topic 8 salts

7/30/2019 Topic 8 Salts

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X INTRODUCTION

Hello and welcome to the final topic of this module! Well, first of all, I am sureyou are aware that chemical salts are part of our daily life. We use sodiumchloride and monosodium glutamate or MSG in food preparation, ammoniumnitrate and urea as fertilisers, mercury (II) chloride and copper(II) sulphates inpesticides, sodium benzoate and sodium nitrate as preservatives in our food andso on.

In addition, most of the medications we take are salts in tablet form. As anexample, for gastric pain due to digestive acids, we take antacids such as natriumhydrogen carbonate, which is a base to neutralise the digestive acids. Anotherfamiliar and popular example is Panadol or paracetamol which we use to relieveheadaches and fever. For indigestion and bloatedness, we use ENO salt.

LEARNING OUTCOMES

By the end of this topic, you should be able to:1. Explain how soluble salts and insoluble salts can be prepared;

2. Explain how cations and anions in salts can be identified;

3. Conduct tests to identify gases;

4. Describe how salt can be prepared by crystallisation; and

5. Identify the ions present in an unknown salt using qualitativeanalysis.

TTooppiicc

88X Salts


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Epsom salts or magnesium sulphate crystals are used for relaxing muscles, as asedative and to draw toxins from our body. The examples can go on and on

because there are so many uses of salts in our daily lives; it is just a matter of

whether we realise it or not.

So, in this topic, we will learn more about salts, how they are prepared, howcations and anions in salts can be identified, how cations and anions in salts can

be analysed through qualitative analysis as well as how salts can be obtained intheir crystalline form through the process of crystallisation. Are you ready tostart now? Let this exciting lesson begin and enjoy this final topic!

PREPARATION OF SALTS

We begin this topic by discussing salt. Before we proceed further, what can yousay to define salt?

There are many different types of salts which exist in nature. The earth crustcontains different types of salt such as calcium chloride, magnesium sulphateand calcium carbonate. There are also salts such as potassium chloride,magnesium chloride and potassium bromide. Keep in mind that not all salts aresoluble in water. There are soluble salts and insoluble salts. In Topic 7, we learntthat the solubility of a substance is the amount of that substance (solute) that can

be dissolved in a given quantity of solvent. Can you still recall? What does itmean by soluble salts?

8.1

are salts that d while s are called i .

A s is an i that contains a c

and an a .


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However, some salts that do not dissolve in water at room temperature dodissolve in hot water such as lead(II) chloride. You can refer to Table 8.1, whichgives you a summary of soluble and insoluble salts.

Soluble and Insoluble Salts

x Nitrate Salts

x Sodium

x Potassium

x Ammonium Salts

x Ethanoate Salts

x Chloride salts: All are soluble except s ,(soluble in hot water) and m

x Sulphate salts: All are soluble except l ,, and c (dissolves a little

bit in water)

x

Carbonate salts: All are insoluble except sodium carbonate,potassium carbonate and ammonium carbonate

We can prepare both soluble and insoluble salts using various methods. Themethods used depend on the salts solubility in water. These methods will bediscussed in the following sections.

8.1.1 Preparation of Soluble Salts

Do you know that there are three methods to prepare soluble salts? These

methods are a , d , and. Then, the salt solutions formed are e ,

, f , and d to obtain dry salt crystals. Figure 8.1 shows thepreparation of soluble salts.


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Preparation of soluble salts

http://wiki.one-school.net


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Now, let us study these three methods further, starting with neutalisationreactions.

(a) NFirstly, let us learn the definition of neutralisation reaction. What does itstand for?

Do you know which salts can be prepared this way? Salts of s ,, and a can be prepared by neutralisation reactions.

How does this method work? This method works by neutralising the acidby a base/alkali, thus producing the end products which are salt and water.This process can be simplified as follows

Acid + Base/Alkali Salt + Water(Neutralisation)

One typical example for this particular method is the reaction betweenhydrochloric acid (acid) and sodium hydroxide (base) to form sodiumchloride (salt) and water.

