Chemistry: particles, reactions and mass
Separate a substance description from a separation method. A pure substance has a definite chemical identity; a mixture contains constituents in variable proportions. Filtration can separate a suitable insoluble solid from a liquid, but dissolved salt passes through ordinary filter paper. Evaporation can recover a nonvolatile dissolved solid. Distillation instead collects a vapor after condensation and can recover a solvent when the volatility difference is suitable. Select the method for the desired component rather than memorizing one method for every mixture.
A chemical equation represents a rearrangement of atoms. Balance it by changing coefficients, never by changing subscripts inside a formula, because subscripts define a substance. Count every element on both sides and distinguish coefficients from the number of particles inside one formula unit. Color change, a precipitate, heat or a gas may suggest a reaction, but a single visible sign alone is not universal proof: boiling also makes bubbles without making a new chemical substance. Interpret the sign together with the full evidence.
Mass conservation applies to the complete closed system. An open-container balance can fall when gas escapes or rise when a reactant enters from the air. Include that transferred matter before claiming a violation. A supplied mass ratio can identify a limiting reactant; the unreacted excess still contributes to the final total. For aqueous acid and alkali tasks, use the specified indicators, concentrations and amounts. Neutralization need not leave pH exactly seven when one reactant is in excess. Ionic equations must conserve charge as well as atoms. Treat a reaction exercise as data analysis and follow appropriate supervised laboratory rules for any actual experiment.