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neurologyYou need to know

• The law of conservation of mass states that in a closed system no atoms are lost or made during a chemical reaction, so the total mass of reactants equals the total mass of products.
• A closed system is one where no substances can enter or leave during the reaction.
• In a chemical reaction, atoms are rearranged to make new substances, but the number and type of atoms stay the same.
• A balanced symbol equation has the same number of atoms of each element on both sides of the equation.
• The reactants are the substances that react together and are written on the left-hand side of a chemical equation.
• The products are the new substances made in a reaction and are written on the right-hand side of a chemical equation.
• State symbols show the physical state of a substance in a chemical equation: solid is `(s)`, liquid is `(l)`, gas is `(g)`, and aqueous solution is `(aq)`.
• An apparent change in mass can happen in an open system when a gas escapes from the reaction mixture or when a gas from the air reacts with a substance.
• If a gas product escapes into the air, the measured mass of the reaction mixture decreases even though mass is conserved overall.
• If a gas from the air reacts with a substance, the measured mass of the reaction mixture can increase even though mass is conserved overall.
• Relative atomic mass, `A_r`, is the mean mass of an atom of an element compared with one twelfth of the mass of an atom of carbon-12.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Relative atomic mass takes account of the masses and relative abundances of the isotopes of an element.
• Relative formula mass, `M_r`, is the sum of the relative atomic masses of all the atoms shown in a chemical formula.
• For a substance made of molecules, relative formula mass may also be called relative molecular mass.
• The concentration of a solution in grams per cubic decimetre, `g/dm^3`, is the mass of solute dissolved in one cubic decimetre of solution.
• One cubic decimetre, `1 dm^3`, is equal to `1000 cm^3`.
• The relationship between concentration, mass and volume is `concentration = mass ÷ volume`, where concentration is in `g/dm^3`, mass is in grams, and volume is in `dm^3`.

• Calculate an unknown mass in a reaction by applying conservation of mass so that total reactant mass equals total product mass.
• Explain apparent mass changes in reactions by identifying whether a gas has escaped or whether a gas from the air has reacted.
• Balance a chemical equation by changing only the coefficients in front of formulae, never by changing the chemical formulae themselves.
• Check a balanced equation by counting the number of atoms of each element on both sides of the equation.
• Calculate relative formula mass by adding the relative atomic masses of every atom in the formula, including atoms inside brackets multiplied by the subscript outside the brackets.
• Calculate relative atomic mass from isotope data using `relative atomic mass = sum of (isotope mass × percentage abundance) ÷ 100`.
• Convert volumes between `cm^3` and `dm^3` by dividing by `1000` when changing `cm^3` to `dm^3`.
• Calculate the mass of solute using `mass = concentration × volume`, ensuring the volume is in `dm^3` when the concentration is in `g/dm^3`.