Conservation of Mass and Balanced EquationsAQA GCSE Chemistry: Flashcards

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State the law of conservation of mass.

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State the law of conservation of mass.: No atoms are lost or made during a chemical reaction, so the mass of products equals the mass of reactants.
What do the numbers in front of formulae in a balanced equation represent?: Multipliers showing how many units (molecules/formula units) of that substance react or form.
Can you change a subscript within a formula to balance an equation?: No — this would change the identity of the substance, e.g. H2O to H2O2 turns water into hydrogen peroxide.
Why might mass appear to decrease during a reaction in an open flask?: A gas produced in the reaction escapes into the air, so the measured mass of what remains is lower.
Why might mass appear to increase during a reaction, such as burning a metal in air?: A gas from the air (e.g. oxygen) joins the reaction, adding mass to the solid product.
Why is a closed system used to demonstrate the law of conservation of mass?: Because no matter can enter or leave, so the total mass measured stays constant, confirming no atoms are lost or made.
Balance this equation: __CH4 + __O2 → __CO2 + __H2O: CH4 + 2O2 → CO2 + 2H2O
How can you use masses of reactants and products to find balancing numbers for an equation? (HT): Convert each mass to moles, then find the simple whole number ratio between them to use as the balancing numbers.
What happens to the atoms during a chemical reaction, according to the law of conservation of mass?: They are rearranged into new substances — none are created or destroyed.
A carbonate is heated in an open test tube and loses mass. Does this break the law of conservation of mass?: No — carbon dioxide gas escapes into the air; if that gas were captured and included, total mass would be unchanged.