SALTS

Keywords

• Acids bases / alkalis
• Crystal
• Crystallization
• Insoluble
• Neutralization
• Salts
• Soluble
• Titration

 By the end of this chapter, you will be able to:

 (a) be familiar with, and be able to carry out neutralization reactions to prepare salts.

(b) Know and appreciate the uses of common salts in everyday life.

Competency

You should be able to appreciate that acids and alkalis form salts.

2.1 Introduction

Most of us are familiar with the word ‘ salt, especially when preparing some foods and during meal time. In everyday life, we use the words table salt which is actually sodium chloride, also referred to as common salt.

Why do you think common salt is important to us? Where else do you think we encounter salts?

Look at the pictures below of the common salt.

 Figure 2.1: Crystals of common salt (sodium chloride)

 Figure 2.2: Processed common salt

So, what do you think is the meaning of salts in Chemistry?

In this chapter, you will find out about the wider meaning of salts, how salts are made, types of salts and the applications of salts in our daily life.

What are salts?

We already know from Chapter 1 that acids and bases (alkalis) can cancel the effect of each other. Do you recall what is / are formed when an acid cancels out the effect of a base or vice versa?

Group discussion

 In small groups:

1. What are the products of the reaction between an acid and a base?

2. Write a general word equation for the reaction between an acid and a base (alkali).

3 What name do you think we give to the above general .word equation of reaction in 2?

4 Write both word and symbolic equations for the reaction between sodium hydroxide and hydrochloric acid.

5. Present your ideas to your teacher and then to the class.

Common salt, which is sodium chloride, has the chemical formula, NaCl.

Therefore, we can say that the salt, sodium chloride, is a compound made up of sodium and chlorine atoms chemically combined.

Can you recall the meaning of the term compound? (You may refer to Book 1 for help.)

The knowledge of salts helps us to make use of some compounds to improve our lives in so many ways.

For example, sodium nitrate and ammonium sulphate are useful fertilizers applied to soils to improve crop productivity. Many medicines are salts, for example, Iron tablets contain iron (II) sulphate (sometimes called ferrous sulphate) as shown in Figure 2.3. The tablets are used to treat and prevent iron deficiency – anemia.

Figure 2.3: Iron tablets that contain iron (II) sulphate (sometimes called ferrous sulphate)

In the following activity, you will find out what salts are.

Activity 2.1 Finding out more about salts in this activity, you will work in groups to find out more about salts.

 What you need

 Note book, library / internet (if available) / video clips.

What to do

In pairs or groups, discuss the questions below and write a report for presentation to the class.  You can do research in the library or through the internet (if available)

1. What do you understand by the term ‘ salts “?
2. Salts are regarded as compounds derived from acids.
3. State how salts are derived from acids.
4. State how salts are named,
5. Copy and complete the following table:

Table 2.1: Some common salts and their parent acids

d) What conclusions can you draw about salts in this activity?

e) Prepare a summary of your findings. Share your findings with your teacher and then, the class .

Assessment 2.1

Sulphurous acid and phosphoric acid have the following chemical formulae H₂SO₃, and H₃PO₄ respectively.

a )Salts formed from sulphurous acid are generally called …. While those formed from phosphoric acid are called ……..

b) Write the chemical name of any five salts that can be formed by sulphurous acid and four salts that can be formed by phosphoric acid .

What are some of the natural sources of salts ?

Just like acids and alkalis , salts are widely distributed around us . Look at Table 2.2 below , which shows natural sources of some salts .

Table 2.2 : Natural sources of common salts

Much as many salts are found to exist in nature , we can also prepare the salts in the laboratory from chemical reactions , as we shall see in the next activity .

2.2 How salts are made in the laboratory

We can make salts in the laboratory using several methods and this depends on whether the salt is soluble or insoluble , or if it is required in an anhydrous state . One of the methods of interest in this chapter is the preparation of soluble salts from the reaction between an acid and a base ( alkali )

The reaction between an acid and a base ( alkali ) is called neutralization reaction . It is important to note that a base can be a metal oxide or a metal hydroxide .

 Bases which dissolve in water are called alkalis .

You can read Chapter 1 for more about alkalis . Now let us look at how to prepare some salts in the laboratory for example , Epsom salts , which contain hydrated magnesium sulphate ( see Figure 2.4 ) . The salts relax muscles , reduce inflammation and help muscle function . The name ‘ Epsom salts ‘ comes from the town of Epsom , which has mineral springs from which hydrated magnesium sulphate was extracted .

