Whenever a chemical change occurs, we can say that a chemical reaction has taken place. Chemical reactions are used to explain the various processes occurring around us. Chemical reactions provide more in-depth information on how molecular interactions and changes occur.
In this Chapter, we will study about the various types of chemical reactions and their symbolic representation.
Chemical Reaction
The process in which a chemical transformation of one set of chemical substances to another occurs is called a chemical reaction. Substances are either chemical elements or compounds. The substances initially involved in the chemical reaction are called reactants or reagents.
The new substances produced as a result of chemical reaction are called products.
A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.
Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting to water and water evaporating to vapour. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water (H2O) is the same compound, with each molecule composed of two atoms of hydrogen and one atom of oxygen.
In a physical change, the only the state of matter changed the identity of the substance remains the same. Melting, boiling, and condensation are the process through which physical properties change.
However, in a chemical reaction, the atoms will be redistributed to give a new substance. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. For example,
If water, as ice, liquid, or vapour, encounters sodium metal (Na), the atoms will be redistributed to give the new substances molecular hydrogen (H2) and sodium hydroxide (NaOH). By this, we know that a chemical change or reaction has occurred.
In a chemical reaction, only the position of the electrons changes due to the formation and breakdown of the chemical bond between the atoms, no change in the nucleus means no change in the elements present.
In a chemical Change:
(i) Atoms in the reactants are rearranged to form one or more different substances.
(ii) Old bonds are broken, new bonds are formed.
(iii) Reactants lose their properties to form product of different properties.
Following are the observations that help us to determine whether a chemical reaction has taken place or not:
(i) The emission of heat and light.
(ii) The formation of a precipitate.
(iii) The evolution of gas.
(iv) Change in colour.
(v) Change in temperature.
(vi) Change in state.
Absolute confirmation of a chemical change can only be validated by chemical analysis of the products.
Chemical Equation
As we observe the changes around us, we can see that there is a large variety of chemical reactions taking place around us. And to describe a chemical reaction in a sentence form is quite long. So, chemical reactions are written in the form of chemical equations. You all must have read equations in mathematics. Here, chemical equations also follow the same principle as in mathematics.
But here in chemical equation arrow sign is used instead of equal sign in mathematics.
Features of a chemical equation:
(i) The reactants are written on the left-hand side with a plus(+) sign between them.
(ii) The products are written on the right-hand side with a plus(+) sign between them.
(iii) An arrow separates the reactants from the products.
(iv) The arrow head points towards the products and shows the direction of the reaction.
Word Equation
The simplest way to write a chemical reaction in the form of a word-equation. For example, when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide. The word-equation for the above reaction would be –
Magnesium + Oxygen
→
Magnesium oxide
Reactants
Product
The substances that undergo chemical change in the above reaction,
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
A word-equation shows change of reactants to products through an
arrow placed between them.
We can still simplify this by using a chemical formula instead of words. The above word equation can be written as
Mg + O2
→
MgO
Such a chemical equation is called a skeletal chemical equation for a reaction.
Components of a Chemical Equation
Followings are the components of a chemical equation:
(i) Reactants or Reagents
(ii) Products
(iii) Arrow sign
(iv) Plus sign
(v) Coefficients
(vi) Symbols
(vii) Physical State
(viii) Reaction conditions
Arrows
To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae.
The following notations are used to represent the states of reactants and products :
→
used to separate the reactants from the products. The arrowhead points towards the products, and shows the direction of the reaction.
Used when the reaction can proceed in both directions - this is called an equilibrium arrow. These reactions are called reversible reactions.
↑
An alternative way of representing a substance in a gaseous state
↓
An alternative way of representing a substance in a solid state
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(↑)
Coefficients
Coefficients are used to show the relative amounts of each substance present in a chemical equation. This amount can represent either the relative number of molecules or the relative number of moles.
It is placed immediately to the left of each formula. A coefficient of 1 is typically omitted.
