Lesson Explainer: Salts Chemistry

In this explainer, we will learn how to determine and interpret the names of simple salts and describe the preparation of soluble and insoluble salts.

Often, the word salt is taken to mean the table salt used to flavor food. In chemistry, however, salt refers to a much more general type of substance. A salt is an ionic compound made of cations bonded with anions.

Definition: Salt

A salt is an ionic compound made of cations and anions.

A cation is a positively charged ion. Some examples of cations include Na+, Li+, Mg2+, K+, Ca2+, Fe2+, Fe3+, Co2+, Cu2+, and Ag+ as well as polyatomic cations such as the ammonium ion NH4+.

An anion is a negatively charged ion. Some examples of anions include O2, Cl, N3, S2, I, Br, F, and P3 and polyatomic anions such as the nitrate anion NO3, the sulfate anion SO24, and the carbonate anion CO32.

There are many reactions that will produce salts including the following:

  • Acid–base reactions
  • Reaction of acids with metals
  • Reaction of acids with carbonates

The following table shows a selection of salts that can be formed from acid–base reactions. The anion from each acid is shown in red and the cation from each base in blue. The salt in each case is made from the combination of the acid anion and the base cation.

AcidAnion from AcidBaseCation from BaseSalt
HClClChlorideMg(OH)2Mg2+MagnesiumMgCl2Magnesium chloride
HClClChlorideKOHK+PotassiumKClPotassium chloride
HNO3NO3NitrateFeO23Fe3+Iron(III)Fe(NO)33Iron(III) nitrate
HNO3NO3NitrateNaOHNa+SodiumNaNO3Sodium nitrate
HSO24SO24SulfateCa(OH)2Ca2+CalciumCaSO4Calcium sulfate
HSO24SO24SulfateCuOCu2+Copper(II)CuSO4Copper(II) sulfate
HPO34PO34PhosphateNHOH4NH4+Ammonium(NH)PO434Ammonium phosphate
HCO23CO32CarbonateCa(OH)2Ca2+CalciumCaCO3Calcium carbonate
CHCOOH3CHCOO3AcetateKOHK+PotassiumKCHCOO+3Potassium acetate

To name a salt, the cation name is written first, and it is the same as the element name. The anion name is written second and is given the suffix -ide for single element ions or -ite or -ate for polyatomic ions containing oxygen.

Example 1: Recalling the Suffixes of Single-Atom Anions

Lithium and fluorine react to produce a salt with the formula LiF. What is the name of this salt?

Answer

The name of a salt can be deduced from its formula, and the formula can be deduced from the name. The question provides the formula of a salt (LiF). By identifying the elements or ions from the formula, we can determine the name of this ionic compound.

The compound contains two elements, lithium (Li) and fluorine (F).

Salts are composed of a metal cation and a nonmetal anion. In this case, lithium forms a cation and fluorine anion, which we call the fluoride anion.

The metal cation name is written first and is the same as the element name. The metal cation name is therefore lithium. The nonmetal anion name is written second.

The salt name is, therefore, lithium fluoride.

Some salts can dissolve in water to produce aqueous solutions that are either acidic, basic, or neutral. The type of solution produced depends on the identity of the salt and its composition, as shown in the table below.

ProduceExamples of CationsExamples of AnionsExamples of Salts
Acidic SaltsAcidic solutionspH < 7NH4+NHBr4
NHNO43
Neutral SaltsNeutral solutionspH 7K+
Ca2+
Li+
Cs+
Rb+
Ba2+
Sr2+
Na+
Cl
I
Br
NO3
SO24
CaBr2
NaSO24
Basic SaltsBasic solutionspH > 7F
CO32
HCO3
O2
NaF
NaCO23

The table shows two examples of salts that produce acidic solutions. The ammonium ion (NH4+) is an example of an acidic cation because it can react with bases. An ammonium salt such as ammonium nitrate is acidic since it produces the hydronium ion (HO3+) when the salt dissolves in water. The following equations show the reactions that occur to produce the acidic hydronium ion: NHNO()NH()+NO()NH()+HO()NH()+HO()434+34+233+saqaqaqlaqaq

Basic salts produce solutions that are basic, with a pH above 7, and react with acids. Some basic anions include the fluoride anion, the carbonate and bicarbonate anions, and the oxide anion. An example of a salt that produces a basic solution when dissolved in water is sodium fluoride. The equations below show that fluoride ions from the dissolved salt can react with water, forming the basic hydroxide ion (OH): NaF()Na()+F()F()+HO()HF()+OH()saqaqaqlaqaq+2

Neutral salts dissolve in water to produce neutral solutions. With a pH of approximately 7, they react with neither acids nor bases. Many metal cations, as well as the chloride, bromide, iodide, nitrate, and sulfate ions, are neutral. An example of a salt that forms a neutral solution when dissolved in water is sodium sulfate.

