Chemical Reactions and Equations

 

 

 

Summary

Some high fat foods such as potato chips are packaged in materials that protect them

from light and oxygen and the containers are flooded with nitrogen to further exclude oxygen

At times, to avoid the presence of oxygen altogether, vacuum packaging is used in some

processed foodstuff

In a physical change the shape, size, appearance or state of a substance may alter, but it

is a temporary change, which is usually reversible. No new substance is formed, hence no

change in the mass of the substance and no energy changes are involved.

In a chemical change, new substances are formed and energy changes are involved.

There is a change in mass during the reaction and a permanent change takes place.

The chemical formula of a substance is the symbolic representation of the actual number

of atoms present in one molecule of that substance

A complete chemical equation represents the reactants, products and their physical

states symbolically and is a balanced account of a chemical transaction.

A chemical equation is balanced so that the numbers of atoms of each type involved in a

chemical reaction are the same on the reactant and product sides of the equation

Equations must always be balanced. Equations are balanced by the hit and trial method.

The rate of a chemical reaction depends on temperature (heat), presence of light,

presence of catalyst, application of pressure and the passage of electric current

Chemical reactions that proceed with evolution of heat energy, that is, in which heat is

given out along with the product, are called exothermic reactions

Chemical reactions that proceed with the absorption of heat energy are called

endothermic reactions

Combination or synthesis is a reaction in which a new single compound is formed by the

direct union of two or more substances

Decomposition reactions are opposite to combination reactions. In a decomposition

reaction a compound breaks down into two or more simple substances by the application of

heat or electricity.

In a displacement reaction an atom or group present in a compound is replaced by

another atom or group, without bringing any change in the structure of the molecule or

compound

In double displacement reactions two different atoms or groups of atoms (ions) of two

compounds exchange their places and give rise to two new compounds

In a precipitate reaction insoluble salts are produced

A reversible reaction is one that can proceed in both the directions depending upon the

conditions of the reaction

A reversible thermal decomposition reaction is called thermal dissociation

Oxidation is a reaction in which oxygen is added to a substance, or hydrogen is removed

from a substance

Reduction is a reaction in which oxygen is removed from a substance, or hydrogen is

added to a substance

Oxidation reactions cause corrosion on metals and rancidity on food stuffs

Question (1): Define a chemical change.

Answer:  A chemical change is a permanent, not easily reversible

change in which the identity of the substance is altered to produce

one or more new substances.

Question (2): Give four examples of a chemical change.

Answer:  1) Burning of coal.

2) Rusting of iron.

3) Photosynthesis in plants.

4) Electrolysis of water into hydrogen and oxygen.

Question (3): Give four characteristics of a physical change.

Answer:  1) A physical change is temporary and is usually reversible.

2) No new substances are formed during a physical change.

3) There is no change in mass of the substance during a physical

change.

4) There is no net absorption or release of energy during a physical

change.

Question (4): Give an example of a chemical change that is

reversible.

Answer:  Action of heat on ammonium chloride is reversible. When

heated, it decomposes to form ammonia and hydrogen chloride.

When the products are cooled, they recombine to form ammonium

chloride.

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Question (5): Define a physical change.

Answer:  A physical change is a temporary and reversible change in

which the physical properties of the substance change without

altering the composition of the substance.

Question (6): Classify as physical or chemical change:

1) Tearing of paper

2) Production of biogas

3) Butter going rancid

4) Magnetisation of iron

5) Clotting of blood

Answer: 

1) Physical - No change in mass.

2) Chemical - New substances with new properties are produced.

3) Chemical - New substances are formed and the original

substances cannot be re-obtained.

4) Physical - Temporary, no new substance is formed.

5) Chemical - New substance with new properties is produced.

Unclotted blood cannot be reobtained in this change.

Question (7): Give four examples of a physical change.

Answer:  1) Evaporation of water.

2) Formation of dew.

3) Crystallisation of sugar from its solution.

4) Ringing of an electric bell.

Question (8): Give four characteristics of a chemical change.

Answer:  1) A chemical change is a permanent and not easily

irreversible.

2) A chemical change results in alteration in the composition of the

molecules of the substance undergoing change and the formation of

one or more new substances.

