AQA · Chemistry · 8462
GCSE Chemistry (AQA): Compounds, Molecules & Mixtures
Topic 1 — Atomic Structure & the Periodic Table. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Compounds, Molecules & Mixtures (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 1.03 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_1.03.
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Why is oxygen (O2) a molecule but not a compound?
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It contains only one type of element; a compound must contain two or more different elements.
Why is helium not classed as a molecule?
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It exists as single atoms, and a molecule needs two or more atoms bonded together.
What is a molecule?
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A group of two or more atoms held together by chemical bonds.
Why can the substances in a mixture be separated by physical methods?
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They are not chemically bonded to each other, only physically mixed, so filtration, crystallisation or distillation can separate them.
How many atoms of each element are in H2SO4?
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Two hydrogen, one sulfur and four oxygen.
Why is sodium chloride not classed as a molecule?
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It exists as a giant ionic structure of millions or billions of atoms, not small separate units.
What does the formula NaCl represent for a giant ionic structure?
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A ratio — sodium and chloride ions are present in a 1:1 ratio.
What is a mixture?
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Two or more substances that are not chemically combined — there are no chemical bonds between them.
In a compound, the elements are always present in the same ?.
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proportions
What is the chemical name and formula of common table salt?
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Sodium chloride, NaCl.
In a chemical formula, what does a number outside brackets mean?
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How many of the whole group inside the brackets there are, e.g. Ca(OH)2 has two OH groups.
What is a compound?
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A substance containing two or more different elements held together by chemical bonds.
What does the subscript number in a chemical formula tell you?
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How many atoms of the element before it are in the molecule. No number means one atom.
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