States of matter, physical vs. chemical properties, and the changes that move a substance between them — the one distinction that decides almost every question here is whether a new substance formed. Every question comes with a written explanation of exactly what changed.
Dissolving sugar in water, melting ice, or crushing a rock all look significant but don't create a new substance — the test is whether a new substance formed, not how dramatic the change looks.
Sublimation is a solid turning directly into a gas, skipping the liquid state entirely (like dry ice); evaporation is specifically a liquid turning into a gas — the two start from different states.
Flammability is a chemical property because you can only confirm it by actually burning the substance — you can describe it, but proving it requires watching the reaction happen.
Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.
Which of the following is an example of a chemical change?
A — Burning a piece of paper. Burning paper produces new substances — ash, carbon dioxide, and water vapor — that weren't present before, which makes it a chemical change. Melting an ice cube (B) is a physical change: the water's chemical identity, H₂O, stays exactly the same whether it's solid or liquid.
Dry ice (solid carbon dioxide) is left out at room temperature and turns directly into carbon dioxide gas, without ever becoming a liquid. What is this phase change called?
A — Sublimation. A solid transitioning directly into a gas, skipping the liquid state, is called sublimation — exactly what happens with dry ice. Evaporation (B) specifically describes a liquid turning into a gas, which doesn't apply here since dry ice never becomes liquid carbon dioxide at normal pressure.
A student burns 10 grams of wood in a sealed, airtight container along with enough oxygen for complete combustion, then weighs everything remaining inside the container — ash, gases, and unused oxygen. Based on the law of conservation of mass, what should the total mass be, and why?
A — Equal to the combined starting mass of the wood and oxygen, because mass is conserved in a sealed system even during a chemical change. Conservation of mass applies to chemical changes just as much as physical ones, as long as the system is closed — no atoms are created or destroyed, they're only rearranged into new substances. Since the container is sealed, the total mass of everything inside equals the combined starting mass of the wood and oxygen. Less than the starting mass, because burning destroys some of the matter and converts it to energy (B) is a common misconception — mass "seeming to disappear" when something burns in an open container is really just the gaseous products escaping into the air, not evidence that mass was actually destroyed.
Matter and properties questions reward one habit above all: asking whether a new substance actually formed before calling something a chemical change.
For every change described, decide whether a new substance was created before labeling it physical or chemical. Untimed practice is where that single question becomes automatic.
Move to timed sessions once physical-vs-chemical classification is instant.
Pair matter and properties with chemical reactions in a mock — both hinge on the same new-substance test.
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