3) The mass of the substance undergoing a chemical change usually

changes.

4) Heat is either absorbed or given out during a chemical change.

Chemical Reactions and Equations

Introduction

Physical Change

Chemical Change

Chemical Equations

Types of Chemical Reactions

Combination or Addition or

Synthesis Reaction

Decomposition Reaction

Displacement or Substitution

Reaction

Double Displacement Reaction

Oxidation Reduction

The Effects of Oxidation

Reactions In Every Day Life

Summary

Question and Answers

Multiple Choice Questions

Question (9): What do you understand by an exothermic chemical

change?

Answer:  An exothermic chemical change is one, which takes place

with the liberation of heat energy. Example: Burning of coal in air.

C + O2 CO2 + heat

Question (10): Explain whether the following are physical or

chemical changes. Give one reason for your answer.

1) Formation of curd

2) Ripening of fruit

3) Bending a glass tube by heating

4) Melting of wax

5) Formation of wine

Answer:  1) Chemical change

New substance (curd) with a different composition is formed from the

milk. Milk cannot be reobtained. Hence the change is permanent.

2) Chemical change

The property of the substance has changed. The hard, sour fruit has

changed to a soft, sweet form. The change is permanent.

3) Physical change

The physical properties changed on heating, but there was no

change in mass.

4) Physical change

On heating the wax melts but on cooling it re-solidifies. So there is no

change in properties or composition. The change is temporary and

reversible.

5) A chemical change

The properties of the substance changed and new substance like

alcohol is produced. The change is permanent and irreversible.

Question (11): Action of heat on blue vitriol is a physical as well as

chemical change. Justify.

Answer:  When blue vitriol is a gradually heated, it undergoes a

physical change to form a white powder. On adding a drop of water to

the white powder, it changes back to blue. Thus the change is a

physical change. On strongly heating to about 800oC, copper

sulphate decomposes to give new substances like copper oxide and

sulphur dioxide. On cooling these, copper sulphate cannot be re-

obtained. Thus it is a chemical change.

Question (12): Is boiling of water, a chemical change? Give reasons

for your answer.

Answer:  Boiling of water is not a chemical change it is a physical

change. When water boils, it absorbs heat energy and changes to

stream. But this change is temporary and reversible. The steam can

change back to water by giving out the same amount of heat. Also

the mass of water boiling is equal to the mass of steam that is

formed. Accordingly, it is a physical change.

Question (13): A person eats chocolate and then digests it. In doing

so a physical and a chemical change takes place. Identify the

changes and give reasons for your answer.

Answer:  When the person chews the chocolate and breaks it into

smaller pieces - it is a physical change. No new substances have

been formed as yet. Once acted upon by saliva and other digestive

juices, the chocolate is broken down into other simpler substances,

which can be absorbed by the blood. This is a chemical change.

Question (14): When we mix iron filings with sulphur and grind it, a

physical change takes place. However if we heat the mixture, a

chemical change takes place. Explain why the former is said to be a

physical change while the latter process is said to be a chemical

change.

Answer:  Mixing iron filings with sulphur is considered to be a

physical change because the mixture can be easily separated and no

new substance is formed. Moreover, no energy changes occur while

mixing iron and sulphur. When the mixture is heated, the two

substances combine to form a new substance known as ferrous

sulphide. The original substance, iron and sulphur, cannot be re-

obtained by cooling the ferrous sulphide. Consequently, it is a

permanent chemical change involving an energy change.

Question (15): State whether the following statements are true or

false. Correct the false statements.

1) Chemical changes are generally accompanied by an exchange of

energy.

2) A physical change is usually reversible.

3) When a magnesium ribbon burns brilliantly in air a physical change

takes place.

4) Ammonium chloride dissolves in water with the absorption of heat.

This is an exothermic reaction.

Answer:  1) True

2) True

3) False Correct statement: When a magnesium ribbon burns

brilliantly in air a chemical change takes place.

4) False Correct statement: Ammonium chloride dissolves in water

with the absorption of heat. This is an endothermic reaction.

Question (16): Identify physical or chemical changes in the following:

1) A rock rolls down a slope

2) Baking of cake

3) Plucking of fruit

4) Burning of L. P. G

5) Cutting of carrots

Answer:  1) Physical change: No change in mass.