Example 2: Identifying an Acidic Salt

Which of the following salts would produce an acidic solution when dissolved in water?

  1. KBr
  2. NHNO43
  3. NaCO23
  4. CHCOONa3

Answer

The identity of the cation and anion in a salt determines whether the salt produces an acidic, basic, or neutral solution when dissolved in water. Of the possible answer options, only the ammonium ion (NH4+), from the salt ammonium nitrate (NHNO43), is acidic. Its counterion, the nitrate ion (NO3), is neutral. The salt NHNO43 will therefore produce an acidic solution according to the following equations: NHNO()NH()+NO()NH()+HO()NH()+HO()434+34+233+saqaqaqlaqaqNHNO43 dissolves to form NH4+ and NO3 in solution, and the NH4+ ion can then react with water to form the acidic hydronium ion (HO3+).

Not all salts are soluble in water. As a result, there are different ways to prepare both soluble and insoluble salts.

Soluble salts can be prepared by neutralization reactions. The general chemical equation for an acid reacting with a base is as follows: Acid+basesalt+water

Reaction: Production of a Salt from the Reaction of an Acid and a Base

Acid+basesalt+water

An example of a neutralization reaction is the reaction between hydrochloric acid (HCl()aq) and sodium hydroxide (NaOH()aq), which produces the soluble salt sodium chloride (NaCl()aq).

How To: Preparing a Salt From a Soluble Base

The experimental setup to prepare a salt from a soluble base is shown below. Here, a salt is being prepared by reacting hydrochloric acid with sodium hydroxide.

An indicator such as phenolphthalein can be used to indicate when the reaction is complete. When the reaction mixture has been exactly neutralized, the indicator will change color. Using this approach, the exact volumes of the acid and alkali that completely neutralize each other can be determined.

The preparation can then be performed as follows:

  1. The acid and the alkali, which are both water-soluble, are mixed. The correct proportions of each reactant are needed to exactly neutralize each other. Therefore, the acid is often added slowly to the base, or the base slowly to the acid, which can be done using a buret.
  2. Heat the reaction mixture to remove most of the solvent water.
  3. Once the solution is saturated and crystals appear at the edge of the solution, turn off the heat.
  4. Leave the saturated solution at room temperature to evaporate to dryness in a wide, shallow vessel.
  5. After some time all the water will evaporate leaving behind crystals of the salt sodium chloride.
  6. The crystals can be filtered to remove any excess solvent or further dried using filter paper or in an oven.

The chemical equation for this reaction is the following:

The anion from the acid and the cation from the base combine to form salt sodium chloride, and the acid donates a hydrogen ion (H+) to the base.

Example 3: Explaining the Steps in the Preparation of a Soluble Salt from Soluble Reactants

Sodium chloride can be prepared by reacting sodium hydroxide with hydrochloric acid. In an experiment, a fixed volume of sodium hydroxide and a few drops of phenolphthalein were added into an Erlenmeyer flask. Hydrochloric acid was then added, from a buret, until the reaction was complete. The reaction was then repeated, without the indicator but using the exact volume of hydrochloric acid determined from the previous experiment. The resulting solution was then heated to produce white crystals of sodium chloride.

  1. Why were a few drops of phenolphthalein added to the solution at the start?
    1. To increase the solubility of the sodium chloride
    2. To increase the basicity of the sodium hydroxide
    3. To remove any impurities in the solution
    4. To act as an indicator and inform us when the reaction is complete
    5. To act as a catalyst in the reaction
  2. Why would this method not work for producing crystals of copper sulfate from copper(II) oxide and sulfuric acid at room temperature?
    1. Copper oxide is insoluble and would not form an aqueous solution.
    2. Copper sulfate only forms when copper oxide reacts with sulfur dioxide.
    3. Copper oxide does not react with sulfuric acid.
    4. Copper oxide is acidic and would not react with sulfuric acid.
    5. Copper sulfate precipitates out of solution and so filtration would be required.
  3. Why could the sodium chloride not be obtained by filtering the solution?
    1. Any insoluble impurities would not be separated through filtration.
    2. Any excess hydrochloric acid would burn the filter paper.
    3. Sodium chloride is soluble and would therefore not be separated through filtration.
    4. Only the sodium ions would be filtered and not the chloride ions.