Figure 2.4 : Epsom salts

Activity 2.2 Preparing magnesium sulphate from magnesium oxide sulphate .

In this activity , you will work in groups to prepare magnesium

Caution : Wear hand protective gear . Acids are corrosive .

What you need

• Magnesium oxide powder and 60 cm² of 2 M sulphuric acid ( dilute sulphuric acid )
• Two 100 cm ( or 250 cm ) glass beakers , evaporating basin , filter funnel , stirring rod , measuring cylinder ( 50 mi / 100 ml ) , retort stand , spatula , tripod stand and wire gauze
• Heat source and two pieces of filter paper .

What to do

1 . Using a measuring cylinder , measure 50 cm³ of dilute sulphuric acid and put it into a clean beaker .

2. Gently heat the acid in the beaker for a few minutes ( do not boil the acid ) .

 3. Using a spatula, add magnesium oxide a little at a time to the hot acid, while stirring the mixture, as shown in Figure 2.5 (a). What do you observe as you add the magnesium oxide? Continue adding the oxide until the mixture becomes milky.

 a) Dissolving magnesium oxide in acid

 b) Filtering the mixture

Figure 2.5: Preparing/making magnesium sulphate

(c)Evaporating the filtrate

Why do you think

• it was necessary to warm the acid before adding magnesium oxide ?
• it was necessary to add the oxide in small amounts while stirring ?
• the mixture became milky after some time ?
• it was necessary to add magnesium oxide until it was in excess ?

4. Filter the mixture as shown in Figure 2.5 ( b ) . Keep the filtrate .

 a ) What is the colour of the residue and the filtrate ?

 b ) What do you think is / are present in :

( i ) the filtrate ?

 ii ) the residue ?

c ) Why do you think it was necessary to filter the mixture ?

d ) Write a :

1.  word equation for the reaction that took place .
2. balanced equation for the reaction that took  place .

5. Heat the filtrate in order to evaporate some water , to obtain a saturated solution ( see Figure 2.5 ( c ) ) .

Remove the mixture from the heat source and allow it to cool slowly .

1. How can one know that a saturated solution has been formed ?
2. Suggest other ways through which the filtrate can be heated to obtain a saturated solution .
3. Why do you think the filtrate was not evaporated to dryness ?
4. What do you notice as the mixture cools ?
5. Why do you think it was necessary to allow the mixture to cool slowly ?
6. What name is given to the process through which the crystals are formed from a solution after evaporation to form a saturated solution ?

6. Filter off the crystals and dry the crystals .

a ) Apart from filtration , what other method do you think can be used to obtain the crystals formed from the cooled mixture ?

b ) Suggest :

( i ) two ways through which the crystals can be dried .

( ii ) one way through which the crystals formed can be made purer .

7. Name two other substances that can be used with dilute sulphuric acid apart from magnesium oxide to prepare magnesium sulphate in the laboratory .

8. Draw a simple flow chart to show the main steps followed to prepare magnesium sulphate from magnesium oxide .

9. Suggest a reason why it was possible to obtain magnesium sulphate using the above procedures .

10. Prepare a summary of your findings . Share your findings with your teacher and then the class . 9 .

Assessment 2.2

1. Draw a flow chart to show the main steps that can be followed during the preparation of a soluble salt by the neutralization method .
2. Write the names of a base and an acid that can be reacted together to obtain the following salts
3. Zinc chloride .
4. Lead(II) nitrate
5. Copper (II) sulphate
6. Aluminium sulphate .

3 . Name other salts that can be obtained in a similar way like magnesium sulphate apart from those in question 2

4.The salt calcium nitrate is used as a fertilizer . It can be made by reacting an excess of calcium hydroxide with nitric acid ,

a ) Why is :

( i ) it important to ensure that the calcium nitrate used in fertilizers is pure ?

( ii ) an excess of calcium hydroxide used ?

b)How is the excess calcium hydroxide removed ?

c) Write a :

( 1 ) word equation for the reaction between calcium hydroxide and nitric acid .

( ii ) balanced equation for the reaction between calcium hydroxide and nitric acid .

What is titration and how can salts be obtained by the titration method ?