2) Chemical change: Properties of the original substance change.

3) Physical change: No change in mass. Properties of the substance

do not change.

4) Chemical change: New substance like carbon dioxide is formed.

Energy change is involved (heat is evolved).

5) Physical change: No change in mass before and after cutting the

carrots.

Question (17): Explain whether the addition of dilute sulphuric acid to

iron is a physical or chemical change.

Answer:  It is a chemical change for the following reasons:

1) Iron reacts with dilute H2SO4 to form ferrous sulphate (a pale green

solution) and liberates bubbles of hydrogen gas. New products are

formed and the change is permanent.

2) Heat energy is evolved in the reaction i.e., it is an exothermic

change.

Question (18): Heating of iodine crystals is considered to be a

physical change. Explain.

Answer:  When iodine crystals are heated, the solid changes to the

vapour state i.e., it sublimes. This is identified as a physical change

since:

1) The vapours formed settle on the cooler parts of the test tube as

solid - so the change is temporary and reversible.

2) There is no change in mass.

3) There is no energy (heat) change involved because the heat

absorbed when the solid iodine changes to vapour, is given out when

the vapour changes back to solid.

4) The chemical composition of the solid is same as that of the

vapour.

Question (19): Burning of a candle may be considered as showing

physical and chemical changes occurring simultaneously. Explain.

Answer:  When the candle burns, the heated wax melts. The melted

wax re-solidifies and no new substance is formed, therefore this is a

physical change. However, the wax and the wick continue to burn

and produce new substances like carbon dioxide and water vapour.

This burning process is a chemical change as it is a permanent

change. Also, energy in the form of heat and light is evolved.

Question (20): Given the following equation:

Zinc + Hydrobromic acid Zinc bromide + Hydrogen

1) Write the equation using symbols and formulae.

2) Name a solid reactant in the reaction.

3) Name a gaseous product formed in the reaction.

Answer: 

1) 2HBr ZnBr2 +

2) Zinc

3) Hydrogen

Question (21): What information is conveyed by a chemical

equation?

Answer:  A chemical equation tells us:

1) The nature of reactants and the products of the chemical change.

2) Number of molecules of each substance taking part in the change

as well as the number of molecules produced after the chemical

change.

3) The quantities of the reactants taking part in the reaction and also

of the products formed as a result of the reaction.

4) The actual result of the chemical change.

Question (22): What information does the following equation

convey?

Answer:  The above equation tells us that,

1) One molecule of calcium carbonate reacts with two molecules of

hydrochloric acid to produce one molecule each of calcium chloride,

water and carbon dioxide.

2) Adding individual atomic weights (in grams), the total mass of a

compound can be calculated. Thus, 100 grams of calcium carbonate

reacts with 73 g of hydrochloric acid to produce 111g of calcium

chloride, 18g of water and 44g of carbon dioxide.

Question (23): Balance the following equations after writing them as

molecular equations.

1) Sodium + Nitrogen Sodium nitride.

2) Aluminium + Chlorine Aluminium chloride

3) Iron (III) oxide + Hydrogen Iron + Water

4) Phosphorous + Oxygen Phosphorous pentoxide

5) Trilead tetra oxide Lead monoxide + Oxygen.

Answer: 

Question (24): What do you understand by

1) thermal decomposition

2) electrolytic decomposition?

Answer:  1) Thermal decomposition is the breaking down of

substances into simpler substances by the action of heat.

2) Electrolytic decomposition is the breaking down of substances into

simpler substances by electricity.

Question (25):

What type of reaction is this? Why is it called so?

Answer:  This is a neutralisation reaction. The hydrogen ion (H+) of

the acid reacts with the hydroxyl ion (OH-) of the alkali to form water.

Neutralisation of acid with base takes place.

Question (26): Give the balanced equation for reactions that take

place when the following are heated:

1) Silver oxide

2) Potassium nitrate

3) Lead nitrate

4) Trilead tetraoxide

5) Copper carbonate.

Answer: 

1)

2)

3)

4)

5)

Question (27): When an iron knife is dipped in a solution of copper

sulphate, a reddish brown layer gets formed on the knife. Explain.