Answer

Part 1

In this experiment, hydrochloric acid and sodium hydroxide react to produce the salt sodium chloride and water, according to the following equation: HCl()+NaOH()NaCl()+HO()aqaqaql2

Both the acid and the base are colorless, as well as the salt, and so it is impossible to observe when enough acid has been added and all the base has reacted. An indicator is therefore used to indicate when the reaction is complete.

Phenolphthalein is an organic compound that is colorless in acidic solutions and pink in alkaline solutions. It can therefore be used as an indicator to indicate whether a solution is acidic or basic.

In this reaction, the initial solution is pink because it is basic. The acid is slowly added to the base until just enough has been added to neutralize the base. At this point, the phenolphthalein turns colorless. The correct answer is D, to act as an indicator and inform us when the reaction is complete.

Part 2

Copper oxide is a black, insoluble solid. When this basic oxide reacts with a warmed acid, a salt and water are formed, according to the following equation: CuO()+HSO()CuSO()+HO()saqaql2442

This method would not work for producing crystals of copper sulfate since one of the reactants is insoluble in water. The correct answer is A. Copper oxide is insoluble and would not form an aqueous solution.

Part 3

Sodium chloride is a soluble salt. If the solution of sodium chloride was filtered, the sodium chloride salt would pass through the filter paper with the filtrate. Filtration is only suitable to separate solid particles from a liquid. The correct answer is C, sodium chloride is soluble and would therefore not be separated through filtration.

Another way to prepare a soluble salt from a neutralization reaction is to react an acid with an insoluble base, such as a metal oxide.

Many metal oxides are insoluble in water, however, and therefore the acid is usually heated slightly to increase the rate of the reaction between the acid and the base. An example is the reaction between nitric acid (HNO3) and the insoluble base copper(II) oxide (CuO). The soluble salt copper(II) nitrate (Cu(NO)()32aq) is produced.

How To: Preparing a Salt from an Insoluble Base

The experimental setup to prepare a salt from an insoluble base is shown below. Here, a salt is being prepared by reacting nitric acid with solid copper oxide.

  1. Add excess insoluble base to the acid and stir while warming gently. All the acid should react because the base is in excess.
  2. Once no more of the base will dissolve, filter the reaction mixture to remove the excess solid reactant.
  3. Heat the filtrate, which contains the dissolved salt product, to remove most of the water.
  4. Once the solution is saturated and crystals appear at the edge of the solution, turn off the heat.
  5. Leave the saturated solution at room temperature to evaporate to dryness in a wide, shallow vessel.
  6. After some time, all the water will evaporate leaving behind crystals of the salt.
  7. The crystals can be filtered to remove any excess solvent or further dried using filter paper or in an oven.

The reaction equation is as follows:

The anion from the acid and the cation from the base form the soluble salt copper(II) nitrate (Cu(NO)()32aq) and the hydrogen ions from the acid and the oxide ion from the base bond to form water.

Example 4: Explanation of the Preparation of a Soluble Salt from an Acid and an Insoluble Reactant

A student wants to prepare a salt using copper(II) oxide, an insoluble base, and sulfuric acid. The student adds excess copper(II) oxide to warm sulfuric acid and stirs. After a while, the solution is observed to turn blue. Once the reaction is complete, the solution is filtered and then slowly heated to induce crystallization. The sample is then left to fully dry, producing blue crystals.

  1. Why is the sulfuric acid warmed?
    1. To decrease the solubility of copper(II) oxide
    2. To remove any impurities in the sulfuric acid
    3. To increase the strength of the sulfuric acid
    4. To stop the solution from freezing
    5. To increase the rate of reaction between copper(II) oxide and sulfuric acid
  2. What is the symbol equation, with state symbols, for the chemical reaction between copper(II) oxide and sulfuric acid?
    1. CuO()+HSO()CuSO()+HO()saqaql2442
    2. CuO()+HS()CuS()+HO()saqaql22
    3. CuO()+HSO()CuSO()+HO()224242saqaql
    4. CuO()+HSO()CuS()+HO()+2O()saqaqlg2422
    5. CuO()+HSO()CuSO()+HO()laqsl2442
  3. Why is the solution filtered?
    1. To dry the sample of copper sulfate
    2. To obtain the copper sulfate
    3. To remove the blue color of the solution
    4. To remove any excess copper(II) oxide
    5. To remove excess sulfuric acid

Answer

Part 1

The reaction between copper(II) oxide and sulfuric acid is slow since one of the reactants, copper(II) oxide, is insoluble in water. Sulfuric acid is therefore warmed to increase the rate of the reaction. The correct answer is E, to increase the rate of reaction between copper(II) oxide and sulfuric acid.