HCl(aq) + NaOH (aq) NaCl(aq) + H2O (l)

From this example, we can also define salt as a compound that is formed

when the hydrogen ion, H+

, from an acid is replaced by a metal ion orammonium ion from the base/alkali.

A salt is also an i which consists of a cation such as a metalion or an ammonium ion from a base and anion from an acid. For example,in the salt sodium chloride, the cation is sodium ion, Na+, and the anion isthe chloride ion, Cl.

are processes in which an ato form s and w .


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Other soluble salts can also be prepared through neutralisation by havingan acid react with a metal oxide, just as shown in the following:

Acid + Metal Oxide Salt + Water(neutralisation)

An example for this method is when you have hydrochloric acid (acid) reactwith magnesium oxide (base) to form magnesium chloride (salt) and water.

HCl(aq) + MgO (aq) MgCl2(aq) + H2O (l)

Table 8.2 shows some examples of salts from different acids.

Examples of Salts from Different Acids

Sulphuric acid Magnesium sulphateAmmonium sulphate

MgSO4(NH4)2 SO4

Nitric acid Potassium nitrateAluminium nitrate

KNO3Al (NO3) 3

Carbonic acid Iron(II) carbonate

Calcium carbonate

FeCO3

CaCO3

Phosphoric acid Iron(III) phosphateAmmonium phosphate

FePO4(NH4) 3PO4

Ethanoic acid Lead(II) ethanoateCopper(II) ethanoate

Pb (CH3COO) 2Cu (CH3COO) 2

Hydrochloric acid Sodium chlorideAmmonium chloride

NaClNH4Cl

(b)

Now, let us move on to the second method of preparing soluble salt displacement reactions. How does this method work? In this method, saltsare prepared by reacting non-oxidising acids such as hydrochloric acid andsulphuric acid with active metals, just as shown in the following:

Acid + Metal Salt + Hydrogen gas


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For example, when zinc is dissolved in sulphuric acid, the reactionproduces zinc sulphate; hydrogen is displaced from the acid and it bubblesoff as gaseous H2, just like the following equation:

H2SO4(aq) + Zn (s) ZnSO4(aq) + H2 (g)

Zinc sulphate solution is then evaporated, cooled and filtered to get solidcrystals. The reaction is an example of a displacement reaction in which oneelement displaces another from a compound. The more active the metal(Zn) displaces, the less active it is for hydrogen from its compound inaqueous solution to form zinc sulphate and hydrogen gas.

(c) RLastly, let us look at the third method, which is reaction of acids withcarbonates. What does reaction of acids with carbonates yield? Reaction ofacids with carbonates yields salt, water and carbon dioxide gas as shown inthe following:

Acid + Carbonates Salt + Water + Carbon dioxide gas

For example, calcium chloride salt is prepared by the reaction betweenhydrochloric acid and calcium carbonate. Carbon dioxide gas bubbles off andthe salt solution is then evaporated, cooled and filtered to obtain calciumchloride crystals.

2HCl(aq) + CaCO3 (aq) CaCl2(aq + H2O (l) + CO2(g)

Let us now learn how to prepare ammonium sulphate salt in Activity 8.1.


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You will need: ammonia solution (2M), dilute sulphuric acid (1M),beaker, glass rod, bunsen burner, tripod stand, wire gauze, filter funnel,and filter paper.

Step:

1. Heat 30 cm3 of dilute sulphuric acid into a beaker.

2. Add ammonia solution little by little into the beaker, stirring thesolution.

3. Do not add any ammonia solution when the solution smellsamniotic.

4. Heat the solution until it becomes one-third of its original volume.

5. Cool the hot saturated solution in a mixture of ice and water.

6. Filter the salt crystals produced.

7. Dry the crystals using filter paper.

Questions:

(a) Which method is used to prepare the salt?

(b) How does the addition of ammonium sulphate affect the pH of thesoil?