 Another way of obtaining soluble salts by neutralization involves the titration method

 In this case , a soluble base ( i.e. an alkali ) is used . The method involves adding an acid to a fixed volume of a dilute solution of an alkali in a beaker until the mixture is neutral ,

 Do you remember what a neutral solution means ? At the neutral point , we say the acid has exactly neutralized the base or vice versa .

 The next activity illustrates how an acid – alkali titration is performed.

Activity 2.3: Demonstrating acid – alkall titration

 In this activity you will work in groups with the help of your teacher or laboratory technician in performing an acid – alkali titration

Caution : Acids and bases cause severe skin burns, handle with care! Handle the glass wares with great care to avoid breakage and body cut! Rinse your mouth with a lot of water in case of contact of the alkali with the mouth.

This activity can be first demonstrated by the teacher or the technician as the learners watch and later the learners can be given the requirements to try it out under supervision. What is formed when an acid neutralizes a base and vice versa?

What you need

• 150 cm³ of 2 M sodium hydroxide solution , 150 cm³ of 2 M hydrochloric acid solution , phenolphthalein indicator ( or litmus solution / methyl orange ) and distilled or deionised water .
• 50cm³ burette , 20/25 ml pipette ( or 50/100 ml measuring cylinder , dropper , two 100/250 ml conical flasks , white tile ( a plain white piece of paper ) , filter funnel and retort stand .

What to do

1. Measure 25/20 cm³ of dilute sodium hydroxide solution using a pipette ( or a measuring cylinder ) and put it into a clean conical flask as shown in Figure 2.6 ( a ).  ( Your teacher or technician will demonstrate to you how to use the pipette with care) . Why do you think a pipette was preferred to a measuring cylinder for this purpose ?

2. Add 2-3 drops of phenolphthalein indicator to the solution in the flask and shake .

What do you observe in the flask ?

Figure 2.6 . a ) : Measuring the alkali with a pipette

Figure 2.6 . b ) : Titration process

 3. using a funnel , fill the vertically clamped clean burette with dilute hydrochloric acid . Adjust the acid level in the burette until the meniscus is at the zero ( 0 cm³ ) mark or just below the zero mark . ( Note : Filling up to the zero mark is possible only if the acid solution provided is sufficient . )

4. Place the flask containing the mixture directly under the burette on a white tile / piece of plain white paper . Suggest a reason why a white tile and a conical flask are used in this practical technique .

5. Run the acid into the mixture in the flask fairly quickly ( in the beginning ) , shaking the flask all the time ( see Figure 2.6 ( b ) ) . Ensure that you keep your eyes on the colour of the solution . When the colour of the mixture in the flask becomes pale , run the acid dropwise into the flask until the mixture becomes colourless .

a ) Read the volume of the acid left in the burette , and then find the volume of the acid needed to neutralize the volume of the alkali measured in procedure 1. Record your results in Table 2.3 of results below .

b)How do we call the point where the solution just turn colourless ?

c ) Why is the conical flask swirled ( shaken all the time ) during a titration ?

Why do you think the solution from the burette was added dropwise around the point where the Indicator changed colour ?

e ) Write both the word equation and balanced equation using the correct symbols and formulae for the reaction that took place .

6. Pour away the solution in the flask and rinse the flask well . Repeat procedures 1 to 5 two times .

Table 2.3 . Table of results

a ) Why do you think it was necessary to repeat procedures 1 to 5 two times? What can you say about the volumes of acid required for neutralization of the alkali obtained in experiments 2 and 3 ?

b ) Work out the average (mean) volume of the acid , V , using values from experiments 2 and 3 .

c ) Why do you think the value of burette reading in experiment 1 should not be used when working out the average volume of the acid?

7.Prepare a summary of findings and share it with your classmates.

From Activity 2.3 , you have learnt how to perform an acid – base titration.

Now , let us discover what is formed when an acid neutralizes a base (an alkali ) and vice versa in the following activity .

Activity 2.4 Finding out what is formed when hydrochloric acid neutralizes sodium hydroxide or vice versa

 In this activity, you work in groups to find out what is formed when hydrochloric acid neutralizes sodium hydroxide and vice versa .