Answer:  Iron is more reactive metal than copper and displaces it

from its solution of copper sulphate. The reddish brown layer seen on

the knife is the deposit of displaced copper that is formed.

Question (28): When hydrochloric acid is converted to chlorine, is

the acid getting oxidised? Explain.

Answer:  Hydrochloric acid is converted to chlorine by removal of

hydrogen from it. The removal of hydrogen from a compound is called

oxidation. Hence the acid is oxidised.

Question (29): Ammonium nitrate when heated disappears

completely. Why?

Answer:  Ammonium nitrate on heating decomposes into colourless

gaseous products namely, water vapour and nitrous oxide.

Both products being gases, escape into the surroundings, leaving

behind no visible products. Thus it seems that ammonium nitrate has

disappeared.

Question (30): Chemical reactions are often accompanied by heat

changes. Explain.

Answer:  Chemical reactions involve breaking of old bonds and

forming of new ones. The breaking of bonds requires energy.

Depending on whether the formation of new bonds release more or

less energy than that involved in breaking old ones, energy is either

released or absorbed during a chemical reaction.

Question (31): State what type of reaction the following are:

1) Splitting of water into hydrogen and oxygen.

2) Burning of hydrogen in air.

3) Action of iron with copper sulphate solution.

4) Action of heat on calcium carbonate.

5) Treating silver nitrate with hydrochloric acid. Give equations with

your answer.

Answer:  1) Electrolytic decomposition reaction

2) Combination or Synthesis reaction

3) Displacement reaction

4) Thermal decomposition reaction

5) Double displacement reaction

Question (32): When ice is exposed to air, it melts to form water and

when hydrogen in burnt in air, it forms water. How do these changes

differ?

Answer:  Ice melting to form water is a physical change. No new

substance is formed in this process and the mass of the melting ice is

equal to the mass of water formed. However, burning hydrogen in air

is a chemical change as a new substance i.e., water is formed. This

change is a permanent change.

Question (33): Blue Copper Nitrate crystals turns black when heated.

Explain

Answer:  Copper nitrate crystals decompose into copper oxide,

nitrogen dioxide and oxygen on heating. The latter two are gases and

escape into the surroundings. The residue left is of copper oxide,

which is black in colour.

Question (34): When chlorine is passed through potassium iodide

solution containing chloroform, purple colour is observed. Explain.

Answer:  Chlorine displaces iodine from the potassium iodide

solution. The displaced iodine dissolves in chloroform giving it a

purple colouration.

Question (35): When silver nitrate is added to seawater, a white

precipitate is formed. Explain.

Answer:  Sea water contains dissolved chlorides of calcium and

magnesium. These react with silver nitrate to form silver chloride by

double displacement. Silver chloride is insoluble in water and thus

appears as a white precipitate.

Question (36): Classify the following reactions into various types.

1) Treating dilute sulphuric acid with sodium hydroxide solution.

2) Heating ammonium chloride.

3) Changing ammonium chloride into sodium and chlorine.

4) Heating a mixture of iron and sulphur.

5) Passing chlorine through a solution of potassium bromide.

Answer:  1) Neutralisation

2) Thermal dissociation

3) Electrolytic decomposition

4) Synthesis

5) Displacement.

Question (37): Define a double decomposition reaction.

Answer:  Double decomposition is a reaction in which the positive

and negative radicals of two substances in solution are mutually

interchanged.

Question (38): What type of change takes place in the following,

physical or chemical? Briefly explain.

1) Heating of zinc oxides.

2) Action of sodium with water.

3) Rain cycle.

4) Heating calcium carbonate.

5) Action of dilute acids on magnesium.

Answer:  1) Physical change occurs. There is only a colour change.

No new substance is formed

2) Chemical change takes place. New substances are formed.

3) Physical change occurs. Change of the state of water occurs.

4) Chemical change occurs and new substances are formed.

5) Chemical change occurs and new substances are formed.

Question (39): Name the following and give equations for these

reactions:

1) A white solid, which when heated, leaves behind a yellow solid and

gives CO2.

2) A bluish green powder, on heating, leaves behind a black solid and

gives CO2.

3) A colourless solid produces a yellow solid when hot and a white

solid when cold and gives a reddish brown gas. Give equations for

each.

