Topic 3 – Chemical changes Flashcards
how can we determine the pH of a solution
Put a drop of universal indicator which changes colour depending on how acidic or alkaline it is
pH of Acids
less than 7
pH of Alkalines
more than 7
pH of neutral substances ( e.g : water )
7
what is an indicator
a dye that changes colour depending on whether it is below or above a certain pH
litmus colour ( acidic , alkaline , neutral )
red , blue , purple
methyl orange colour ( acidic , alkaline , neutral )
red , yellow , yellow
phenolphthalein colour ( acidic , alkaline , neutral )
colourless , pink , colourless
what ions do acids release in solution
H+ ions
what ions do alkalis release in solution
OH- ions
Concentration of H+ ions
The higher the concentration of H+ ions, the more acidic it is and so the lower the pH
Concentration of OH- ions
The higher the concentration of OH- ions, the more alkaline it is and so the higher the pH
pH scale and H+ ions relationship
As pH decreases by 1 unit, concentration of H+ ions increases by x10
Strong acids
Strong acids almost completely ionise in water, releasing H+ ions
As there are more H+ ions being formed strong acids are more acidic and so have a lower pH ( 0-2 )
Weak acids
Weak acids only partially ionise in water, if you put a sample of weak acid only some of the acid molecules would release H+ ions
As there are less H+ ions being formed weak acids are less acidic and so have a higher pH
