Acids and Alkalis

Lesson Objectives

By the end of the lesson, students sbat:

•Describe acids and alkalis– Meaning of an acid and examples– Meaning of an alkali and examples

•State the properties of acids and alkalis

•State the reaction of acid with metal to produce hydrogen gas

•State the reaction of acid with carbonate to produce carbon dioxide gas.

Lesson Objectives

• State the reaction of acid with alkali to produce salt and water.

• State the uses of acids and alkalis in daily life

• Describe indicators – Litmus indicator and simple test for acids and

alkalis– Universal indicator (idea of pH)– pH meter and pH sensor

Lesson Objectives

• Show an understanding that indicators are substances that change colour when an acid or alkali is added to them.

Recap

• Mixtures can be classified as solutions and suspensions.

•  The differences between a solution and a suspension are:

Properties Solutions SuspensionsFormation A solute dissolves in the solvent Insoluble solids mixed

with a liquid / gasAppearance Clear, transparent Cloudy, opaqueHomgeneous Yes NoWhen left to stand

Solute particles do not separate from solvent.

Solid particles sink to bottom

When filtered No residue is left on filter paper Residue is left on filter paper

Examples Salt dissolved in water Chalk in water

Acids

• An acid is an example of a solution.

• Common acids

Name Chemical formula

Explanation

Hydrochloric acid

HCl Hydro- from hydrogen, -chlor from chlorine

Nitric acid HNO3 Nitr- from nitrogen (contains nitrogen)

Sulfuric acid H2SO4 Sulfur- contains sulfur

Acids

• What is common in the formula?– They all have H (hydrogen) in them.

• Hence, an acid is a substance which produces hydrogen ions in water.

Properties of Acids

corrosive sour taste good conductors of

electricity

Chemical Properties of Acids

• Acids turn blue litmus red

Chemical Properties of Acids

• Acids react with metals to produce salt and hydrogen gas.

• When a piece of metal is placed into an acid, a gas, hydrogen, is formed.

• Effervescence in the solution will be observed.

Hydrogen is a colourless and odourless gas.

acid + metal → salt + hydrogen

• To confirm that gas produced is hydrogen,

Test: Place a lighted splint in the gas.

Observation: The splint will be extinguished with a ‘pop’ sound.

Think

• Why is vinegar often sold in glass bottles rather than tin cans?

Vinegar is an acid. The acid will react with the metal containers, it will corrode the metal containers.

Chemical Properties of Acids• Acids react with carbonates to produce salt, carbon dioxide and water.

• Observation:– Effervescence is observed

acid carbonate+ → carbon dioxide

+salt + water

Test for Carbon Dioxide

• How do we know that the gas is carbon dioxide?

• Test: – Bubble the gas into

limewater.

• Observation:– White precipitate is

observed in limewater.

limewater

acid + carbonate

delivery tube

Think!

Indigestion pills often contain the substances, sodium bicarbonate, calcium carbonate and magnesium carbonate, to react with the excess acid in the stomach. Why is it that when these pills are consumed, may result in frequent burping?

When the stomach acid reacts with the carbonates, carbon dioxide is formed.

Alkalis

• Common alkalis:– Sodium hydroxide NaOH– Potassium hydroxide KOH

– Calcium hydroxide Ca(OH)2

– Ammonium hydroxide NH4OH

• What do you see in common?– Hydroxide ions

An alkali is a substance that produces hydroxide ions in water.

Physical Properties of Alkalis

corrosive soapy feel good conductors of

electricitybitter taste

Chemical Properties of Alkalis

• Alkalis turn red litmus blue

pH

If can burn skin,

can this burn skin as well?

Why?

pH

• pH is a measure of acidity an alkalinity

• pH range is from 0-14

pH of common substances

pH

• The smaller the pH value, the more acidic the substance.

• The bigger the pH value, the more alkaline the substance.

• pH 7 indicates that the substance is neither acidic nor alkaline (neutral)

More Properties

• Acids have a pH of less than 7.

• Alkalis have a pH of more than 7.

• When an acid (pH <7) reacts with an alkali (pH >7) the resulting mixture is neutral.

• The reaction is known as neutralisation.

 acid + alkali → salt + water

NeutralisationApplications ExplanationBrushing teeth with toothpaste

Toothpaste is slightly alkaline. It neutralises the acid left when bacteria feeds on the food left in the teeth.

Control of pH of soil Lime (calcium oxide) is added to soil to neutralise the acidic soil. Some plants grow better in slightly alkaline soil. E.g. Turnips, cabbage, chrysanthemum.

Using conditioner after shampooing hair 

Shampoo is alkaline. It makes hair feel dry and coarse. Conditioners are slightly acidic, it neutralises the shampoo left in the hair.

Taking indigestion pills for indigestion

Indigestion is usually a result of excessive acid in the stomach. Taking indigestion tablets which contain sodium bicarbonate neutralises the excessive acid.

Indicators

• Indicators are substances which indicate if a substance is acidic or alkaline.

• Examples of indicators:– Litmus

• In an acid, blue litmus turns red• In an alkali, red litmus turns blue

Indicators

– Universal Indicator• measures pH• green in neutral solutions.• Red/orange in acidic solutions.• Blue/purple in alkaline solutions.