Acids, Alkalis and Indicators

Prepared by Mdm RY LeowThe Chinese High School

AcidsAcids

• What causes the sour taste in these beverages?

The presence of citric acid.

Some Common Acids

• Organic acids are found in living things. • Which of the acids listed above are organic acids? • Hydrochloric acid is an mineral acid (inorganic acid) and is commonly

found in science laboratories.

Acids 1 All solutions of acids contain hydrogen

ions,H+

2 Examples of acids commonly found in the laboratories.

• Hydrochloric acid (HCl)

• Sulphuric acid (H2SO4)

• Nitric acid (HNO3)

• Ethanoic acid (CH3COOH)

Physical properties of acidic solutions

Concentrated acids are corrosive.

Note: All acids only show acidic properties in the presence of water!

Physical properties of acidic solutions1. Acids taste sour.

2. Acids turn blue litmus paper red but

have no

effect on red litmus paper.3. They have pH less than 7.

4. They are good electrical conductors.

*All acids can only show their acidic

properties in the presence of water.

Chemical properties of acids1.Dilute acids react with some metals (K, Na,

Ca, Mg, Zn, Fe) to produce hydrogen gas.metal + acid salt + hydrogen

*Dilute acids do not react with copper, silver, gold

eg Mg + 2HCl MgCl2 + H2

Mg + H2SO4 MgSO4 + H2

Chemical properties of acids2. An acid reacts with a carbonate to produce

2HCl + CaCO3 CO2 + CaCl2 + H2O

Carbon dioxide gasis bubbled into

limewater

White precipitate is formed

acid + carbonate carbon dioxide gas + salt + water

a salt, water and carbon dioxide.

Chemical properties of acids

3. An acid reacts with a base toform a salt and water

only.

The reaction between an acid and a base to form a salt and water only is called neutralisation

acid + base salt + water

HCl + NaOH NaCl + H2O

HNO3 + KOH KNO3 + H2O

H2SO4 + Ca(OH)2 CaSO4 + 2H2O

H2SO4 + CuO CuSO4 + H2O

Some Uses of Acids

Hydrochloric acid

Sulphuric acid

Nitric acid

Ethanoic acid

Citric acid

Cleaning metallic surfaces before they are painted or coated with other metals

Used as an electrolyte in car batteries making fertilisers, and plastics

Making fertilisers and explosives

Preserving food and cooking

Making health salts

Alkalis• Many household products such as

toothpaste, soap and detergents contain alkalis.

Click the picturefor uses

of alkalis.

Alkalis

1 All solutions of alkalis contain hydroxide ions, OH -

2 Examples of common alkalis found in the laboratories are

Sodium hydroxide solution NaOH

Potassium hydroxide solution KOH

Calcium hydroxide solution Ca(OH)2

Ammonia solution NH3.H2O

Physical Properties of Alkaline solutions

• Strong alkalis are corrosive

liquids.

*All alkalis can only show their alkaline properties

in the presence of water.

Physical Properties of alkaline solution

1. Alkalis taste bitter.

2. They feel soapy.

3. They turn red litmus paper blue but have no effect on blue litmus paper.

4. Alkalis are good electrical conductors.

5. They have pH greater than 7.

NaCl + H2O + NH3

Chemical Properties of Alkalis

1. An alkali reacts with an ammonium salt to produce

2. An alkali neutralises an acid to form a salt and water. alkali + acid salt + water

ammonia gas, salt and water.alkali + ammonium salt salt + water + ammonia gas

KCl + H2O

e.g. NaOH + NH4Cl

e.g. KOH + HCl

AlkalisAlkalis Common names Uses

Sodium hydroxide caustic soda Making soap, washing powder, drain cleaner

Potassium hydroxide caustic potash Making dyes for fabric and paint removers

Calcium hydroxide slaked lime Reducing acidity of soil and making mortar

Aqueous ammonia Making fertilisers and (Ammonia solution) household cleaning liquids

Indicators• Indicators are dyes which change colour with

the acidity or alkalinity of a solution.

• Some common indicators are (litmus, methyl orange, phenolphthalein, universal indicator)

pH scale

The pH scale is a range of numbers from 0 to 14, which indicates how acidic or alkaline a substance is.

pH scale

Other indicator such as pH paper can be used

A pH meter is an electronic device that can be use to measure the acidity or the alkalinity of solutions very effectively.

(a) pH meter being used to measure the pH of water in swimming pool

(b) pH meter is used to measure pH of the soil

Neutralisation

1 Neutralisation occurs when an acid neutralises a base or an alkali to form salt and water.(An indicator can be use to find the pH 7 of the solution)

*acid + base salt + water H2SO4 + MgO MgSO4 + H2O

*acid + alkali salt + water HNO3 + KOH KNO3 + H2O

•Acid and alkali are reactants.•salt and water are products.

Neutralisation

2 Example of everyday uses of neutralisation

• Shampoo (a mild alkali) and conditioner (a mild acid)

• Toothpaste (alkaline) neutralises acids produced by bacteria

• wasp sting (alkaline) is treated with vinegar to

relieve pain

• Excess acid in the soil is neutralised by adding

slaked lime (calcium hydroxide)

Salts• Salts are metal compounds that can be made

from acids.* All acids contain hydrogen.

*When the hydrogen in an acid is replaced by a metal, we get a salt.

nitric acid

hydrochloric acid

sulphuric acid

nitrates

chlorides

sulphates

Preparation of Salts

1 The four methods of preparing salts are:(a) precipitation of an insoluble salt (mixing)

HCl + AgNO3 AgCl + HNO3

(b) neutralisation of an acid with a base (excess insoluble oxide or hydroxide) CuO + H2SO4 CuSO4 + H2O

(c) neutralisation of an acid with an alkali (soluble hydroxide)

HCl + KOH KCl + H2O

(d) reaction of an acid with excess insoluble carbonate

CuCO3 + 2HCl CuCl2 + H2O + CO2

Flowchart to decide which method to use

Start

Is the salt soluble?(a) PRECIPITATION METHOD

Mix solutions of two soluble salts to precipitate an insoluble salt.eg NaCl + AgNO3 AgCl + NaNO3

React (b) acid with metal oxide

or (c) acid with alkalior(d) acid with metal carbonate

Are all reactants soluble in water?

(c) Mix acid with alkali to produce salt and watereg HCl + NaOH NaCl + H2O

(b)Mix acid and excess metal oxideor (d) excess metal carbonate to produce salt?

eg CuO + H2SO4 CuSO4 + H2OMgCO3 + H2SO4 MgSO4 +H2O+CO2

No

Yes

No

Yes

SaltsSoluble Insoluble

All sodium, potassium and

ammonium salts

All nitrates

All chlorides except lead(II) chloride and silver chloride.

All sulphates except lead(II) sulphate, barium sulphate and

calcium sulphate

sodium carbonate, All other carbonates

potassium carbonate

and ammonium carbonate