Part 2

Copper(II) oxide has the formula CuO and it is an insoluble solid; thus, it has ()s for its state symbol. Sulfuric acid is water-soluble and has the chemical formula HSO24 and state symbol ()aq. They react with each other to produce water and the soluble salt copper(II) sulfate, with the chemical formula CuSO4. The correct chemical equation for this reaction is therefore A, CuO()+HSO()CuSO()+HO()saqaql2442.

Part 3

The solution is filtered to remove any remaining, unreacted copper(II) oxide, which is added in excess at the beginning of the reaction. As the copper(II) oxide is solid, then filtration is a good technique to remove it from the solution. The correct answer is D, to remove any excess copper(II) oxide.

Insoluble salts can be made by reacting two soluble salts together. The insoluble salt precipitates out of solution, and therefore this is called a precipitation reaction.

Definition: Precipitation Reaction

A reaction that forms an insoluble solid product from the reaction between two soluble substances in solution.

An example of how an insoluble salt is produced from a precipitation reaction is the reaction between solutions of silver nitrate and sodium chloride. Both the reactants are soluble in water but when mixed together an insoluble salt of white silver chloride is formed.

How To: Preparing an Insoluble Salt from Two Soluble Salts

The experimental setup to prepare an insoluble salt from two soluble substances is shown below. Here, a salt is being prepared by reacting a solution of sodium chloride with a solution of silver nitrate.

  1. Prepare aqueous solutions of the two reactants. Here, they are sodium chloride and silver nitrate.
  2. Mix the two solutions together. Immediately, an insoluble salt, silver chloride, forms and precipitates out of solution as a fine white powder.
  3. Filter the solution—the precipitate will remain on the filter paper.
  4. Rinse the precipitate with distilled water.
  5. Dry the precipitate; this can usually be done in a warm oven.

The word equation for this reaction is as follows: Silvernitrate+sodiumchloridesilverchloride+sodiumnitrate

And the chemical reaction equation is as follows:

In a precipitation reaction, the two soluble salts exchange anions. This is also called a double displacement reaction. The chloride anion (Cl) from sodium chloride bonds to the silver cation (Ag+) from silver nitrate, forming an insoluble salt, silver chloride (AgCl).

Example 5: Writing the Symbol and the Ionic Equations for a Precipitation Reaction

A precipitate of barium sulfate can be formed by reacting barium chloride with magnesium sulfate.

  1. What is the symbol equation, with state symbols, for this reaction?
  2. What is the net ionic equation for this reaction?

Answer

Part 1

Barium chloride has the chemical formula BaCl2 and magnesium sulfate is MgSO4. Both are soluble in water and therefore have ()aq as their state symbols. The anions “switch” places in this precipitation reaction, and the products are barium sulfate (BaSO4) and magnesium chloride (MgCl2). Barium chloride is a precipitate, which is insoluble in water and therefore has the state symbol ()s. Magnesium chloride is soluble in water and thus has a state symbol ()aq. The chemical equation is as follows: BaCl()+MgSO()BaSO()+MgCl()2442aqaqsaq

Part 2

Consider the chemical equation for the reaction: BaCl()+MgSO()BaSO()+MgCl()2442aqaqsaq

It can be written as a total ionic equation to show which salts are dissociated into separate ions in solution. The two reactants, as well as the product magnesium chloride, are dissolved in water and exist as ions. The product barium sulfate is insoluble and does not dissociate into ions. The total ionic equation is as follows: Ba()+2Cl()+Mg()+SO()BaSO()+Mg()+2Cl()2+2+2442+aqaqaqaqsaqaq

Those chemical species that appear on both sides of the equation are spectator ions. Spectator ions do not take part in the reaction and can therefore be eliminated from the equation: Ba()+2Cl()+Mg()+SO()BaSO()+Mg()+2Cl()2+2+4242+aqaqaqaqsaqaq

The net ionic equation for the formation of the precipitate is therefore the following: Ba()+SO()BaSO()2+244aqaqs

Key Points

  • A salt is an ionic compound made of cations and anions.
  • The identity of the cation and anion in a salt will determine whether a salt is soluble in water or not.
  • Soluble salts can be prepared by neutralization reactions using two methods:
    • The reaction of a soluble acid with a soluble base
    • The reaction of a soluble acid with an insoluble base
  • Insoluble salts can be prepared by reacting two soluble salts together in a precipitation reaction.
  • The name of a salt has the cation name written first and the anion name written second. The anion name is given the suffix -ide for single element ions or -ite or -ate for polyatomic ions containing oxygen.
  • When a salt dissolves in water, it can produce an acidic, basic, or neutral solution.

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