Salts | Chemistry Form 2

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Salts - Chemistry Form 2

In this lesson, we will discuss preparation of soluble salts.


By the end of this lesson you should be able to:

i) Describe the preparation of a soluble salt.
ii) Explain the terms saturated solution, crystallization and neutralization.

The table shows a summary of the solubilities of various salts.Use the scroll bar to view the solubility of different salts.


Soluble salts can be prepared by : Action of acid on metal Action of acid on an alkali Action of acid on an insoluble base Action of acid on carbonate Direct synthesis.

Click to play the video to observe how Copper(II) Sulphate crystals are prepared.



A saturated solution is one that cannot dissolve any more solute at a given temperature. On cooling the saturated solution, crystals form. This process is referred to as crystallization.

The reaction between a base and water only is called Neutralization reaction.
From the experiment, copper( II) sulphate solution is heated to saturation.
A saturated solution is one that cannot dissolve any more solute at a given temperature.
The saturated solution form crystals on cooling. This process is referred to as crystallization.


 

The reaction between a base and water only is called Neutralization reaction.

From the experiment, copper II sulphate solution is heated to saturation.

A saturated solution is one that cannot dissolve any more solute at a given temperature.

The saturated solution form crystals on cooling. This process is referred to as crystallization.

In this lesson, we will discuss how to prepare insoluble salts by precipitation and direct combination of elements.


An insoluble salt is prepared by using two soluble salts. A soluble salt and insoluble salt are formed.

By the end of this lesson, you should be able to:

i) Describe the preparation of salts by precipitation.
ii) Write ionic equations for precipitation reactions.
iii) Describe the preparation of salts by direct combinations.


Click to play the video to observe the preparation of Lead (II) Iodide.



The insoluble solid formed when two soluble salts react is called a precipitate. Reactions in which two soluble salts react to form a soluble salt and an insoluble salt are refered to as double decomposition or precipitation reaction.Other salts that can be prepared through preciptitation reaction are Barium Sulphate, copper (II) carbonate, silver chloride and lead (II) chloride.


Salts can also be prepared by direct combination of elements.Dry chlorine gas is passed over heated Iron Wool in a combustion tube.Chlorine gas is poisonous hence experiment should be carried out in a fume chamber. Anhydrous calcium chloride is used to absorb moisture.This is because Iron (III) Chloride is deliquescent. It absorbs moisture to form a solution.

In this method two elements combine to form a salt only. This method of salt preparation is called direct combination or synthesis. Direct combination reactions do not produce crystals. Other salts that ca be prepared by this method include: sodium chloride, Iron (II) sulphide and Aluminium Chloride.


In this lesson we will discuss the effect of heat on salts.

Some salts decompose on heating while others do not.

By the end of the lesson, you should be able to:

i) Describe the effect of heat on various salts

ii) Explain the observations made when salts are heated

iii) State some uses of salts.


Effect of heat on Nitrates

When nitrates are heated, they decompose to form different products.Two spatulaful of Lead (II) nitrate are placed in a dry boiling tube.Lead (II) nitrate is then heated. Click to play the video to observe the reaction.



Lead (II) nitrate decomposes on heating to give brown fumes, a colourless gas, and a yellow solid on coooling Yellow solid is Lead (II) oxide Brown fumes are Nitrogen (IV) oxide. A gas that relights a glowing splint is produced. The gas is oxygen.

Two spatula ful of copper (II) nitrate are placed in a dry boiling tube. Copper (II) nitrate is heated gently then strongly.

Click to play the video to observe what happens when Copper (11) nitrate is heated.



Copper (II) nitrate decomposes on heating to form a black solid, brown fumes and a colourless gas .Black solid is copper (II) oxide while brown fumes is Nitrogen (IV) oxide gas. A gas that relights a glowing splint is produced. The gas is oxygen.


