Acidity and Alkalinity
How do we tell an acid from a
base and what is it good for?
What is an acid?
• A substance that donates a hydrogen ion
(proton) when dissolved in water.
• The hydrogen
ion goes with
water to make
hydronium
(H3O)
• It tastes sour
What is a base?
• A substance that accepts a hydronium ion
(H30) when in water.
• It releases hydroxide ions OH- when
dissolved in water
• It tastes bitter
• Examples include: Ammonia, sodium
hydroxide and baking soda
• What about bases make them react with
acids to make salt?
• Tomato is at 4 on the pH scale and lemon is at 2.
How many times more acidic than tomato do you
think the lemon is?
• 100times more! Each step on the pH scale
works out to be 10 times more acidic or basic!
Indicators
• There are several substances which react
with acids and bases
• They can tell us whether the substance is
acidic or basic
• Examples include: litmus paper,
bromothymol blue and red cabbage juice
Litmus paper
• Litmus paper is used to tell whether a solution is an acid
or a base.
• It cannot tell you how strong the acid or base is
• It comes in 2 colours: red and blue
• Red paper turns blue in a base
but will not change otherwise
• Blue paper turns red in an acid
but will not change otherwise.
• Why do you think we sometimes
need to test an acid or a base
with both colours of paper before
knowing what it is?
Bromothymol Blue
• Bromothymol Blue is a liquid that reacts in acids
and bases
• It turns yellow in acid
and blue in base
• It takes on a green
tinge in a neutral
solution
• It is good for telling
the exact pH when
the substance is only
slightly acidic or basic
Red Cabbage Juice
• Red cabbage juice is another liquid indicator that
will change colour in an acid or base
• It can be made by
boiling red cabbage
and keeping the juice
• It will change a wide
range of colours and so
can be used to accurately
determine the aciditiy
of a substance.
Classify each of the following as
either an acid or a base
Bibliography of images
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http://science.nayland.school.nz/SimonPa/Webpage/Year10/Acid_base_images/acids
_and_bases_phscale.jpg
http://www.mhhe.com/physsci/chemistry/chang7/esp/folder_structure/cr/m3/s3/assets
/images/crm3s3_1.jpg
http://www.biologycorner.com/resources/dissociation.gif
http://oldwww.mtlsd.org/senior/science/JPtachcinski/Lab_Proc/Basic%20Science%20
Lab%20Techniques_files/litmus.jpe
http://www.farm.ucl.ac.be/tpao/instrumentation/titrimetrie/Indicateurs/HBtB.jpg
http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg
http://wetheadmedia.com/wp-content/uploads/pictures/2009/03/Arm-Hammer-BakingSoda.jpg
http://cojent7.files.wordpress.com/2010/05/coca-cola.jpg
http://www.bized.co.uk/images/battery.jpg
http://beautifullyused.com/wp-content/uploads/2009/03/lemon.jpeg
http://www.mochadad.com/wp-content/uploads/2008/12/soap-on-a-rope.jpg
http://www.savingwithshellie.com/wp-content/uploads/2009/04/ammonia.jpg