CH4 + 2O2 → CO2 + 2H2O
It is common practice to use the smallest possible whole-number coefficients in a chemical equation. However, these coefficients represent the relative number of reactants and products, and therefore, they may be correctly interpreted as ratios.
In the above example, Methane and oxygen react to yield carbon dioxide and water in a 1:2::1:2 ratio. This ratio is satisfied if the numbers of these molecules are, respectively, 1-2-1-2, or 2-4-2-4, or 3-6-3-6, and so on.
Likewise, these coefficients may be interpreted with regard to any amount or number, and so this equation may be correctly read in many ways.
One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules.
One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of CO2 molecules and 2 moles of water molecules.
Symbols
There are a few special symbols used above and/or below the arrow in the equation to represent certain features.
▵
Triangle above arrow indicates that the reaction is being heated.
CaCO3(s)
▵
CaO(s) + CO2(g)
Physical State
To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae.
The following notations are used to represent the states of reactants and products :
Solid state - s
Liquid state - l
Gaseous state - g.
Aqueous state - aq
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
The word aqueous (aq) is written if the reactant or product is present as a solution in water.
Usually physical states are not included in a chemical equation unless it is necessary to specify them.
Reaction Conditions
Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example,
CO(g) + 2H2(g)
340atm
CH3OH
6CO2 (aq) + 12H2O(l)
SunlightChlorophyll
C6H12O6(aq) + 6O2(aq) + 6H2O(l)
2H2O2 (aq)
MnO2
2H2O(l) + O2(g)
Formula written above the arrow is used as a catalyst in the reaction. A catalyst is a chemical which speeds up a chemical reaction without being itself consumed or produced.
Writing a Chemical Equation
The symbolic representation of a chemical reaction in the form of symbols and formulae is called a chemical equation.
Mg + O2 → MgO
Count and compare the number of atoms of each element on the LHS and RHS of the arrow. If the number of atoms of each element is the same on both sides, then the equation is called a balanced equation. If not, then the equation is unbalanced because the mass is not the same on both sides of the equation.
2Mg + O2 → 2MgO
Balanced Chemical Equation
Even though chemical compounds are broken up and new compounds are formed during a chemical reaction, neither atoms in the reactants disappear, nor do new atoms appear to form the products.
As we know, mass can neither be created nor destroyed in a chemical reaction.
That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
Like that in chemical reactions, atoms are never created or destroyed. The same atoms that were present in the reactants are present in the products—they are merely reorganized into different arrangements.
In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.
No. of atoms of an element on LHS = No. of atoms of that element on the RHS.
Let us learn about balancing a chemical equation step by step.
Let us balance the following chemical equation:
Fe + H2O → Fe3O4 + H2
Step - 1: To balance a chemical equation, first draw boxes around each
formula. Do not change anything inside the boxes while balancing the
equation.
To balance a chemical equation, it must be remembered that you cannot change subscripts in a chemical formula.
Step - 2: List the number of atoms of different elements present in the
unbalanced equation.
Elements
Number of atoms
Reactants (LHS)
Products (RHS)
Fe
1
3
H
2
2
O
1
4
Step - 3:
It is often convenient to start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum
number of atoms. Using these criteria, we select Fe3O4 and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS.
Remember here, you can change only the coefficients.
To balance oxygen atoms we can put coefficient ‘4’ as 4 H2O and not H2O4 or (H2O)4.
Step - 4:
Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms. To equalise the number of H atoms, make the number of molecules
of hydrogen as four on the RHS. Put coefficient ‘4’ to H2.
Step - 5:Now, only one element is left to be balanced, that is, iron. There are 3 Fe atoms on the RHS and only one on the LHS.
So, put coefficient ‘3’ to Fe on LHS.
Step - 6: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.
3Fe + 4H2O → Fe3O4 + 4H2
The numbers of atoms of elements on both sides of the above equation are
equal. This equation is now balanced. This method of balancing chemical
equations is called hit-and-trial method as we make trials to balance
the equation by using the smallest whole number coefficient.
