of suggesting suitable healthy diets
to patients and to optimize a food item in terms of nutrient content or taste or
shelf life.
Let’s clear some space. Air, soil, and river samples, for
example, are analyzed in environmental studies by environmental chemists for things
like toxins and their levels.
One last example of where we use
chemical analysis is in engineering. Mechanical engineers and
metallurgists may take samples of alloys from machinery components to study their
elemental composition and related strength. Doing this analysis helps them
improve material properties. For example, they can design
stronger or more-corrosion-resistant machine components.
These are just a few of the many
examples of chemical analysis in real life.
Now, let’s have a look at types of
chemical analysis. We can analyze the chemical
substances or analytes in a sample in two ways: qualitatively or quantitatively. Qualitative analysis, coming from
the word quality, is the identification of substances in a sample, in other words
identifying what constituents the sample is composed of, which elements or ions or
compounds or what mixture of these is the sample composed of. Many samples are mixtures of
different components, but some samples are pure. Pure table salt, for example,
contains Na+, sodium, and Cl−, chloride, ions. If we were to qualitatively analyze
pure water, we’d find only H2O molecules.
The qualitative analysis of a blood
sample would reveal that blood is a mixture containing red blood cells; white blood
cells; water; hormone compounds; ions like Ca2+, the calcium ion, sodium ions, Na+,
chloride ions, Cl−; various sugar compounds; gases like carbon dioxide; as well as
many other compounds such as urea, cholesterol, etcetera. These are all analytes that can be
identified through the qualitative analysis of a blood sample. Even air is a mixture which we can
qualitatively analyze. The components of air that we can
identify in an air sample could be nitrogen gas; oxygen; gas; water vapor; carbon
dioxide gas; argon; as well as NO𝑥 gasses or nitrogen oxide gasses, which are
pollutants; and other pollutants such as carbon monoxide; or even solid particulates
such soot or dust.
So far, we’ve said that qualitative
analysis helps us identify the components in a sample and whether the sample is pure
and consists of only one component or a mixture. A pure sample has unique
characteristic physical properties, for example, melting point, boiling point,
solubility, and molar mass. Let’s think of pure water. We know that it appears colorless
with a faint hint of blue and that it boils at exactly 100 degrees Celsius at one
atmosphere pressure. In this way, we can identify pure
water. Impure or contaminated water
samples will boil at temperatures slightly higher than 100 degrees Celsius.
Mixtures, on the other hand, first
need to be separated into their components, and then these components can be
identified. Pure substances can be broadly
classified into organic and inorganic substances. We can identify an organic analyte
by the elements present as well as the functional groups present in the
compound. Many types of electromagnetic
radiation are used in modern instruments for qualitative analysis. Organic functional groups in
particular are often studied using infrared spectroscopy, which uses infrared light,
as each functional group interacts differently with infrared light, giving a
signature pattern unique to that functional group.
Inorganic substances such as metal
ions can be identified by the color that they give off in flame tests. Some ions can be identified through
their unique reactions. For example, we can identify a
chloride ion in a solution by adding some silver nitrate and observing the formation
of a white precipitate. Bromide ions, however, react
differently and form a cream-colored precipitate with silver nitrate. And iodide ions can be identified
by the formation of a pale-yellow precipitate, while fluoride ions don’t produce a
precipitate with silver nitrate. These are just some examples of how
to test for the presence of certain substances. Usually two or more tests are
necessary to confirm the identity of a substance in a sample. Doing only one qualitative test is
not conclusive enough.
Now let’s have a look at
quantitative analysis. The term quantitative comes from
the word quantity, which means amount. Scientists often need to know how
much of each component is present in the sample. So quantitative analysis is the
determination of the quantity or abundance of an analyte in a sample. The quantity of a constituent can
be measured in terms of mass, concentration, number of moles, or relative
abundance. The various units for these
quantities include grams, milligrams, and micrograms for mass; mole per liter or
molarity for concentration, as well as parts per million, percent by mass, or
percent by volume; moles for moles; and percentage for relative abundance.
A commonly used and important tool
in quantitative analysis is the analytical balance. Analytical balances are sensitive
mass balances used in laboratories to measure small masses accurately.
Now it’s time to practice.
Which of the following is not an
example of qualitative chemical analysis? (A) Determining the concentration
of a compound in solution. (B) Identifying the cationic groups
in a compound. (C) Identifying the anionic groups
in a compound. (D) Identifying the functional
group in a molecule. Or (E) determining the elemental
composition of a molecule.
There are two ways to analyze a
chemical sample: by qualitative analysis or by quantitative analysis. Qualitative analysis is the
identification of substances in a sample, in other words identifying what
constituents a sample is composed of, be it elements, ions, or molecules or a
mixture of these, whereas quantitative analysis is the determination of the quantity
or amount or abundance of a substance, be it elements, ions, molecules in a
sample. So qualitative analysis involves
finding out the identity of an analyte, while quantitative analysis involves finding
out the quantity of the analyte.
The question asks, which of the
following is not an example of qualitative chemical analysis? In other words, they are asking us,
which answer is an example of quantitative analysis where we measure the quantity of
an analyte? There are various ways to measure
the amount of an analyte in a sample. We can measure its mass, or we can
determine its concentration, or we can express its amount in terms of moles or as a
percentage in relative abundance.
So determining the concentration of
a compound in solution is an example of quantitative analysis. Identifying cationic groups is a
qualitative analysis. Identifying anionic groups is also
a qualitative process. Identifying which functional groups
are present is not a measurement of the amount of analyte, and so we can rule this
answer option out too. And lastly, determining the
elemental composition is all about determining which elements are present and not
the amount of each element.
Finally, which of the following is
not an example of qualitative chemical analysis? The answer is (A) determining the
concentration of a compound in solution.
Now let’s summarize what we’ve
learnt about chemical analysis. We learnt that an analyte is a
substance in a sample under investigation and that a sample is a small quantity of a
larger bulk material with the same or very similar physical and chemical properties
as the original source material. In other words, a sample must be a
reliable representation of the original material.
We learnt that chemical analysis is
used in many areas of life and industry, including medicine, agriculture,
engineering, the food industry, and even in environmental studies. We also learned that there are two
broad types of chemical analysis, qualitative analysis and quantitative
analysis. In qualitative analysis, we
identify the analyte or analytes. And these can be elements, ions, or
molecules, or even the identification of part of a molecule such as a functional
group.
In quantitative analysis, we
quantify or determine the amount of an analyte or analytes. And this can be done in terms of
mass or concentration or moles or relative abundance.