CAUTION: ACIDS AND BASES CAUSE SEVERE SKIN BURNS. HANDLE WITH CARE ! HANDLE GLASSWARE WITH GREAT CARE TO AVOID BREAKAGE AND BODY CUTS! RINSE YOUR MOUTH WITH A LOT OF WATER IN CASE OF CONTACT BETWEEN THE ALKALI AND THE MOUTH,

What you need

• 150 cm³ of 2 M sodium hydroxide solution , 150 cm³ of 2M hydrochloric acid solution , phenolphthalein indicator ( or litmus solution / methyl orange ) and distilled or deionised water .
• 50 cm³ burette , 20/25 ml pipette ( or 50/100 ml measuring cylinder , dropper , two 100/250 ml conical flasks , white tile ( a plain white piece of paper ) , evaporating basin , filter funnel , stirring rod , retort stand , tripod stand and wire gauze .
• Heat source , steam bath apparatus and a piece of filter paper .

What to do

1. Perform an acid – base titration as shown in Activity 2.3.
2. Note and record the volume of acid, V cm³, required to solution . neutralize the measured quantity of sodium hydroxide.
3. Repeat the titration process without an indicator by adding to 25 cm³ of the same alkali solution, an equal volume, V , of the same acid solution from the burette.
4. Evaporate the resultant solution to obtain a saturated solution using a steam bath as shown in Figure 2.7 .

Figure 2.7 : Evaporating the salt solution

• Allow the saturated solution to cool slowly for the crystals to form . What do you observe as the solution cools ? You can taste the product and compare the taste with that of the common salt / table salt provided by the teacher .

 ( Remember ; tasting things in the laboratory is against the laboratory rules and regulations ) .

• Draw a flow chart to show the main steps followed during the preparation of sodium chloride .
• Name other salts which can be obtained in a similar way like sodium chloride .
• For each salt named in step 7 , name a pair of compounds that can be used to prepare the salt .
•  What conclusions can you draw from your findings ?
• . Prepare a summary of your findings . Share your findings with your teacher and then the class .

Assessment 2.3

1. Both sodium nitrate and ammonium nitrate cannot easily be obtained by the method described in Activity 2.3 and Activity 2.4 . Explain this statement .

2. Write the names of an alkali and an acid that can be reacted together to obtain the following salts :

1. Potassium chloride
2. Sodium nitrate ,
3. Ammonium sulphate.

3(a ) Write a chemical equation to represent the neutralization of sulphuric acid by sodium hydroxide .

b ) Name the salt formed in ( a ).

Solubility of salts

The amount of salt that can dissolve in water depends on several factors like type and amount of the salt used , amount of solvent used and the temperature . We use the term ‘ solubility to describe the behavior of salts in water , Thus , we can define the term solubility of a salt as the amount of salt in grammes that can dissolve in 100 grammes of water at a given temperature . The solubility of salts in water differs . Some salts are very soluble ( i.e. dissolve completely in water and form a solution ) . Others are partly soluble and yet the others are insoluble . It is important to note that the salts we describe as insoluble in water dissolve in water to a very small extent and a large amount remains undissolved  

The salts of interest are sulphates , chlorides , nitrates and carbonates . The following activity tests whether the given salt is soluble , sparingly soluble or insoluble in water at room temperature .

 Activity 2.5  Investigating whether a given salt is soluble , sparingly soluble or insoluble in water

In this activity , you will work in groups to investigate the solubility of salts in water .

CAUTION : TASTING OF ANYTHING IS AGAINST LABORATORY RULES AND REGULATIONS

How do we know that a given salt is soluble or not in water ?

What you need

• About 1.0 g of each of the following salts : sodium nitrate , potassium sulphate , anhydrous sodium carbonate , sodium chloride , ammonium chloride , silver chloride , lead ( II ) chloride , potassium carbonate , copper ( 11 ) sulphate , barium sulphate , magnesium sulphate , zinc carbonate , calcium sulphate , lead ( II ) sulphate , lead ( II ) nitrate , ammonium carbonate , potassium nitrate , ammonium nitrate , potassium chloride and zinc nitrate .
• Distilled / deionised water
• Spatula , stirring rod , thermometer and weighing balance .
• Heat source .
• 250 ml beaker .

What to do

1. Note and record the room temperature .
2. Measure 100 cm³ of distilled / deionised water and put it into a clean beaker ( see Figure 2.8 ) .
3. Add about 0.5 g of sodium nitrate , shake the mixture carefully and allow it to stand .