Practice Time
Balance the following equations.
H2 + Cl2 → HCl
N2(g) + O2(g) → NO2(g)
Hg(l) + O2(g) → Hg2O2(s)
C2H4 + O2 → CO2 + H2O
C6H14 (l) + O2(g) → CO2(g) + H2O(g)
C7H16 + O2 → CO2 + H2O
C8H18 + O2 → CO2 + H2O
Pb(NO3)2(aq) + NaCl(aq) → NaNO2(aq) + PbCl2(s)
Mg(NO3)2(s) + Li(s) → Mg(s) + LiNO3(s)
BaCl2(aq) + Al2(SO4)3(aq) → BaSO4(s) + AlCl3(aq)
Na(s) + H2O(l) → NaOH(aq) + H2(g)
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + NaCl(aq)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Types of Chemical Reactions
There are many different types of chemical reactions. Chemists have classified the many different reactions into general categories. The chemical reactions we will explore are a representation of the types of reactions found in each group. There is a general description of the main reaction types and specific examples provided in the selection boxes.
(i) Combination Reaction
(ii) Decomposition Reaction
(iii) Displacement Reaction
(iv) Double-Displacement Reaction
(v) Oxidation and Reduction
A reaction in which two or more reactants combine to form a new product is known as a combination reaction. It is also called synthesis reaction.
This type of reaction is represented by the following equation.
A + B → AB
A and B represent the reacting elements or compounds while AB represents a compound as the product.
The following examples are representative of combination reactions.
Calcium Oxide and Water
Formation of slaked lime(calcium hydroxide) :
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
Formation of water :
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
2H2(g) + O2(g) → 2H2O(l)
Hydrogen + Oxygen → Water
Decomposition Reaction
A reaction in which a single reactant breaks down to give two or more simpler products is called a decomposition reaction.
A decomposition reaction can be represented by the following equation.
AB → A + B
The following examples are representative of decomposition reactions.
Decomposition of Ferrous sulphate (FeSO4)
Formation of ferric oxide, sulphur dioxide and sulphur trioxide :
Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to from products.
Formation of lead oxide, nitrogen dioxide and Oxygen:
When lead nitrate is heated over a flame, it gives yellow colored lead (II) oxide, brown fumes of nitrogen dioxide and a colorless gas of oxygen.
2Pb(NO3)2(s)
Heat
2PbO(s) + 4NO2(g) + O2(g)
Lead nitrate → Lead oxide + Nitrogen dioxide + Oxygen
Decomposition of Silver chloride (AgCl)
Formation of silver and chlorine:
When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas. This reaction is used in black and white photography.
Formation of silver and bromine:
When silver bromide is exposed to light, it decomposes to form silver metal and bromine gas. This reaction is used in black and white photography.
2AgBr(s)
Sunlight
2Ag(s) + Br2(g)
Silver bromide → silver + bromine
Decomposition of Water (H2O)
Formation of Hydrogen and Oxygen:
This process is called electrolysis of water. When a direct current is passed through water it decomposes to form oxygen and
hydrogen. The volume of hydrogen gas produced at the negative electrode is twice the volume of the oxygen gas formed at the positive electrode. This
indicates that water contains twice as many hydrogen atoms as oxygen atoms, which is an illustration of the law of constant composition.
2H2O(l)
electricity
2H2(g) + O2(g)
Water → Hydrogen + Oxygen
Decomposition of Nitrogen Triiodide (N2I)
Formation of Nitrogen and Iodine:
Nitrogen triiodide is extremely unstable when it is dry. Touching it with a feather causes it to
decompose explosively. The explosion occurs as chemical energy is released by
the decomposition of nitrogen triiodide to N2 and I2.
Violet iodine vapor can be observed after the explosion.
Decomposition reactions which require heat are known as thermolytic.
