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.
Be an International Online Tutor
Top of Form
Name
Subject
Phone
Get tutoring in Math, Algebra,
Trigonometry, Geometry, English,
Physics, Chemistry and Biology
Top Clicked
tissues gravitation Valence Shell
isotopes science
questions types of
Respiration photosynthesis onlin
e math tutor
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.
<>
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