Nitrates of group 1 elements decompose on heating to give the metal nitrate and oxygen gas.
Nitrates of potassium and sodium decompose on heating to form their metal nitrate and oxygen gas.
Nitrates of silver and mercury decompose on heating to form their metal, Nitrogen (IV) oxide and oxygen gas. Silver nitrate and mercury nitrate salts decompose on heating to form their metals, Nitrogen IV oxide and oxygen.Nitrates are categorized on the basis of the products formed on heating e.g.
Nitrates of very reactive metals such as sodium and potassium form corresponding metal nitrates, and oxygen gas.
Nitrates of moderately reactive metals such as calcium, zinc, Magnesium, Lead and copper decompose to form metal oxide, Nitrogen (IV ) oxide and oxygen gas.
Nitrates of less reactive metals such as silver and mercury decompose to form their metals, Nitrogen (IV) oxide and oxygen gas.
However, Ammonium nitrate decomposes on heating to form Ammonia gas, Nitrogen (IV) oxide and water vapour.

The following table shows the effects of heat on various Nitrates. Use the scrollbar to view the products.

Summary.

Nitrates are categorized on the basis of the products formed on heating e.g.

I.    Nitrates of very reactive metals such as sodium and potassium form corresponding metal nitrates, and oxygen gas.

II.    Nitrates of moderately reactive metals such as calcium, zinc, Magnesium, Lead and copper decompose to form metal oxide, Nitrogen IV oxide and oxygen gas.

III.    Nitrates of less reactive metals such as silver and mercury decompose to form their metals, Nitrogen IV oxide and oxygen gas.

IV.    How ever, Ammonium nitrate decomposes on heating to form Ammonia gas, Nitrogen IV oxide and water vapour.

NH4NO3(s)    heat    NH3(g) + NO2(g) + H2O.

Potassium and sodium carbonates do not decompose on heating, while other metal carbonates decompose on heating. Aluminium and iron (III) carbonate do not exist.

Potassium carbonate

Effect of Heat on Carbonates

Two spatulaful of solid copper (II) carbonate are placed in a dry boiling tube. The Gas produced is bubbled through calcium hydroxide solution. Click to play the video to observe this reaction.



Green copper (II) carbonate decomposes on heating to form black copper (II) oxide and carbon (IV) oxide gas. Carbon (IV) oxide forms a white precipitate when bubbled through calcium hydroxide solution. The White precipitate is Calcium Carbonate.

Effect of heat on lead II carbonate.

i.     Place a spatulaful of solid lead II carbonate into a dry boiling tube.

  .show transfer of solid white lead II carbonate into a dry boiling tube.

 

 

 

 

 

 

ii. Bubble the gas produced on heating through calcium hydroxide solution as shown.

*show

-bubbling of gas in calcium hydroxide solution.

-formation of white ppt.

-white solid turning red when hot and yellow on cooling.

-Remove delivery tube before stopping to heat.

-stop heating on formation of white

Two spatulaful of solid lead (II) carbonate is placed into a dry boiling tube.The gas produced is bubbled through Calcium Hydroxide solution.Click to play the video to observe what happens.



White lead (II) carbonate decomposes on heating to form red lead (II) oxide and carbon (IV) oxide gas.On cooling, the red lead (II) oxide turns to yellow. Similarly, potassium and sodium hydrogen carbonates decompose on heating to form their metal carbonates, carbon (IV) oxide and water . Sodium and Potassium carbonates formed do not decompose further on heating.

The following table shows the effects of heat on various Nitrates.

Use the scrollbar to view the products.

Effect of heat on sulphates.

Metal sulphates do not decompose easily on heating unless under very strong heating. When heated, the sulphates of alkali metals and of alkaline earth metals do not decompose.

V.O.(insert after first paragragh).

Hydrated sulphates of iron , copper lose their water of crystallization. On further heating they decompose to their metal oxides. Sulphur IV oxide and sulphur VI oxide.

Animated.

AL2(SO4)3(s)  heat     AL2O3(s)     +    3SO3(g)

Fe2(SO4)3(s)     heat        Fe2O3(s)      +    3SO3(g)

FeSO4(s)     heat     Fe2O3(s)      +    SO2(g)          +    SO3(g)

Some metal sulphates decompose only on very strong heating.However sulphates of alkaline earth metals do not decompose .On heating Hydrated sulphates of Iron and Copper lose their water of crystallization. On further heating they decompose to their metal oxides.



Heating Copper (II) Sulphate.


Most of the salts are useful in our daily lives.Click on each of the box to observe some of these uses.


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