ACTIVITY 8.1


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8.1.2 Preparation of Insoluble Salts

Let us move on to insoluble salts. How do we prepare it?

What are the steps involved for this method? Figure 8.2 shows the steps involved

in the preparation of insoluble salts.

Preparation of insoluble salts by precipitationhttp://www.docbrown.info/page03/AcidsBasesSalts06.htm

are prepared by p ,which are processes in which t are m to form a soluble salt and an insolublesalt as a precipitate that can be obtained by filtration.


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A typical example of precipitation is the preparation of the insoluble salt, bariumsulphate. This is shown in Figure 8.3.

Preparation of barium sulphatehttp://wiki.one-school.net

You may notice that both starting solutions of barium chloride and magnesiumsulphate are soluble in water. The reaction between the two solutions yields aprecipitate, barium sulphate, and a soluble salt, magnesium chloride, as shown inthe following:

BaCl2 (aq) + MgSO4(aq) BaSO4(s) + MgCl2(aq)


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How do we predict whether a precipitation reaction will occur or not? In order topredict whether a precipitation reaction will occur on mixing solutions of twosubstances, you must know the solubilities of the potential products. This can be

done by using the solubility guidelines in Table 8.1 shown earlier. Can you stillrecall?

Based on Table 8.1, we can conclude that if a substance has a low solubility inwater, it is likely to precipitate from an aqueous solution. If a substance has ahigh solubility in water, no precipitate will form from this reaction.

IDENTIFICATION OF CATIONS ANDANIONS

Thus far, we have learnt about the formation of ions in Topic 5. An ion is an atomor group of atoms that carries an electric charge. Ions that possess a positivecharge, such as sodium ion, Na+, are called cations. Those carrying a negativecharge, such as chloride ion, Cl, are called anions. Ions also exist as polyatomicions or groups of atoms that bear an electric charge.

The first atom in the formula is usually the central atom to which the other atomsare bonded to make a stable unit. Examples include the ammonium ion, NH4+,the sulphate ion, SO42 and the nitrate ion, NO3.

8.2

Search in the Internet and find out the uses of salt in:

(a) Agriculture;

(b) Medical field; and

(c) Food industries.

ACTIVITY 8.2


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Table 8.3 shows the formula, ionic charges and names of some common ions.

Some Common Cations and Anions

Li+ 1+ Lithium F 1 Flouride

Na+ 1+ Sodium Cl 1 Chloride

K+ 1+ Potassium Br 1 Bromide

Ag+ 1+ Silver/argentum I 1 Iodide

NH4+ 1+ Ammonium OH 1 Hydroxide

Cu+ 1+ Copper(I) MnO4 1 Permanganate

Cu2+ 2+ Copper(II) CH3COO 1 Ethanoate/acetate

Mg2+ 2+ Magnesium NO3 1 Nitrate

Ca2+ 2+ Calcium O2 2 Oxide

Zn2+ 2+ Zinc O22 2 Peroxide

Hg22+ 2+ Mercury(I)/mercurous CO32 2 Carbonate

Hg2+ 2+ Mercury(II) ofmercuric

CrO42 2 Chromate

Fe2+ 2+ Iron(II) Cr2O72 2 Dichromate

Pb2+ 2+ Plumbum(II) S2 2 Sulphide

Sn2+ 2+ Tin(II)/stannous SO42 2 Sulphate

Fe3+ 3+ Iron(III) SO32 2 Sulphite

Al3+ 3+ Aluminium CO32 2 Carbonate

Cr3+ 3+ Chromium PO43 3 Phosphate

In the following sections, we will learn how to identify anions and cations.


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8.2.1 Identification of Anions

Do you know that anions are identified by the reaction of the salt or salt solution

with silver nitrate solution or barium chloride solution? These are shown inTable 8.4.