Photolytic Reaction
Decomposition reactions which require light are known as photolytic.
Electrolytic Reaction
Decomposition reactions which require electricity are known as electrolytic.
Displacement Reaction
A reaction in which one element replaces another similar element in the compound is called a displacement reaction.
It is also called single-replacement reaction. Displacement reactions can be represented by the following equations.
In a displacement reaction, more active element displaces less active element from its salt solution.
In the reactivity series, the most reactive metal is placed at the top and the lowest reactive metal is placed at the bottom.
Potassium is the most reactive metal so placed at the top in the reactivity series. Gold is least reactive so, placed at the bottom of reactivity series.
The following examples are representative of displacement reactions.
Iron and Copper sulphate
Formation of Iron sulphate and Copper:
When an iron nail is dipped in the blue colour of copper sulfate solution, the iron is displaced from the copper sulfate solution because iron is more reactive than copper. Therefore, the colour of the copper sulphate solution changes from blue to light green and the iron nail turns brownish.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Iron + Copper sulphate → Iron sulphate + Copper
Zinc and Copper Sulphate
Formation of zinc sulphate and copper :
When zinc is added to copper sulphate (CuSO4) solution due to more reactivity of zinc, cooper is replaced by the zinc and forms zinc sulphate. During the process, the colour of the solution changes from blue to colourless.
Formation of Lead chloride and copper :
When lead powder is added to copper chloride (CuCl2) solution due to more reactivity of lead, cooper is replaced by the lead and forms lead chloride. During the process, the green color of the solution becomes colorless and a brown ring is formed on top due to the presence of copper.
Pb (s)+ CuCl2(aq) → PbCl2(aq) + Cu(s)
Lead + Copper chloride → Lead chloride + copper
Iron (III) Oxide and Aluminum
Formation of Iron and Aluminum Oxide:
This reaction is an example of thermite Reaction. The thermite reaction is an exothermic reaction between the metal and metal oxide.
In the thermite reaction, Al reduces Fe2 to Fe in an extremely exothermic reaction in which Al is oxidized to
Al2O3. The reaction produces enough heat to melt the iron. Because of the
extreme heat produced in the thermite reaction, it is used industrially to weld iron. This reaction is used to connect railway tracks.
Fe2O3(s) + 2Al(s) → 2Fe(s) + Al2O3(s)
Iron (3) Oxide + Aluminum → Iron + Aluminum Oxide
Copper (II) Oxide and Carbon
Formation of Copper and Carbon dioxide:
When black carbon and black copper oxide are heated together the Cu2+
ions are reduced to metallic Cu and a gas carbon dioxide is evolved. When the gas is collected in Ca(OH)2 a white precipitate of CaCO3
is formed. The reaction which occurs involves the reduction of Cu2+ions by carbon which is oxidized
to CO2
2CuO + C → 2Cu + CO2
Copper(II) oxide or cupric oxide + Carbon → Copper + Carbon dioxide
Silver Nitrate and Copper
Formation of Silver Crystals :
When a copper wire is placed in a solution of AgNO3, the silver nitrate reacts with copper to form hairlike crystals of silver metal and a blue solution of copper nitrate.
Formation of Tin Crystals :
If a small zinc rod is put into a solution of tin(II) chloride (SnCl2) tin will precipitate on the zinc rod and zinc will go into solution as Zn2+ ions.
SnCl2(aq)+ Zn(s) → ZnCl2(aq) + Sn(s)
Tin (II) Chloride + Zinc → Zinc chloride + Tin
The presence of acid in the solution means that hydrogen gas will also be formed.
When acidified Sn(II)Cl2 is added to a beaker containing a
piece of Zn, some of the Sn2+ reacts with H+ in the solution to produce H2 gas.
Immediate changes can also be observed on the surface of the Zn as it quickly becomes coated with Sn crystals. After the reaction has progressed for a time, hairlike crystals of Sn can be observed on the surface of the Zn.
