Everything you can touch is made of atoms — about ninety naturally occurring kinds, called elements, and the periodic table is simply the chart of that alphabet. Atoms bond into molecules the way letters build words: two hydrogens and an oxygen make water; the same letters in other arrangements make everything else. (Where the atoms themselves were made, you already know if you’ve read Round Trip to the Big Bang: inside stars.)
The same stuff can wear three states. In a solid, the particles hold formation; in a liquid, they slide past each other; in a gas, they fly free. What moves matter between states is heat — and here is a quiet key that unlocks many questions: heat is the motion of particles, and temperature is the measurement of that motion. Ice melting is water’s molecules being shaken loose, nothing more.
Now the distinction the test asks about constantly: physical change versus chemical change. Melt ice, tear paper, dissolve sugar — physical changes: the substance is rearranged but remains itself. Burn wood, rust iron, bake batter into bread — chemical changes: bonds break and reform, and new substances exist that didn’t before. The tell-tale signs of the chemical kind: heat or light given off, gas bubbling out, a color that won’t change back.
Behind both kinds of change stands the rule that runs the whole domain: conservation. In ordinary changes, matter is neither created nor destroyed. Burn a log and it seems to vanish — but weigh the ash, the smoke, and the gases, and every atom is accounted for; a balanced chemical equation is nothing but that bookkeeping written out. Energy obeys the same accounting: it is not made or lost, only converted — chemical energy in gasoline to motion in the wheels to heat in the brakes. Whenever a question asks where something “went,” the answer is an accounting question, and the books always balance.
Then, motion. Three plain rules, three centuries old, still undefeated. A thing keeps doing what it’s doing — resting or moving — until a force changes it; that stubbornness is called inertia, and it’s why you lurch when the bus brakes. Second, the more mass a thing has, the more force it takes to change its motion — shopping carts versus trucks. Third, every push pushes back: the gun recoils, the rocket rises by throwing exhaust down. And through it all runs gravity, the attraction of every mass toward every other — the same pull that drops your keys holds the moon in its orbit.
Energy’s daily costumes finish the tour: electricity is charges moving through a wire; sound and light travel as waves, energy on the move without the stuff moving with it. Different faces, same ledger — which is the point of the whole domain: a handful of rules, wearing examples.