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 acidand 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 byboiling 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• 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-Baking-

Soda.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