Reactions of Anions

Bromide, Br Pale yellow precipitate No reaction No reaction

Chloride, Cl White precipitatesoluble in ammonia

No reaction No reaction

Iodide, I

Yellow precipitate Red colouration No reactionNitrite, NO2 White precipitate Reddish brown

precipitate on heatingNo reaction

Sulphate, SO42 White precipitate ifsolution isconcentrated

No reaction White precipitateinsoluble in diluteHCl

Sulphide, S2 Black precipitate Black precipitate inalkaline solution

No reaction

Sulphite, SO32 White precipitate

soluble in excesssulphite solution

Red colouration or

brown precipitate

White precipitate

soluble in diluteHCl

8.2.2 Identification of Cations

How about cations? How do we identify them? Cations are identified by reactionof the salt or salt solution with sodium hydroxide or ammonia solution as shownin Figure 8.4.


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Test for cationshttp://wiki.one-school.net


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In addition, cations can be identified by a flame test where a small amount ofthe salt is heated on a nichrome wire moisten with concentrated sulphuric acidto produce a flame characteristic of the cations. The characteristic is shown in

Table 8.5.

Flame Test of Some Cations

Green Barium ion, Ba2+

Red Calcium ion, Ca2+

Blue Copper(II) ion, Cu2+

Yellow Sodium ion, Na+

Violet Potassium ion, K+

IDENTIFICATION OF GASES

We turn our attention now to gases. How do we identify gases? First of all,you have to understand that many important chemical substances are gasesat ambient conditions. The earths atmosphere is a mixture of gases such asnitrogen and oxygen with smaller concentration of other gases. When somechemical compounds are heated, gases are also evolved.

For example, when a carbonate salt is heated, carbon dioxide gas is evolved. Eachof these gases has characteristic physical properties and can be identified by itscolour, smell, and test with damp litmus paper as shown in Figure 8.5.

8.3


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Identification of gaseshttp://wiki.one-school.net


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The gases that are evolved can be further identified by carrying out confirmatorytests as shown in Table 8.6.

Confirmatory Test for Gases

Carbon dioxide Turns lime water or calcium hydroxide milky due to theformation of calcium carbonate.

Ca(OH)2(aq) + CO2 (g) CaCO3(s) + H2O(l)

Sulphur dioxide Decolourises acidified potassium permanganate solution.

Hydrogen sulphide Decolourises acidified potassium permanganate solution,smells like rotten eggs.

Chlorine Bleaches wet litmus paper.Ammonia Thick, white fumes of ammonium chloride when in contact

with concentrated hydrochloric acid.

NH3(g) + HCl(g) NH4Cl(g)

Oxygen Burns when a lighted splinter is placed inside a gas jar ofoxygen.

CRYSTALLISATION

Let us start this subtopic by defining crystallisation. Do you know what it standsfor?

8.4

is the formation of s from a.


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Do you know that there are three steps of doing qualitative analysis of anunknown salt? These steps are shown in Figure 8.7.

Three steps of qualitative analysis

8.5.1 Preliminary Examination of the Salt

What can we do to preliminary examine the salt? Preliminary examination of thesalt involves these five following tests as listed in Figure 8.8.

Five tests of preliminary examination of salt


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Now, we are going to look at these steps further, starting with the first step physical examination of the salt.

(a) We can do preliminary examination by looking, touching and smelling. Bydoing so, we can determine the physical properties of the salt such as itsphysical state, colour and smell. We can always check our observationsagainst Figure 8.9 to infer the salt.

Colour of saltshttp://wiki.one-school.net


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(b) SWe can check the solubility of the salt by dissolving it in water. This can bedone by putting a small amount of the salt in a test tube. Then, we can refer

to Table 8.1 to check the solubility of the salt in water.

(c)This can be done by heating the salt and then observing the colour changeof the salt and the gases that are evolved. These observations can infer thetype of that salt as shown in Table 8.7 and Table 8.8 respectively.

