Classify matter as an element, compound, or mixture, and distinguish physical from chemical properties and changes.
Chemistry is the study of matter -- anything that has mass and takes up space -- and the changes it goes through. The water in your glass, the metal in a spoon, and the air you breathe are all matter, and chemistry explains what each is made of and why it behaves the way it does.
Matter sorts into three basic types. An element is a pure substance made of only one kind of atom (oxygen, gold) and cannot be broken down into anything simpler by ordinary means. A compound is two or more elements chemically combined in a fixed ratio (water, H2O, always 2 hydrogen atoms to 1 oxygen atom) -- break that bond and you get something with completely different properties than the elements it came from. A mixture is two or more substances physically combined, each keeping its own identity, and it can usually be separated again by physical methods like filtering, evaporating, or letting it settle.
Every substance has physical properties (observed or measured without changing what it is -- color, density, melting point, hardness) and chemical properties (how it behaves when it reacts and becomes something new -- flammability, reactivity with acid). A physical change alters appearance but not identity: ice melting is still H2O. A chemical change produces a genuinely new substance: burned wood becomes ash, smoke, and gas, and there is no getting the wood back.
Mastery looks like: They classify a given substance as an element, compound, or mixture, and correctly identify whether a described change is physical or chemical, with a reason.
Common stumbles: Assuming any blend must be a compound (mixtures do not need a fixed ratio); assuming a color change always means a chemical change (some, like melting crayon wax, are physical).