Chemistry · Class 9-10 · Chapter 1 · The idea of chemistry
How far chemistry reaches.
The usual way to ask about scope is: where is chemistry? The answer runs so long that it is easier to turn the question over. Where is chemistry not? That list is much shorter, and if you try to make it you will find it almost empty.
1. Where the boundary runs
The boundary was drawn earlier in the chapter. Chemistry works on matter, and matter is whatever has mass and takes up space. So wherever there is something with mass, there is a question for chemistry: what is it made of, how does it behave, and how does it change.
The ten boxes below are ten parts of your life. Tap any of them to see exactly which chemistry is at work there, and you will get at least one real substance named each time.
the reachtap any of them
Where chemistry turns up
Air. A mixture of nitrogen, oxygen, carbon dioxide and water vapour. Without the oxygen there is no respiration and no burning, which means neither your body nor the stove.
Water. The universal solvent. Nearly every reaction inside your body happens through water, and it is the most used solvent in industry as well.
Food. Starches, proteins, fats, vitamins, minerals: compounds, all of them. Digestion means breaking those large molecules into small ones the body can take in. Keeping food from spoiling, with vinegar or a preservative, is chemistry too.
Fertiliser. Plants take nitrogen, phosphorus and other elements out of the soil. Harvest the crop and those elements leave with it, and fertiliser puts them back. Urea is one of them.
Medicine. The medicines for cholera, typhoid and tuberculosis are chemistry's work. Making one means deciding which compound will interrupt which reaction in the body.
Cloth. Thread from the cellulose of cotton, and man made fibres like polyester and nylon straight out of a chemical plant. Dyeing the cloth is a chemical process as well.
Paper. Paper is made by treating the cellulose of wood chemically. The exercise book in front of you came out of a chemical works.
Fuel. Natural gas, petrol and diesel are all hydrocarbons. Burning them gives heat, and makes carbon dioxide and water along with it.
Building. Cement, lime, steel, paint: a separate chemical process behind each one. Keeping the steel from rusting is chemistry's job too.
The environment. Here one chemistry cuts both ways. Burning fuel gives heat, and the carbon dioxide from that same burning feeds air pollution and a changing climate.
All ten. Now try it the other way round: think of something with no chemistry in it at all. That list does not get long.
2. A territory that large has to be divided
Since the territory is more or less everything, chemistry is divided up for study. The divisions are called branches, and each branch has a question of its own. Learning the questions is easier than learning the names, and more useful, because an exam asks you to recognise a branch rather than list one.
Open the tree. There is one word at the root, chemistry, and eight branches growing out of it. Under each branch is the question that belongs to it.
branchesopen a branch
The branches of chemistry
Chemistry
Organic how is the carbon joined up?
hydrocarbons, alcohols, acids
plastics, medicines, dyes, man made fibre
Inorganic which element, which salt?
metals, non metals, salts, minerals
getting a metal out of its ore
Physical how fast, at what energy?
rates of reaction and energy
gas laws, thermodynamics, equilibrium
Analytical what is in it, and how much?
qualitative: what is in it
quantitative: how much of it
adulterated food, arsenic in water
Biochemistry what is happening inside a body?
proteins, sugars, fats, enzymes
DNA and RNA, the molecules of heredity
Industrial how does it scale to a factory?
fertiliser, cement, paper, sugar, cloth
taking a bench reaction to the plant
Environmental what is polluting, and how is it fixed?
pollution of air, water and soil
the ozone layer, greenhouse gases
Nuclear what happens in the nucleus itself?
radioactivity and nuclear reactions
uses in medicine and in generating power
The divisions are there to make the subject readable, not because nature is divided that way. A real problem usually needs three or four branches at once. Making a new medicine takes organic chemistry to build the molecule, analytical chemistry to check that it is pure, and biochemistry to work out what it will do once it is inside a body.
check yourselfpick one answer
Is the pair matched
Which of these pairs is correctly matched?
Extracting a metal from its ore is inorganic chemistry; there is no living body anywhere in it. Biochemistry is about reactions inside one.
Right. Plastics and man made fibres are both carbon based polymers, so this is organic chemistry. The question is how the carbon is joined up.
Detecting adulteration is analytical chemistry: what is in it and how much. Nuclear chemistry is about reactions inside the nucleus.
An enzyme is a molecule of a living body, so that is biochemistry. Environmental chemistry is about pollution and what to do about it.
3. The branch follows the question, not the object
There is a trap here that almost everybody falls into once. Branches are decided by the question, not by the object. “Food means biochemistry” will get you into trouble, because everything depends on what is being asked about the food.
Sugar is an organic compound. But “why does sugar dissolve faster in hot water” is not an organic chemistry question, because it is not about the structure of the sugar, it is about a rate. That is physical chemistry. Same object, different question, different branch.
sort themchip, then bin
Whose job is it
Pick up a chip, then drop it in a bin. Do not ask what the thing is; ask what the question is.
Right. Next one.
Not there. Is the question about structure, about a rate, or about the inside of a body?
All in place. Notice that the fridge chip could have gone to biochemistry, since spoiling is the work of microbes. But the question was about a rate, so it is physical chemistry. The branch follows the question, not the object.
check yourselfpick one answer
Sugar, hot water
“Sugar dissolves faster in hot water.” Which branch of chemistry does that observation belong to?
This is the common trap. Sugar is indeed an organic compound, but the question is not about its structure. The object does not decide the branch.
Right. The question is how fast it dissolves, which is a question about rate. Rates, energy and equilibrium belong to physical chemistry.
An analytical question would be “is there sugar in this, and how much?” How fast it dissolves is a different question.
Biochemistry would be the answer if the question were what happens to sugar inside a body. There is no body inside the cup.
4. And one branch you will need yourself
Of all the branches, analytical chemistry is the one ordinary people meet most often, and the one whose name they hear least. It has two questions, both very plain: what is in it, and how much. Whether there is arsenic in the tubewell, whether the milk has been adulterated, which element the soil is short of: all of those answers live inside those two words.
This is where knowing chemistry and using chemistry become the same thing. Catching adulteration takes chemistry too, and the chapter leans harder on that point a little further on.
check yourselfpick one answer
The tubewell water
Finding out whether there is arsenic in tubewell water, and how much, is the work of which branch?
Nuclear chemistry is about radioactivity and reactions in the nucleus. Arsenic is not turning up here as a radioactive substance; it is turning up as a poison.
Industrial chemistry is about making things at the scale of a factory. Nothing is being made here; something is being measured.
Right. “What is in it” and “how much of it” are between them the definition of analytical chemistry. Arsenic is a non metal so inorganic chemistry is involved too, but the measuring is analytical.
Arsenic is a non metal, so inorganic chemistry is not irrelevant. But the question is not about the properties of the element, it is about how much of it is in the water.