Chemistry · Class 9-10 · Chapter 1 · The idea of chemistry
What chemistry is.
The book does not introduce chemistry as a list of topics. It introduces it as three questions: what a thing is made of, how it behaves, and how it turns into something else. Once those three questions are in your hand, the rest of the book stops being a heap of separate facts.
1. A nail on a wet windowsill
Leave an iron nail on a damp windowsill for a month. Come back and it is not shiny any more: brown, rough, flaking in places. Nothing else in the room has changed, only the nail. All three questions of chemistry can be asked about this one event, and asking them is what the whole chapter is for.
Drag the handle to put the two states side by side. Everything in the drawing except the nail is identical on both sides: the same sill, the same drop of water, the same arrow of air. Only the nail is different.
From iron to rust
The drawing is a model, not a scale drawing.
Rust needs two things, water and the oxygen of the air. Iron stays bright for a long time in dry air, and it also stays bright in water from which the oxygen has been driven out. That is why the drop and the arrow are both in blue ink: those two together are the cause.
2. The book's definition, in three words
The book puts the definition like this:
The branch of science in which the composition of matter, the properties of matter and the changes of matter are discussed is called chemistry.
It is a long sentence, but there are only three words inside it: composition, properties, change. The rest is grammar. Three words, three different questions, and the answer to each comes from a different place.
Composition is what a thing is made of. Water is a compound of hydrogen and oxygen, H₂O. Properties is how a thing behaves. Water freezes at 0 °C, boils at 100 °C, and dissolves salt. Change is whether a thing turns into something else. Iron with water and air becomes rust, Fe₂O₃·nH₂O, and that rust is no longer iron.
The definition is easy to memorise and hard to use. Every statement below is answering one of the three questions. Tap the three boxes first to see what each question is asking, then tap each statement to see which box it lands in.
The three questions of chemistry
The three words of the definition
Which three aspects of matter does chemistry mainly discuss?
3. The word the whole subject stands on
One word keeps coming back in all three questions: matter. Matter is anything that has mass and takes up space. You cannot see air, but it has mass and it occupies space, so air is matter. Light and sound have no mass and take up no space, so they are not.
This is not a definition to recite. Chemistry works on matter, so if a question is not about matter, its answer is not going to come from inside chemistry. The book in your hand, the hand itself, the air in the room, the water in the glass: all matter, all with a composition, all with properties, and all able to change.
4. Physical change and chemical change
The third question, the question of change, splits in two, and the rest of the book leans on that split.
Ice melts into water. The stuff is still H₂O, only its state has changed, and cooling it brings the ice back. That is a physical change: no new substance was made, and it can usually be undone.
A candle burns. What is left is not wax, and no one gets the candle back out of the smoke and the soot. That is a chemical change: a new substance has been made, and it does not come undone easily. The second kind is what the book is really interested in.
Telling them apart needs one question: has a new substance been made? Each card below has an everyday event on the front. Decide your answer before you turn it over, then tap.
Physical or chemical
There is a rhythm inside the cards. Melting, boiling, breaking, dissolving, bending: the substance survives, so these are physical. Burning, rotting, rusting, digesting, ripening: something new appears every time, so these are chemical. A change of colour, a change of smell, a gas coming off, heat going in or out, these four are often the signature of a chemical change. A signature is not a proof though, and the real question stays the same one: has a new substance been made.
Which one makes something new
Which of these is a chemical change?
5. Chemistry does not only describe, it puts to work
If the definition stopped at three questions, chemistry would be description and nothing more. The book adds one thing: chemistry learns the properties of the substances in nature and then puts that knowledge to work for people and for the environment.
That extra clause explains the rest of the chapter. Medicines, fertiliser, pesticide, soap, dye, plastic, fuel, purifying water, keeping food from spoiling: all of it is one job wearing different faces, which is learning what a substance does and then changing it into something useful. And that is exactly why the chapter ends up talking about safety, because a thing that is useful when handled well does harm when it is not.
6. Where the word comes from
The English word Chemistry comes from Chimia or Alchemy, and behind those stands the Arabic al-kimiya. In mediaeval Arabia the practice of chemistry was called alchemy. How that practice turned into the chemistry we have now is the next part of the chapter.
7. Chemistry outside the laboratory
Chemistry is not glassware and a white coat. Most of what you did this morning was chemical change. Digesting your food, breathing, a plant photosynthesising, cooking on the stove, a rubbish heap rotting, gas burning at the burner: in every one of them a new substance is made.
Holding on to that takes a lot of weight off the subject. What you are reading in class is not the law of some other world. It is the law of the one you have been in all day, written out with names and formulas attached.
8. Where the chapter goes next
If chemistry is composition, properties and change, the remaining questions stand up by themselves. How far does it reach, which is its scope. Which other sciences does it work alongside. What is gained by learning it. How is new chemistry actually found, which is the steps of an investigation. And last, how is all of this done without getting burned. The rest of the chapter follows exactly that order.
That nail again
An iron nail left in a damp place turns brown after a month. Which question of chemistry is this an example of, and what is the substance that forms?
This chapter's board questions, with answers and reasons →
Source: Chemistry, Classes 9-10 (NCTB), Chapter 1 “The idea of chemistry”, the section introducing chemistry.