C4 Chemical Changes, Acids and Bases (page 129) Flashcards
What is a pH scale?
its a measure of how acidic or alkaline a solution is
What does it mean ‘The lower the pH of a solution’ mean?
The more acidic it is
The higher the pH of a solution means what?
the more alkaline it is
What do ‘neutral’ mean on the pH scale?
pure water - a neutral substance has pH 7
See pH chart on page 129
How can you measure the pH of a solution?
An indicator is a dye that changes colour depending on whether it’s above or below a certain pH.
Some indicators contain a mixture of dyes that means they gradually change colour over a broad range of pH. These are called wide range indicators and they’re useful for estimating the pH of a solution. for example a universal indicator gives the colours between reds/yellows/neutral/blues/purples
A pH probe attached to a pH meter can also be used to measure pH electronically. The probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value, meaning it’s more accurate than an indicator.
What is an acid?
an acid is a substance that forms aqueous solutions with a pH of less than 7.
What do Acids form in water?
H+ ions
What is a Base?
a Base is a substance with a pH greater than 7
What is an Alkali?
an Alkali is a base that dissolves in water to form a solution with a pH greater than 7.
What do Alkalis form in water?
OH- ions
What is the reaction called between acids and bases?
neutralisation:
acid + base»_space;» salt + water
Neutralisation between acids and alkalis can be seen in terms of H+ and OH- ions, write the equation?
H+(aq) + OH-aq) »_space;» H2O(l)
(Hydrogen (H+) ions react with hydroxide (OH-) ions to produce water
When an acid neutralises a base (or vice versa), the products are neutral, what do this mean?
they have a pH of 7
An indicator can be used to show that a neutralisation reaction is over
A student uses universal indicator to test the pH of some lemon juice. What colour would you expect the indicator to turn? (1 mark)
red/orange
the pH of an unknown solution is found to be 8. Is the solution acidic or alkaline? (1 mark)
alkaline (1 mark)