Answer:  1) Lead carbonate

2) Copper carbonate

3) Zinc nitrate

Question (40): Reaction of carbon monoxide with heated copper

oxide is a reduction reaction. Explain.

Answer:  Carbon monoxide removes the oxygen from copper oxide

leaving behind metallic copper. Removal of oxygen is called

reduction.

Question (41): What do you observe when concentrated sulphuric

acid is added to sugar?

Answer:  The sugar changes into a black spongy mass of carbon,

which swells up. The sulphuric acid acts as a dehydrating agent

removing all the water molecules from sugar.

Question (42): What do you observe when zinc oxide is heated and

then cooled?

Answer:  When white zinc oxide is heated, it turns yellow. On cooling

it turns back to white. This is a physical change.

Question (43): What do you observe when trilead tetraoxide is

heated?

Answer:  On heating, the red powder of trilead tetraoxide

decomposes and produces a colourless gas. As this gas can rekindle

a glowing splinter, it is oxygen. The residue left behind is orange-

brown when hot but changes to yellow on cooling and fuses with the

glass.

Question (44): What happens when ammonium carbonate is kept

open?

Answer:  Ammonium carbonate on exposure to air decomposes to

give ammonia, water and carbon dioxide.

Question (45): Why is the reaction between steam and red hot iron

said to be a reversible reaction?

Answer:  If steam is passed over red hot iron, it is converted into its

magnetic oxide and hydrogen gas. If on the other hand, hydrogen is

passed over heated magnetic oxide, it will change back to iron and

steam. This reaction can be made to proceed in both directions, so it

is called reversible reaction.

Question (46): Give an example of a redox reaction involving a

metallic oxide and a neutral gas as the only reactants.

Answer: 

Here H2 reduces CuO to Cu and itself gets oxidised to water. Hence it

is a redox reaction.

Question (47): Classify the following reactions as exothermic and

endothermic.

Answer: 

1) Exothermic

2) Exothermic

3) Endothermic

4) Endothermic

5) Endothermic.

Question (48): State the effect of:

1) An endothermic reaction

2) An exothermic reaction on the surroundings.

Answer:  1) When an endothermic reaction takes place, the

surroundings cool down as the reactants absorb heat.

2) When an exothermic reaction occurs, the temperature of the

surroundings increase as heat is released into the surrounding.

Question (49): Predict the products for the following reactions and

balance.

Answer: 

Question (50): Complete the following double displacement

reactions.

Answer: 

Question (51): Classify the following reactions as thermal

decomposition and thermal dissociation.

Answer: 

1) Thermal decomposition

2) Thermal decomposition

3) Thermal decomposition

4) Thermal dissociation

Question (52): Define a reversible reaction.

Answer:  A reversible reaction is one in which the products formed

after the reaction, react together once again to form the original

reactants. The conditions for the reverse reaction may or may not be

the same as the conditions for the forward reaction.

Question (53): Predict the type of each reaction.

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Answer:  1) Synthesis

2) Decomposition

3) Displacement

4) Combination

5) Double decomposition.

Question (54): How can nitrogen stop food items containing high fat

and oil from turning rancid?

Answer:  Some high fat foods such as potato chips are sealed in

packages that are filled with nitrogen. Nitrogen displaces oxygen gas

in the packet and prevents the deterioration in fats and fatty foods

because of oxidation process, which is the main cause of rancidity.

Question (55): Explains what happens when iron metal is left

exposed in moist weather.

Answer:  Iron easily reacts with substances like moisture and air

when left for some time in their presence. In the presence of air and

water or moisture exposed iron gets oxidized and gets coated with a

reddish brown powder. The oxide formed does not firmly adhere to

the surface of the metal causing it to flake off easily. This process is

called corrosion or rusting. Corrosion causes deterioration of

essential properties in a material.

Question (1): Which among the following is not a physical change? 

1. Melting of solids to liquids

2. Vaporisation of liquids to gases

3. Liquefaction of gases to liquids

4. Decay of matter

Ans:  4

Question (2): Which among the following is not a chemical change? 

1. Melting of ice

2. Carbon cycle

3. Dehydration of substances

4. Fermentation of substances

Ans:  1

Question (3): Physical changes are _______. 