Action of Heat on Salts and Colour Change

Greenish blue Brown Copper(II) chloride

Blue White Copper(II) sulphate

Violet Green Chromium(III) salts

Green White Iron(II) salts

Bright yellow Iron black Iron(III) chloride

Bright green Yellow Nickel(II) salts

White Yellow when hot, white when cold Zinc salt or zinc oxide

Yellow Yellow when cold, brownish-redwhen hot

Lead(II) oxide

White White sublimate Ammonium salts

Yellow Yellow sublimate Sulphur or sulphurcompounds such asthiosulphates, sulphites


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Action of Heat on Salts and Gases Evolved

Water vapour Hydrated salt, hydrogencarbonate, hydroxideViolet vapour Iodine or iodine compounds

Oxygen Oxides, peroxides, nitrates, chlorates and others

Carbon dioxide Carbonates, bicarbonates, oxalates and organiccompounds

Ammonia Ammonium salts

Sulphur dioxide Sulphites, sulphates, thiosulphates

Hydrogen sulphide Hydrated sulphides, acid sulphides

Chlorine Hypochlorites, some chlorides

Dark brown or reddish fumes Nitrogen dioxide gas from nitrates, bromine fromcertain bromides

Confirmatory tests are then conducted for specific gases as shown in Table8.6 (Subtopic 8.3).

(d)Now, let us move on to see the reaction of the salt with dilute hydrochcloric

acid. We can do this by adding dilute hydrochloric acid to a small amountof the salt in a test tube. If there is no reaction, then we need to heat themixture slowly. Identify any gases that are evolved to infer the type of saltas shown in Table 8.9.

Action of Dilute Hydrochloric Acid

Reddish brown fumes Nitrogen dioxide from nitrites

Odour of vinegar Acetic acid from acetates

Yellowish green gas which bleaches wet litmus paper Chlorine from hypochlorites

Colourless gas which discolourises acidifiedpotassium permanganate solution (no rotten smell)

Sulphur dioxide from sulphites

Colourless gas which discolourises acidifiedpotassium permanganate solution

(rotten smell)

Hydrogen sulphide gas fromsulphides

Colourless gas which turns lime water milky Carbon dioxide fromcarbonates or bicarbonates


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(e) RLastly, let us look at the reaction of the salt with concentrated sulphuricacid. This can be done by adding concentrated sulphuric acid to a small

amount of the salt in a test tube. If there is no reaction, heat the mixtureslowly. Identify any gases that are evolved to infer the type of salt as shownin Table 8.10.

Action of Concentrated Sulphuric Acid

White precipitate Insoluble sulphates from barium, calciumand lead(II) salts

Colourless gas which fumes in air or

when in contact with ammonia solution

Hydrogen chloride from chlorides

Violet vapour Iodine from iodides

Colourless gas burns with blue flame Carbon monoxide from formates oroxalates

Colourless gas which burns when alighted wooden splinter is placed

Oxygen from peroxides, permangantes,chromates and dichromates

Colourless gas which discolourisesacidified potassium permanganatesolution (no rotten smell)

Sulphur dioxide from sulphites

Colourless gas which discolourisesacidified potassium permanganatesolution

(rotten smell)

Hydrogen sulphide gas from sulphides

Colourless gas which turns lime watermilky

Carbon dioxide from carbonates orbicarbonates


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8.5.2 Identification of Anions and Cations in the Salt

How do we identify anions and cations in the salt? Well, this can be done by

preparing an aqueous solution of the salt and do testing for anions and cationspresent in the salt as discussed in Subtopic 8.3.

8.5.3 Confirmatory Tests for Specific Anions andCations

How about confirmatory tests for specific anions and cations? Confirmatory testsfor specific anions and cations can be conducted as described in Table 8.11 andFigure 8.10 respectively.

Confirmatory Tests for Anions

Nitrate ion,NO3

Brown ring test. Put an amount of a solution in a test tube; then,slowly pour concentrated sulphuric acid down the side of the testtube. If the solution is a nitrate, a brown ring is obtained where theacid meets the nitrate solution.

Bromide ion, Br With chlorine water or bleaching powder, a brown colouration isobtained.

