C2.1 (purity and separating mixtures)

• what can masses of subatomic particles be described by?


  Relative masses (their masses compared to the mass of a proton)
• how can masses of atoms be described?

  Relative atomic masses (masses compared to a standard atom)
• how can relative atomic mass be described in relation to a carbon -12 atom?


  The relative atomic mass of a carbon-12 atom is 12.0 exactly, so if the relative atomic mass is below 12.0 then it has less mass than a carbon-12 atom and if it is above 12.0 then it has more mass than a carbon-12 atom
• what does the periodic table show?

  The relative atomic (Ar) masses for each element, the bottom number represents the relative atomic mass for that element's atoms
• what does chemical formula tell you?

  How many atoms of each element there are in a unit of a substance, for example, the chemical formula of water is H2O so it tells you that each molecule of water contains 2 hydrogen atoms and 1 oxygen atom joined together
• what do the brackets in aluminium sulfate show you?

  Al2(SO4)3

  the 3 tells you that there are 3 times the number of each atom inside the brackets, a unit of aluminium sulfate contains 17 atoms: 2 aluminium atoms , (3x1) 3 sulfur atoms and (3x4) 12 oxygen atoms
• what does the term relative formula mass refer to?

  All the substances that consist of 2 or more atoms
• what does the term relative molecular mass refer to?

  For substances that exist as molecules instead of elements
• what is the symbol for both relative formula mass and relative molecular mass?

  Mr
• what can you not find in the periodic table?

  Relative formula mass values, instead you work them out , you add together the relative atomic mass values for all of the atoms in the formula for the substance because Ar and Mr values have no units
• calculate the relative formula mass of magnesium hydroxide , Mg(OH)2

  1, work out the relative atomic mass values of the elements in the compound: Mg=24.3 O=16 and H=1

  2, work out the number of atoms of each element in Mg(OH)2 : Mg=1, O=(2x1) 2 and H=(2x1) 2

  3, multiply these values then add them together to calculate the relative formula mass: = (1x24.3) + (2x16) + (2x1) = 58.3
• what does empirical formula show?

  The simplest whole number ratio of the atoms of each element in a compound
• find the empirical formula for butane (C4H10)

  1, find the highest common factor (2 in this case)

  2, divide the chemical formula by the highest common factor: C=4/2=2 and H=10/2= 5

  3, empirical formula: C2H5
• calculating empirical formula from diagrams ?

  1, work out the number of atoms of each element in the molecule by counting how many times each chemical symbol is shown

  2, divide by the highest common factor

  3, write down the empirical formula
• what does a balanced chemical equation show?

  The formulae and number of units for all of the substances in a reaction eg, C + O2 = CO2

  C=12 and O2 = 32 and CO2 = 44, so this equation is balanced
• how can you work out the relative formula mass using a balanced equation?

  If you know the other relative formula masses that have been used, then just either add them up to find a total or subtract the other numbers from the total to find the missing number
• what does a pure substance mean?

  a substance that just consists of one element or compound
• are mixtures pure substances?


  No they are impure because they contain more than one element or compound
• why is it hard to find pure substances?

  Just one atom or molecule of another substance makes that substance impure
• what is an alloy?


  A mixture of a metal with one or more other elements
• what is an example of an alloy?


  Jewellery is normally made of gold and copper because gold on its own is very soft
• what is the melting point of a substance?

  The temperature at which it changes from the solid state to the liquid state
• what is the melting point of a pure substance?


  A single temperature
• what happens to the melting point of an impure substance?

  Its melting point is less than that of a pure substance and it often melts over a range of temperatures not just one temperature
• how can you determine purity levels from melting points?

  The greater the difference between the measured melting point for a substance and its accepted melting point, the lower the purity is likely to be
• what must you do when measuring melting points?

  Heat the substance slowly and stir the substance as it melts
• what does the temperature of a pure substance do as it melts?

  Remains constant
• how can you measure melting points?

  Heating the substance and either measure the temperature at which it melts or measure its temperature at regular time intervals and look at any changes
• what does heating a substance slowly allow it to do?

  Allows the temperature of the whole sample to increase
• how does mixing help when measuring melting points?

  It ensures the entire sample is at the same temperature
• how do you improve the accuracy of a measurement of a melting point?

  Mixing the substance and heating it slowly
• when does a solution form?

  When one substance dissolves in another
• what is a solute?


  The substance that dissolves
• what is a solvent?

  The substance that it dissolves in
• when is a substance soluble?

  If a substance can dissolve in a particular solvent
• when is a substance insoluble?


  If the substance cannot dissolve in a particular solvent
• what does filtration do?

  Separates an insoluble substance in the solid state from substances in the liquid state
• how does filtration work?

  The filter paper has tiny holes, so when you filter a mixture of sand and water, the water molecules are small enough to pass through but the large and insoluble grains of sand cannot
• what is residue?


  The things left in the filter paper
• why do chemists 'flute' their filter paper?

  It provides a larger surface area for a filtrate to pass through