Is brass a compound or a mixture? I'm confused about how to classify pure substances vs mixtures
Meera Patel11th Sample
27 Aug 2026 · 1 min read
Normally I do not have trouble with this chapter but a classification question in our last test has made me doubt myself.
The question was: 'Which of the following is a homogeneous mixture?' with options air, brass, sugar solution and... all of the above? I chose air and sugar solution and marked 'all of the above' wrong, because I thought brass is an alloy of copper and zinc, so a compound of the two.
My understanding so far (please correct):
- Pure substance: fixed composition, cannot be separated by physical methods. Elements and compounds.
- Mixture: variable composition, components keep their properties, separable by physical methods. Homogeneous or heterogeneous.
- Compounds: elements chemically combined in a fixed ratio by mass.
The things I'm unsure about: (1) alloys, (2) ozone and oxygen, are they a mixture, a compound, or what? (3) Is Hg an element, even though it is a liquid and 'looks like a mixture of silver'? (4) What do we call a solution of NaCl in water: pure substance? Also a plain-language test to tell a compound from a mixture would help.
6 answers & replies
Sign in to join the conversation.
[+][–]Zoya Khan12thSample· 1mo ago
Here is how I check, in order. Ask: can I get the components back using only physical methods (filtering, evaporation, distillation, magnet, even melting)? If yes, it's a mixture. And ask: does it have a fixed composition with properties different from its parts? If yes it is a compound. For example iron and sulphur powder mixed together can be separated with a magnet, but iron sulphide formed by heating them cannot, and it is not magnetic like iron anymore. Compounds also follow the law of definite proportions: water always has H:O = 1:8 by mass.
[+][–]Ishaan Bose11thSample· 1mo ago
Brass IS a compound, it has a fixed Cu and Zn in it and you can't see the two metals separately, so it's chemically combined. Chemistry people name everything so confusingly. My answer: all of the above is wrong because brass is a compound 😅
[+][–]Kavya NaikDropperSample· 1mo ago
Ishaan, the key is that brass is a homogeneous mixture (a solid solution). The composition of brass is variable: different grades have different Cu:Zn percentages, and the metals keep their individual chemical properties. A compound would have one fixed ratio. You can't see the two metals separately only because they are mixed uniformly at the atomic level, 'looks uniform' means homogeneous, not compound. So Meera, 'all of the above' was correct.
[+][–]Meera Patel11thSample· 1mo agoAuthor
Understood, thank you. The 'variable composition' part is what I was not applying to alloys. Will fix my notes.
[+][–]Aman YadavDropperSample· 1mo ago
(2) O₂ and O₃ are both elements, they are allotropes of oxygen. Only oxygen atoms present, just arranged differently. (3) Hg is an element, a metal that happens to be liquid at room temperature. (4) NaCl solution is a homogeneous mixture, not a pure substance, because you can change the amount of salt, the composition varies. Pure substance means fixed composition throughout, nothing else.
[+][–]Tanvi Deshmukh12thSample· 1mo ago
Flowchart I use in notes!!
Matter → pure substance or mixture
- Pure → element (one kind of atom, e.g. Na, O₂, Hg, S₈) or compound (fixed ratio, e.g. H₂O, NaCl, CO₂)
- Mixture → homogeneous (one phase, e.g. air, brass, salt solution) or heterogeneous (sand + salt, oil + water)
Trick: if you have a ratio that can be changed freely, it's a mixture. If it can't be changed unless you change the formula, it's a compound.