Figure 2.8 : Investigating solubility of common salts in water

Figure 2.9 :

4. Note and record whether the salt dissolves completely or not .

5.Repeat procedures 1 to 4 for all salt samples provided . In the case of lead ( II ) chloride , you may heat the mixture and allow it to slowly cool .

You may use the table below to record your results of experiment

Table 2.4 : Table of results

• Basing on your results in Table 2.4 :

a ) List all the salts that are :

• soluble in water .
•  insoluble in water .
•  sparingly soluble in water .

 b ) What general conclusion can you make about the solubility of the following salts in water ?

(i) Zinc sulphate .

( ii) Lead ( II ) chloride .

 iii ) Magnesium carbonate .

iv ) Silver nitrate

• Suggest a reason why it is important to know the general rule for the solubility of salts in water .
• Explain why it is not possible to prepare a sample of lead ( II ) sulphate by just reacting lead ( II ) oxide or lead ( II ) hydroxide with warm dilute sulphuric acid .
• Name the pairs of compounds that can be used to prepare the following salts :

a ) Barium sulphate .

b ) Lead ( II ) chloride .

c ) Calcium carbonate .

 10. What name is given to the method used to prepare salts that do not dissolve in water ?

11. Prepare a summary of your findings . Share your findings with your teacher and then the class . You may use a table of the form in 2.4 for summary writing

2.3 Uses of salts to our society

 Salts are used in many different fields. Look at the following pictures showing the uses of some salts

Figure 2.10: Photos showing the applications of salts

Plaster of Paris is made of calcium sulphate and is used to keep broken limbs in place

Cement is made from rocks containing calcium carbonate

Toothpaste contains tin (II) fluoride

Dry cells contain ammonium chloride

Silver bromide is used in photographic films

Ammonium nitrate fertilizer

To further understand the importance of salts, let us carry out the following activity.

Activity 2.6 Finding out more about uses of common salts

Are salts important to us?

In this activity, you will work in groups to find out uses of common salts.

What you need

• Assortment of pictures of products containing salts / video clips.
• Library and internet (if available).

 What to do

1. Table 2.5 below gives a list of some salts.

Use the internet and library to find out uses of salts.

Copy and complete Table 2.5.

Table 2.5: Some common salts

• Share your ideas with your teacher and then the class.

Chapter Summary

 In this chapter, you have learnt that:

• Salts are all around us.
• Acids and bases (alkalis) react to form salts and water.
• When an acid reacts with an alkali, we call the reaction neutralization.
• the amount of an acid that neutralizes an alkali can be determined by titration.
• the salts are compounds derived from acids when their hydrogens are replaced.
• the method used to prepare a given salt sample depends on whether the salt is soluble or insoluble.
• Soluble salts can be prepared by the neutralization method.
• a neutral substance has a pH = 7, the soluble salts are recovered from a solution formed from the neutralization reaction by careful evaporation to dryness of the solution.
• a saturated solution contains more of the salt than water.
• the insoluble salts are prepared by precipitation method where two soluble salts are mixed to form an insoluble salt.
• the salts can be classified as an acid salt or normal salt.
• the main groups of salts are sulphates derived from sulphuric acid, chlorides derived from hydrochloric acid, nitrates derived from nitric acid and carbonates derived from carbonic acid.
• the solubility of salts in water varies depending on several factors like the type of salt, amount of salt, temperature and amount of solvent used.
• all sulphates are soluble except barium sulphate, calcium sulphate and lead (II) sulphate.
• all chlorides are soluble except silver chloride and lead (II) chloride.
• lead (II) chloride dissolves in hot water and reforms on cooling.
• all nitrates are soluble.
• all carbonates are insoluble except sodium, potassium and ammonium carbonates.
• all sodium, potassium and ammonium salts are soluble.
• the salts are widely used in homes and in industries for consumption, cleaning and making fertilizers and drugs.
• salts can be prepared by the reaction between acids and bases.
• neutralization is the reaction between an acid and a base to form a salt and water only
• salts get their names from the acids they are derived from
• some of the physical properties of salts including different colors, and solubility in water.
• some alsts are soluble in water and some are not soluble in water.
• all carbonates except those of potassium, sodium and ammonium are insoluble in water. it is important to note that anhydrous solid carbonates normally exist in powder form.
• all slats of sodium and potassium are readily soluble in water.

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