Chemical Changes Flashcards
what is the pH scale?
- measure of how acidic/alkaline solution is
the lower the pH scale…
the more acidic it is
the higher the pH scale…
the more alkaline it is
what has a pH of 1?
stomach acid
what has a pH of 3?
lemon juice
what has a pH of 8?
washing up liquid
what has a pH of 12?
bleach
what 2 things can you use to measure pH?
- indicator
- pH probe
how can you use an indicator to measure pH?
- changes colour depending on whether its above/below certain pH
- wide range indicators: contains mixture of dyes gradually change colour over pH
- useful for estimation
how can you use a pH probe to measure pH?
- measure electronically
- attached to pH probe in solution
- displays numerical value
- more accurate than indicator
what is an acid?
- forms aqueous solution of pH less than 7
- forms H+ ions in water
what is an alkali?
- base that dissolves in water to form solution
- with pH greater than 7
- forms OH- ions in water
what happens when acid mixes with base?
- neutralisation
acid + base
salt + water
what is the ionic equation of neutralisation?
H+(aq) +OH-(aq) –> H2O(l)
what pH is the product of a neutralisation reaction?
- pH 7
what is titration used for?
what can this then help you work out?
- find volume of acid needed ro neutralise a quantity of alkali
- concentration of acid/alkali
how do you carry out titration to find out the concentration of an alkali?
- using pipette and pipette filler, add set volume of alkali to conical flask
- add 2/3 drops of indicator
- use funnel to fill burette with some acid of known concentration
- record initial volume of acid in burette
- using burette, add acid to alkali gradually - regularly swirl flask
- go slow when you think end-point is near
- indicator changes colour when all alkali neutralised
- record final volume of acid left in burette to calculate volume of acid used to neutralise alkali
what is a safety precaution when carrying out titration?
- fill burettte with acid below eye level
- acid may spill over
- wear safety glasses
how do you increase the accuracy of titration?
- several consistent readings to spot anomalies
- first is rough to get approximate idea of end-point
- repeat to get same number within 0.1cm3
- calculate mean of results ignoring anomalies
what is universal indicator?
- mixture of different indicators
- estimate pH of solutions cuz can turn variety of colours
- each colour is narrow range of pH values
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what happens when UI is used in an acid or alkali?
goes from red to violet
why is indicator used in titration not UI?
want to see sudden colour change between alkali and acid
what does litmus change to in acids?
blue in alkali
red in acid