s1.1 the particle model

Question 1

what are the characteristics of matter?

Answer 1

1. made up of particles
2. has a mass
3. occupies a volume in space
4. has particles in constant motion

Question 2

how has the atomic theory develop over time?

Answer 2

1. greek philosophers democritus and leucippus - matter is composed of tiny particles
2. dalton - elements exist in small packets of matter
3. thompson - measured mass of cathode rays, discovered electrons, plum pudding model
4. rutherford, geiger & maarsden - alpha particle experiment, small positive mass in centre of atom with rest being empty space
5. bohr - planetary model, electrons in orbit around nucleus

Question 3

what is an element?

Answer 3

an element is a pure substance that cannot be broken down into a simpler substance by chemical means

Question 4

what is a compound?

Answer 4

a compound is a pure substance formed from two or more different elements joined in a fixed ratio. compounds have different properties from the elements they are made up of

Question 5

what is a mixture?

Answer 5

mixtures contain more than one element and/or compound that are not chemically bonded together and so retain their individual properties

Question 6

what is a homogenous mixture?

Answer 6

a homogenous mixture has the same uniform appearance and composition throughout

Question 7

what is a heterogenous mixture?

Answer 7

a heterogenous mixture consists of visibly different substances or phases

Question 8

how do you pick a separation technique?

Answer 8

the choice of the method of separation depends on the nature of the substances being separated. all methods rely on there being a difference of some sort, usually in a physical property such as boiling point, between the substances being separated

Question 9

what is solvation?

Answer 9

solvation can be used to separate a heterogenous mixture of two solids based on differences in solubility. for a difference in solubility, a suitable solvent must be chosen (e.g. water is a suitable solvent for a mixture of sand and salt)
solvation involves the formation of ion-dipole forces between a solute and solvent. when a salt is dissolved the ions are pulled apart from the lattice structure and surrounded by water molecules. the forces of attraction between the solute particles and the water molecules are ion-dipole forces

Question 10

what is filtration?

Answer 10

filtration involves the separation of an insoluble solid from a liquid or solution. the mixture is poured through filter paper: the liquid filtrate is removed but the solid residue is left in the filter paper

Question 11

what is evaporation/crystalisation?

Answer 11

crystalisation is used to separate a soluble substance from the solvent in the solution. heat the solution so that the solvent evaporates. the saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases and crystals will grow. the crystals are collected by filtering the solution and they are then washed with distilled water to remove any impurities and is then allowed to dry

Question 12

what is distillation?

Answer 12

distillation can be used to separate a mixture of miscible liquids that have different boiling points. an example is ethanol and water. water and ethanol are miscible, meaning that they fully dissolve. the mixture is heated until the more volatile liquid is vaporised and removed

Question 13

what is chromatography?

Answer 13

chromatography is a separation technique used to separate a mixture of substances that have different solubilities

Question 14

what is recrystallisation?

Answer 14

recrystallisation is a purification technique used to purify a solid. this technique uses the varied solubilities of solids at different temperatures to separate them. this is used to purify sugar crystals from sugar cane juice and is also used by pharmaceutical companies to remove any impurities that could contaminate medication. once the solution has cooled down to room temperature and crystallised, product crystals can be recovered by filtration using buchner apparatus

Question 15

what are the steps of recrystallisation?

Answer 15

1. place a boiling tube and glass filter funnel in the oven so that it is hot
2. set up a water bath in a 250cm3 beaker and start heating the water
3. place a boiling tube which is about two thirds full of deionised water in the water bath
4. weigh out about 1g of the product into another boiling tube and place this boiling tube in a water bath
5. once the water is hot, add water from the boiling tube of deionised water to the impure product. dissolve the product in a minimum volume of hot water
6. collect the hot glassware from the oven and place a piece of fluted filter paper in the funnel. as quickly as possible, pour the solution of the product through the filter
7. leave the filtrate to cool to allow the product to crystallise
8. once it has cooled and crystallised, filter off the crystals under reduced pressure. wash with some cold deionised water and dry the crystals in a warm oven

Question 16

what is paper chromatography and how is it done?

Answer 16

this technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink). a pencil line is drawn on chromatography paper and spots of the sample are placed on it. pencil is used for this as ink would run into the chromatogram along with the samples
the paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so that the samples don’t wash into the solvent container. the solvent travels up the paper by capillary action, taking some of the coloured substances with it. this will show the different components of the dye

Question 17

what are Rf values and how do you calculate them?

Answer 17

the retention factor (Rf) value is a ratio that describes how far a component in a mixture travels in relation to the distance traveled by the solvent. it is calculated using the formula:
Rf = distance travelled by component / distance travelled by solvent
components in the mixture that are more soluble in the mobile phases will have a larger Rf value

Question 18

how do you identify colourless compounds in chromatography?

Answer 18

for chromatography to be useful, the chemist needs to be able to see the components move up the paper. this is not the case for colourless substances, such as amino acids or sugars. locating agents such as ninhydrin or the use of a UV lamp can be used to see the spots. they react with the sample and produce a visible/coloured spot for the products

Question 19

what are the properties of solids?

Answer 19

1. fixed shape because particles vibrate around a fixed point
2. fixed volume because particles vibrate around a fixed point
3. not compressible because particles are very closely packed
4. not fluid because particles vibrate around a fixed position

Question 20

what are the properties of liquids?

Answer 20

1. no fixed shape because particles can move around
2. fixed volume because particles are closely packed
3. not compressible because the particles are closely packed
4. fluid because particles can move around each other

Question 21

what are the properties of gases?

Answer 21

1. takes same shape of container because particles move quickly in all directions
2. no fixed problem because particles are far apart
3. compressible because particles are far apart
4. fluid because particles can move around each other