chemistry - elements, compounds and molecules

solubility

what is meant by the term solute

the substance being dissolved in the solvent

what is meant by the term solvent

the liquid the solute is dissolving in

what is meant by the term solution

the substance produced when the solute is dissolved in the solvent

what is meant by the term saturated solution

when the maximum amount of solute is dissolved in a certain amount of solvent at a certain temperature

what does solubility in the units per 100g of solvent mean?

the measurement of the maximum amount of solute that can dissolve in exactly 100g of solvent at a certain temperature

how to plot and interpret solubility curves

plot:

• measure the solubility (maximum amount) of solute that can dissolve in the solvent at a certain temperature

• do this multiple times at different temperatures, and then plot and draw the curve

interpret:

• if the curve is going up as the temperature increases, the solute was a solid

• if the curve is going down as the temperature increases, the solute was a gas

explain the dilution of coloured solutions and the diffusion of gases

practical: investigate the solubility of a solid in water at a certain temperature

method:

• set the water bath to a certain temperature, measure a certain amount and add the solid

• continue to add more solid until it stops dissolving and remains solid at the bottom of the beaker (ensures it is saturated)

• weigh the evaporating basin and then pour a portion of the solution (with no remaining solid) in it

• weigh the evaporating bassin and the solid together to get the total mass

• use a bunsen burner to evaporate the solvent until only crystals of the solute are left

• weigh the evaporating basin + solute crystals

• use these masses to calculate solubility

• repeat the experiment at different temperatures

what should happen:

• as the temperature increases, the time taken for the solid to dissolve should decrease

• this is because as the particles re heated up, they gain kinetic energy, which reduces the sxtrength of the intermolecular forces and causes the particles to break apart, making them more soluble

how to classify substances as an element, compound and mixture

element - substance made of one type of atom

compound - a substance where 2 or more different atoms chemically bonded together

mixture - a substance where 2 or more different atoms NOT chemically bonded together

melting/boiling points of pure substances vs mixtures

• a pure substance will have a fixed melting and boiling point

• a mixture will have a range for their melting/boiling point

separation of mixtures

simple distillation

a mixture of a soluble solid and a liquid

as the substances have different boiling points, the solution is heated until one of the substances reaches their boiling point and evaporates

the evaporated liquid rises, adn the gas is eventually condensed in the condenser back into a liquid, and pours into a separate flask

fractional distillation

used to separate 2 or more miscible liquids

the solution is heated and the substance with the lowest boiling point will evaporate and rise

it will travel through a fractionating column, whcih will cause the liquid to heat and condense many times, ensuring the purest separation

the gas will pass through a condenser, and pour into another flask

filtration

used to separate an insoluble solid from a solution

a piece of filter paper is placed in a funnel, and the solution is poured into the funnel

the liquid will travel through the filter paper into a beaker, while the undissolved solid will be caught in the paper as residue

crystallisation

used to separate a soluble solid from a solution

the solution is heated, allowing the liquid to evaporate so that you are left with a saturated solution

the saturated solution will be left to cool, so that crystals willf orm due to decreasing solubility

the mixture will be filtered and you will be left with the solid

paper chromatography

used to separate substances with different solubilities in a certain solvent

draw a pencil line about 1cm from the edge of the filter paper, and put sample sof the different dyes on it

fill a beaker with water and place the paper in it, ensuring that the water level is below the level of the solvent

the solvent travels up the paper, taking the different dyes with it (in different amounts based on their solubility in the solvent), allowing you to see the different dyes

how does a chromatogram provide information about the composition of a mixture

because the different dyes have different solubilities in water, they will travel at different rates, therefore travellign different distances up the paper, identifying the different substances

caluclating the Rf value

distance travelled by dye/ distance travelled by solvent