Matter, Atoms and Chemical Changes

What is Chemistry?

Chemistry is the study of matter — what it is made of, how it behaves and how it changes. RRB NTPC General Science regularly asks basic chemistry questions on elements, symbols, acids and bases, and physical vs chemical changes.

Matter and its states

• Anything that has mass and occupies space is matter.
• Three common states: solid (fixed shape), liquid (fixed volume, no fixed shape), gas (neither fixed shape nor volume).

Atoms and elements

• An atom is the smallest particle of an element. It has protons (+), neutrons (no charge) and electrons (−).
• Atomic number = number of protons. Mass number = protons + neutrons.
• Common symbols: H (hydrogen), O (oxygen), Na (sodium), K (potassium), Fe (iron), Au (gold), Ag (silver).

Physical vs chemical change

• Physical change: no new substance forms (melting ice, dissolving sugar, breaking glass).
• Chemical change: a new substance forms (rusting of iron, burning of paper, curdling of milk).

Acids, bases and pH

• pH scale runs from 0 to 14. pH 7 is neutral (pure water), below 7 is acidic, above 7 is basic.
• Examples: lemon juice (acidic), soap solution (basic).

Quick example

Q. Is rusting of iron a physical or a chemical change?

A. Chemical — iron reacts with oxygen and moisture to form a new substance, iron oxide.

Analogy

Analogy: A kitchen of LEGO bricks

Think of chemistry as a kitchen where everything is built from LEGO bricks.

• Each type of brick is an element, and a single brick is an atom.
• Snapping bricks together makes a molecule or compound — like H₂O built from two hydrogen bricks and one oxygen brick.
• Moving a LEGO model from the table to a box changes only its place or shape — that is a physical change (like ice melting into water).
• Pulling the bricks apart and rebuilding them into a completely new model is a chemical change (like iron turning into rust).

If you end up with the same model, it's physical; if you end up with a new model, it's chemical.