ACIDS and BASES

Why ‘acids and bases’?

•Scientists like to classify things and donating them as acids or bases is one way to do that.

•If a solution is not an acid or a base, it is half way between the two and known as a neutral solution.

What are some properties of Acids?

•The word acid comes from the Latin word acere, which means "sour." All acids taste sour. •Well known from ancient times were vinegar, sour milk and lemon juice. •By the 1884s acids defined by Svante Arrhenius (Sweden), as a material that can release hydrogen ions (H+).•Acids turn litmus paper red.•Acids are corrosive.

Some Common Acids

Name Chemical formula Uses

Hydrochloric acid

HCl Cleaning brickwork

Sulfuric acid H2SO4 Car batteries

Nitric acid HNO3 Fertiliser and dyes

Acetic acid CH3COOH Vinegar

Carbonic acid H2CO3Carbonated drinks

Citric acid C6H8O7Citrus fruits

Lactic acid C3H6O3Milk products

What are some properties of Bases?

•All bases taste bitter. •Mustard tastes bitter. Many medicines such as cough syrup taste bitter. Cough syrups are advertised as having a great taste. This is because sweeteners are added in order to cover the bitterness of the active ingredient in cough syrup.•Many bases are cleaning products and they have a slippery feel to them.•Bases release OH- ions into solution.•Bases turn litmus blue.•Bases are corrosive.

Some Common Bases

Name Chemical Formula Uses

Sodium hydroxide NaOH Soap-making, oven cleaners

Potassium hydroxide KOH Soft soap making

Calcium hydroxide Ca(OH)2Cement, plaster

Ammonium hydroxideNH4OH Household cleaners

Magnesium hydroxideMg(OH)2

Indigestion cleaners

Sodium carbonateNa2CO3

Glass-making

Sodium hydrogen carbonate NaHCO3

Baking soda

The pH of a solution

•This scale is logarithmic : It means that every time you go down 1 level on the scale, the solution is 10x stronger.•For example an acid of pH 1 is 10 times more acidic than an acid with pH 2 and 100 times more acidic than an acid of pH 3.•It also means that a base of pH 11 is 10 times more basic (stronger basicity) than a base of pH 10 and 100 times more basic than a base of pH 9.

70 14

More acid, strong acid

Less acid,Weak acid

Less basic,Weak base

More basic, strong baseNeutral

The pH scale enables us to classify acids and bases

The pH scale 

pH Example

Acids

0 HCl : Hydrochloric acid

1 Stomach acid

2  Lemon juice

3  Vinegar

4 Soda, tomato juice

5 Black coffee

6 Milk, rain water

Neutral 7 Pure water

Bases

8 Egg whites

9 Baking Soda

10 Limewater / antacid

11 Ammonia

12 Mineral Lime - Ca(OH)2

13  Drano®

14 NaOH : sodium hydroxide

10X less acidic

100x more acidic

Indicators

•Indicators are substances that undergo colour changes in either acids or bases or in both

•Because of these properties, scientists can identify whether a solution is an acid or a base by noting the colour change

•Some popular indicators are: phenolpthalene, litmus paper, universal, methyl blue

Indicators

70 14

More acid, strong acid

Less acid,Weak acid

Less basic,Weak base

More basic, strong baseNeutral

What does a neutral solution look like?

H HO

HO

-H

++

+-

+-

+-

+-

•The glass of pure water is neutral (pH=7). •There are as many “H+” ions as there are “OH-” ions.•When a water molecule splits into hydrogen and hydroxide ions it is said to be dissociated.

Water molecules in a glass jar

Some water molecules have a natural tendency to split into OH- and H+ ions (see inside red oval shape)

What does an acid look like in solution?

Add some acid to the water

H+

OH-

The acid dissociates leaving more H+ than OH-. The liquid is therefore Acidic

H+OH-

H+ H+

H+

Hydrochloric Acid (HCl)

H+ H+

H+

Cl- Cl-

Cl-

Start with a neutral solution of water

What does a base look like in solution?

H+

OH-

Base: Sodium Hydroxide(NaOH)

The base dissociates leaving more OH- than H+ : the liquid is therefore Basic

Add some base to the water

OH- OH-

OH-

Na+ Na+

Na+

H+OH-

OH-

OH-

OH-

Start with a neutral solution of water

•When there are as many OH- as H+, the solution becomes neutral as the acid has been neutralised by the base.

Base: NaOH

OH- OH-

OH-

Na+ Na+

Na+

H+OH-

H+H+

H+

Cl-

Cl-

Cl- H+

OH-

OH-

OH-

OH-

H+

H+

H+

Na+

Na+

Na+

Cl-

Cl-

Cl-

Neutralisation

Start with an acid solution i.e. HCl: more H+ than OH-

Add a base i.e. NaOH

The OH- from the base associates with the H+ from the dissociated acid

•When there are as many H+ as OH-, the solution becomes neutral as the base has been neutralised by the acid.

Acid: HCl

H+OH-

OH-

OH-

OH-

Na+

Na+

Na+

H+

OH-

OH-

OH-

OH-

H+

H+

H+

Na+

Na+

Na+

H+ H+

H+

Cl-

Cl-

Cl-

Neutralisation

Start with a Basic solution i.e.NaOH: more OH- than H+

Add some acid i.e.HCl

The H+ from the acid associates with the OH- from the dissociated base

Some Common Salts

Acid + Base Salt + Water

Base Acid Salt Use

Sodium hydroxide

Hydrochloric acid

Sodium chloride

Table salt

Potassium hydroxide

Nitric acid Potassium nitrate

Explosives, fertilisers

Calcium hydroxide

Sulfuric acid

Calcium sulfate

Plaster

Salts

What is a salt?A salt is an ionic compound made up of a metal and a non metal

Non metal

Metal

H+

Cl-

Cl-

H+

H+Cl- H H

HydrogenGas

Fe+

Metal and acid reactions

Fe

e-

e-

e- e-

e-e-

e- e-

e-

A metal lattice of iron with many electrons

HCl is added and dissociated

The H+ ions join to form Hydrogen gas

The chloride joins with the metal to make iron chloride

Overall reaction for metals and acids:

Metal + Acid Hydrogen + Salt

Metal and acid reactionsMetal + Acid Hydrogen + Salt

•When sulfuric acid forms a salt it is a sulfate salt

•When nitric acid forms a salt it is a nitrate salt

•When acetic acid forms a salt it is an acetate salt

•When hydrochloric acid forms a salt it is a chloride salt

Sulfuric acid + magnesium Magnesium sulfate + hydrogen

Nitric acid + zinc Zinc nitrate + hydrogen

Acetic acid + potassium Potassium acetate + hydrogen

Hydrochloric acid + sodium Sodium chloride + hydrogen

ACIDS and BASESThe End