Zn(s)+ 2HCl(aq)→ H2(g) + ZnCl2(aq)
Double Displacement Reaction
A chemical reaction in which the ions of two reactants exchange their places in an aqueous solution to form two new products is called a double displacement reaction.
Double displacement reactions are also called double replacement reactions, or metathesis reactions.
A double-replacement reaction can be represented by the following equation.
The following examples are representative of double displacement reactions.
Sodium sulphate and Barium chloride
Formation of barium sulphate, and sodium chloride :
Take about 3 mL of sodium sulphate solution in a test tube.
In another test tube, take about 3 mL of
barium chloride solution.
Mix the two solutions.
A white substance is formed.
The white precipitate of BaSO4 is formed by the
reaction of SO4−2 and Ba+2. The other product formed is sodium chloride
which remains in the solution.
Formation of potassium nitrate and lead(II) iodide :
When an aqueous solution of Potassium Iodide is added to an aqueous solution of Lead (II) Nitrate, Potassium iodide reacts with lead(II) nitrate and produces lead(II) iodide and potassium nitrate.
The formation of a precipitate occurs when the cations of one reactant combines with the anions of the other reactant to form an insoluble or slightly insoluble compound.
Potassium nitrate is water soluble. However, lead(II) iodide is only partially soluble in water. Most of the lead(II) iodide precipitates out of the solution as a yellow solid.
Formation of silver chloride and sodium nitrate :
When an aqueous solution of silver nitrate is added to an aqueous solution of sodium chloride, the silver ions combine with the chloride ions to form a white precipitate of silver chloride.
Respiration is an exothermic process. We all know that we need energy to stay alive. We get this energy
from the food we eat. During digestion, food is broken down into simpler
substances. For example, rice, potatoes and bread contain
carbohydrates. These carbohydrates are broken down to form glucose.
Burning of Glucose in our body
Formation of carbon dioxide, water and energy:
Glucose combines with oxygen in the cells of our body and provides energy.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
Glucose + Oxygen → Carbon dioxide + water + energy
The decomposition of vegetative matter into compost is also an example of an exothermic reaction. Because microbes require energy to break the bond of the vegetable, it generates heat energy.
Precipitation Reaction
Any reaction that produces a precipitate can be called a precipitation reaction. Precipitation reactions are usually double displacement reactions involving the production of a solid form residue called the precipitate.
Potassium chloride and silver nitrate
Formation of a white precipitate called silver chloride or AgCl and Potassium Nitrate KNO3:
A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white precipitate called silver chloride or AgCl is formed.
The precipitation reaction undergoes in aqueous solutions or medium in an ionic state. These are used for the extraction of magnesium from the seawater.
Combustion Reaction
In a combustion reaction, a substance combines with oxygen, releasing a large amount of energy in the form
of light and heat. For organic compounds, such as hydrocarbons, the products of the combustion reaction are carbon dioxide and water.
CH4 + 2O2 → CO2 + 2H2O
Hydrogen and Oxygen Reaction
The combustion of hydrogen yields water vapor as a reaction product. Hydrogen gas is very flammable and yields explosive mixtures with air and oxygen.
Various Substances with Oxygen
Magnesium, steel wool, white phosphorus, and sulfur burns in oxygen. The resulting reactions are combination reactions in which two substances react to form one product.
Products formed in these reactions are MgO, Iron (III) oxide or ferric oxide (Fe2O3), Phosphorus pentoxide(P4O10) and SO2.
All of these combustion reactions are very exothermic.
Oxidation Reaction
A reaction in which, if a substance gains oxygen during the reaction, it is called oxidation and the substance is called oxidised.
When copper powder is heated in a china dish, the surface of copper powder becomes coated with a black-coloured substance called copper(II) oxide. This is because oxygen is added to copper and
copper oxide is formed.
