Ms. Hennessey Rev A 05/13

Acids and BasesIntroduction

Junior Science

Lesson Objectives

• Know the properties of Acids and Bases

• Understand what is meant by the pH and the pH scale

• Classifying substances as acidic, basic or neutral

• Test for and identify the pH of a variety of common substances

Chemical Compounds

• Chemical compounds are form when two or more elements chemically combine (ionic or covalent bond).

• Acids, Bases and Salts form three of the most important classes of chemical compounds.

Acids Bases

Alkali

Salt are formed when acids and bases

neutralise each other

Alkalis are bases that dissolve in water

Salts

Properties of Acids

• All acids have a sharp sour taste

• Acids are a group of common chemical compounds which donate Hydrogen Ions (H+)

• Acids can be dangerous because theyare corrosive and eat away at metal, skin and clothing.

• Acids neutralise bases.

Common Household Acids

Vinegar (Ethanoic Acid)

Vitamin C(Ascorbic Acid)

Fizzy DrinksCarbonic Acid

Citrus Fruit(Citric Acid)

Common Laboratory Acids(Inorganic - Mineral Acids)

Hydrochloric Acid HCl

Sulphuric Acid H2SO4

Nitric Acid HNO3

These common laboratory acids are called ‘Mineral Acids’ because they can be made

from minerals in the earth. They are harmful if ingested.

Discovered by the Swedish chemist Carl Wihelm Scheele,organic acids make up a group of substances with acidic

properties that have carbon atoms in their formula.

Common Household Acides(Organic Acids)

Not all acids are corrosive, examples include:

Carbonic Acid – Fizzy drinks

Ethanoic Acid – Vinegar

Citric Acid – Oranges

Properties of Bases

• Bases have a bitter taste.

• Bases are chemically opposite to acids and contain hydroxide (OH-) ions.

• They are corrosive, burn the skin and have a soapy feeling.

• Bases that are soluble in water are called Alkali’s.

• Bases neutralise acids.

Common Household Alkali’s

Many household cleaning agents are bases.

Common Laboratory Alkali’s

Sodium Hydroxide NaOH

Calcium Hydroxide Ca(OH)2

Ammonia NH3

Potassium Hydroxide KOH

pH Scale

The pH scale runs from 0 -14

Acid solutions pH values < 7

Neutral solutions have a pH = 7

Alkali solutions have pH > 7

pH is a measure of hydrogen ion

concentration H+, a measure of the

acidity or alkalinity of a solution.

Indicators

An indicator is a chemical substancewhich shows by means of a

colour change, whether a substance is an Acid or an Alkali.

What is Litmus?

• Litmus is the oldest and most commonly used indicator to show whether a substance is an acid or a base

• Litmus is a mixture of coloured organic compounds obtained from several species of lichen that grow in The Netherlands, particularly

Litmus turns RED in acidic solutions and BLUE in alkaline solutions.

Lecanora tartarea and Roccella tinctorum.

Litmus Paper

Acids turn bluelitmus RED

Bases turn redlitmus BLUE

Universal Indicator

Universal indicator is used to find the exact pH of a substance.

It changes colour depending on the pH of the substance.

What does pH Measure?

pH represents the effective concentration of hydrogen ions (H+)

in water.

Importance of pH

• Soil pH is very important in Farming

• pH in Body fluids

– blood and digestive juices

Effects of pH on Plants- Hydrangea

ph 6.0-6.5(red) pH 5.0-5.5 (Blue)

At low pH Hydrangea can absorb Aluminium from the soil which turns the flowers blue.

Summary

• An acid is a substance that turns blue litmus paper to red.

• Acids can be strong (e.g. hydrochloric acid, sulfuric acid) or weak (e.g. vinegar, lemon juice).

• A base is a substance that turns red litmus paper to blue.

• Strong bases include sodium hydroxide and potassium hydroxide, weak bases include soap and baking soda.

• Bases that dissolve in water are called alkalis.

• The pH scale is a scale ranging from 0 to 14. It is used to measure how strong an acid or base is.

• Acids have a pH less than 7, bases have a pH above 7 and a substance with a pH of 7 is neutral.

• pH can be measured using universal indicator paper. The colour of the paper gives the pH of the substance.