1. temporary

2. permanent

3. irreversible

4. endothermic

Ans:  1

Question (4): An example of a chemical change is _____. 

1. formation of clouds

2. glowing of an electric light

3. dropping sodium into water

4. dissolving of salt in water

Ans:  3

Question (5): Which of these will cause a chemical change to occur? 

1. Grinding of wheat into flour

2. Lighting of a gas stove

3. Evaporation of water from a lake

4. Ringing of an electric bell

Ans:  2

Question (6): Chemical changes are __________. 

1. temporary, reversible and a new substance is produced

2. always accompanied by exchange of light

3. permanent, irreversible and a new substance is produced

4. never accompanied by exchange of light and heat energy

Ans:  3

Question (7): Which of the following is a physical change? 

1. Solubility in water

2. Combustibility

3. Aerial oxidation

4. Reaction with water

Ans:  1

Question (8): Which of the following information is conveyed by a chemical reaction? 

1. The colour changes taking place

2. The structure of the reactants and products

3. The absorption of energy only

4. The masses of the reactants and products involved in the reaction

Ans:  4

Question (9): Which is the correct symbol for manganese? 

1. M

2. Ma

3. Mn

4. Mg

Ans:  3

Question (10): The symbol H stands for ________of hydrogen. 

1. one atom

2. one molecule

3. one ion

4. two atoms

Ans:  1

Question (11): The correct formula for nitrogen dioxide is __________. 

1. NO

2. N2O

3. NO2

4. N2O5

Ans:  3

Question (12): The correct formula for ammonium sulphate is __________. 

1. NH4SO4

2. (NH4)2SO4

3. (NH3)2SO4

4. (NH4)2(SO4)2

Ans:  2

Question (13): Which of the following is an incorrect formula? 

1. NaCl2

2. BaSO4

3. H2CO3

4. P2O5

Ans:  1

Question (14): In one molecule of ammonium sulphide there are ___________. 

1. 2 atoms of N, 8 atoms of H, and 1 atom of S

2. 1 atom of N, 4 atoms of H, and 1 atom of S

3. 1 atom of N, 4 atoms of H, and 2 atoms of S

4. 2 atoms of N, 8 atoms of H, and 2 atoms of S

Ans:  1

Question (15): The correctly balanced equation for is _________. 

1.

2.

3.

4.

Ans:  4

Question (16): The sign used to indicate a reversible reaction is _______. 

1.

2.

3.

4.

Ans:  4

Question (17): Breaking of lead bromide into lead and bromine is an example of __________. 

1. decomposition reaction

2. synthesis reaction

3. displacement reaction

4. neutralisation reaction

Ans:  1

Question (18): In the equation

, the substance undergoing oxidation is ________. 

1. lead dioxide

2. hydrochloric acid

3. hydrogen

4. lead chloride

Ans:  2

Question (19): is an example of ________. 

1. neutralization reaction

2. redox reaction

3. double displacement reaction

4. decomposition reaction

Ans:  3

Question (20): In the reaction:

, the white precipitate seen is due to ______. 

1. ZnCl2

2. BaSO4

3. BaCl2

4. ZnSO4

Ans:  2

Question (21): A white solid which is yellow when hot but changes to white again on cooling is _____. 

1. PbO

2. CaO

3. Ag2O

4. ZnO

Ans:  4

Question (22): From the given list, the nitrate which gives metal, NO2 and O2 on heating is _______. 

1. NH4NO3

2. NaNO3

3. AgNO3

4. Cu(NO3)2

Ans:  3

Question (23): The products of a burning candle are ________. 

1. ash and water vapour

2. CO2 and water vapour

3. wax and water vapour

4. only melted wax

Ans:  2

Question (24): Which of these metals do not corrode? 

1. Lead

2. Copper

3. Platinum

4. Silver

Ans:  3

Question (25): The formula for rust is________. 

1. CuO

2. Fe2O3.xH2O

3. Al2O3

4. AgS

Ans:  2

Question (26): The main cause of rancidity in foods is 

1. Bacteria

2. Proteins

3. Antioxidants

4. Oxidation of the fatty acid molecule

Ans: 4