2 Cu + O2
Heat
2 CuO
Copper + oxygen → Copper II Oxide (Black )
2 Mg(s) + O2 (g) → 2MgO (s)
In oxidation reaction, addition of oxygen or removal of hydrogen or loss of electron takes place.
Reduction Reaction
A reaction in which, if a substance loses oxygen during the reaction, it is called reduction and the substance is called reduced.
If hydrogen gas is passed over this heated material (CuO), the black
coating on the surface turns brown as the reverse reaction takes place
and copper is obtained.
CuO(s) + H2 (g)
Heat
Cu(s) + H2O
Copper II Oxide + Hydrogen → Copper + Water
In reduction Reaction addition of hydrogen or removal of oxygen or gain of electron takes place.
Redox Reaction
A chemical reaction that involves both oxidation and reduction simultaneously is called a redox reaction.
In redox reaction electrons are transferred between two reactants, most often one reactant undergoes oxidation while the another reactant is reduced. The oxidation states of atoms are usually changed in a redox reaction.
It is also called oxidation-reduction reaction.
In the above reaction, the copper(II) oxide is losing oxygen and
is being reduced. The hydrogen is gaining oxygen and is being oxidised.
In the above reaction, carbon is oxidised to CO and ZnO is reduced to Zn.
In the above reaction, HCl is oxidised to Cl2 whereas MnO2 is reduced to MnCl2.
If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised.
If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced.
A substance that loses oxygen or gains hydrogen is known as an oxidising agent.
A substance that loses hydrogen or gains oxygen is known as a reducing agent.
An oxidising agent gets reduced whereas a reducing agent gets oxidised.
Corrosion
When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.
Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. Corrosion degrades the useful properties of materials and structures including strength, appearance, etc.
Iron – The most common type of iron corrosion occurs when it is exposed to oxygen and the presence of water, which creates a reddish brown
hydrated iron (III) oxide (Fe2O3. xH2O) commonly called rust. Rust can also effect iron alloys such as steel.
Copper – reacts with moist carbon dioxide in the air and gets a green coat of copper carbonate.
Silver – it reacts with sulphur in the air to form silver sulphide and turns black.
Corrosion causes damage to car bodies, bridges, iron railings, ships
and to all objects made of metals, specially those of iron.
Aluminium is a corrosion resistant metal that naturally generates a protective coating. The coating formed is extremely thin and is generated when aluminium comes into contact with an oxidising environment. This protective aluminium oxide Al2O3 layer helps protect the surface of the metal from corrosion.
Rancidity is the process of oxidation or hydrolysis of fats and oils when exposed to air, light, moisture or by bacterial action, resulting in unpleasant taste and odor (smell). Usually substances which prevent oxidation
(antioxidants) are added to foods containing fats and oil. Keeping food
in air tight containers helps to slow down oxidation. Chips manufacturers usually flush bags of chips with gas such as Nitrogen to prevent chips from being oxidized.
Rancidity can be prevented by:
Adding anti-oxidants.
Storing the food in air-tight containers.
Replacing air by Nitrogen.
Refrigeration of food stuf.
Questions
Question: An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why? Answer:
As we know iron is more reactive than copper. So, it displaces Cu from CuSO4 solution and forms ferrous sulphate which is of Light Green Colour.
CuSO4 (aq) + Fe (s)→ FeSO4 (aq) + Cu(s) Blue colour light green colour.
Question: A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution? Answer:
We know that copper is more reactive than silver, so it will displace silver from its salt solution. Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s). So the solution will turn blue due to the formation of copper nitrate.
Question: What is lime-water test for the detection of carbon dioxide? Answer:
Carbon dioxide reacts with calcium hydroxide solution to produce a white precipitate of calcium carbonate. Limewater is a solution of calcium hydroxide. If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white.
Question: Why cannot a chemical change be normally reversed ? Answer:
In a chemical change some bonds are broken and some bonds are formed. The products are quite different from the reactants. Therefore it normally can’t be reversed.
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