Chloride ion, Cl A mixture of solid chloride and solid dichromate on heating withconcentrated sulphuric acid. It liberates reddish-brown fumes ofchromyl chloride.

Iodide ion, I Same method as bromide ion but the organic layer is violet.

Nitrite, NO2 With dilute hydrochloric acid or dilute sulphuric acid, it evolvesbrown fumes of nitrogen dioxide.

How about cations? Cations in the salt solution are confirmed by reacting itwith potassium iodide (KI), potassium thiocyanate (K4Fe(CN)6), and potassium

hexacyanoferrate(III) solution (K3Fe(CN)6) as shown in Figure 8.10.


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Confirmatory tests for cationshttp://wiki.one-school.net

You are given a magnesium sulphate solution.

1. Carry out a qualitative analysis on the magnesium sulphatesolution; and

2. Describe briefly the chemical tests done to identify and confirmthe presence of anions and cations in the solution.

ACTIVITY 8.4


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x A salt is an ionic compound that contains a cation other than H+ and an anionother than OH or O2. It is formed when the hydrogen ion, H+, from an acidis replaced by a metal ion or ammonium ion from the base/alkali.

x Soluble salts include nitrate salts, salts of sodium, potassium, and ammonium,

and ethanoate salts.

x All chloride salts are soluble except silver chloride, lead(II) chloride (solublein hot water) and mercury(II) chloride.

x All sulphate salts are soluble except lead(II) sulphate, barium sulphate andcalcium sulphate (dissolves a little bit in water).

Answer the questions below:

1. Ammonia gas is bubbled into a test tube filled with a copper(II)sulphate solution. A blue precipitate is formed. What willhappen to the blue precipitate when ammonia gas iscontinuously bubbled into the test tube?

2. How would you differentiate a barium chloride and a bariumnitrate solution?

3. Salt X is soluble in water. The solution is divided into two

portions:

(a) The first portion produces a white precipitate with asodium hydroxide solution. The precipitate will dissolvewhen placed in excess sodium hydroxide solution; and

(b) The second portion gives a yellow precipitate withpotassium iodide.

Based on the above description, name salt X.

SELF-CHECK 8.1


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x All carbonate salts are insoluble except sodium carbonate, potassium carbonateand ammonium carbonate.

x

Soluble salts can be prepared by acid-base neutralisation reactions,displacement reactions, and reaction of acids with carbonates.

x Insoluble salts are prepared by precipitation reactions.

x Anions are identified by the reaction of the salt or salt solution with silvernitrate solution or barium chloride solution.

x Cations are identified by reaction of the salt or salt solution with sodiumhydroxide or ammonia solution.

x Gases can be identified by their colour, smell and a test with damp litmuspaper.

x Crystallisation is the formation of solid crystals from a homogenous solution.

x Qualitative analysis is a procedure for identifying the ions present in anunknown salt.

x Qualitative analysis of an unknown salt involves the following steps:preliminary examination of the salt, identification of anions and cations in the

salt, and confirmatory tests for specific anions and cations.

Anions

Carbonates

Cations

Chloride saltsCrystallisation

Displacement reaction

Gases

Insoluble salts

Nitrates

Neutralisation reaction

Precipitation reaction

Qualitative analysisSalt

Soluble salts

Solubility of salts

Sulphates


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Briggs, J. G. R. (1992). Science in focus chemistry for GCE O Level. Singapore:Pearson Education.

Brown, T. L., Lemay, H. E., & Bursten, B. E. (2000). Chemistry: The central science(8th ed.). New Jersey: Prentice Hall.

McMurray, J., & Fay, R. C. (2001). Chemistry(3rd ed.). New Jersey: Prentice Hall.

Ralph, A. B. (2003). Fundamentals of chemistry. New Jersey: Prentice Hall.

Whitten, K. W., Davis, R. E., Peck, M. L., & Stanley, G. G. (2010). Chemistry(9th ed.). Belmont: Brooks/Cole.

topic 8 salts

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