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CONTENTS

1.      The language of Chemistry

2.      Acids, Bases and Salts

3.      Changes and Reactions

4.      Fibers from Animals

 

 

 

BASIC SCIENCE CLASS VII

1. CH- The language of chemistry

Q.1:- What does the formula of a substance tell you?

Ans: The formulae of an element or compound represent the no of atoms and their

types present in one molecule of the element or compound.

Q.2: What do you mean by the valency of an element? www.rsmaths99.com

Ans: The capacity of an element to combine with other elements is known as its valency.

Q3:- What is the following called? A group of atoms of two or more elements behaving like an atom and having a valency.

Ans: Compound Radical is a group of atoms of two or more elements behaving like an atom and having a valency, in chemical reaction. www.rsmaths99.com

Q.4:- Name two elements that have variable valency. Give the formulae and name of two compounds formed by each of these elements.

Ans:- Two elements having variable valency and the compound formed by them are as

follows:

i)                    Iron (Fe)

Ferrous oxide ( iron ii oxide) FeO

Ferric oxide ( iron iii oxide) Fe2 O3

ii)                  Copper (Cu)

Copper (i) chloride CuCl

Copper (ii) chloride CuCl2 OR

iii)                Copper (Cu)

Coprous oxide (copper (i) oxide) Cu2O

Cupric oxide ( copper(ii) oxide) CuO www.rsmaths99.com

Q.5:- What is a balanced chemical equation? Why should a chemical equation be

balanced?

Ans:- The no. of atoms of each element on the reactant side must be the equal to

the no. of the atoms on the product side in a chemical reaction.

(Remember: No. of Atoms LHS = No. of Atoms RHS).

We have to satisfies the rule that no atoms is lost during a chemical reaction so the

chemical equation be balanced.

Eg. N2 + H2 ----- NH3 ( unbalanced)

N2 + 3H2 ----- 2NH3 ( balanced)

B. Find the valencies of the underlined elements or radicals in the following compounds.

Valency

BaCl2 2

PCl3 3

PCl5 5

NH3 3

MgSO4 2 other answers- 2, 1, 2, 2, 2, 3, 3, 1, 3, 1.

C

Q1. What are compound radicals? Give example of a few redicals, along with their

valencies. Also mention some compounds containing compound redicals. Give the valencies of the parts that make up the compound.

Ans: A group of atom of two or more elements, which behaves like a single atom during a chemical reaction and has a valency, is known as a compound redical.

Compound redical Valency

OH 1

NO3 1

SO4 2

CO3 2

Some compound containing comp. redicals:- www.rsmaths99.com

Compounds name compound redicals valency

NaOH Sodium hydroxide OH 1

HNO3 Nitric acid NO3 1

KNO3 Potassium nitrate NO3 1

H2 SO4 Sulphuric acid SO4 2

Na2 CO3 Sodium carbonate CO3 2

(NH4 )2 SO4 Ammonium sulphate SO4 2

D. 1. symbol 2. Formulae 3. Interchanged 4. Gained

E. 1 (c) Au 2. ( c) Suiphate 3. (b) 2 4. ( b) 3

F. Match the columns A and B

Ans: Helium He, Mercury ----Hg, Copper ----Cu, Calcium ---Ca, Phosphorus ---P,

Lead--- Pb.

G. True or False? www.rsmaths99.com

Ans 1.---T, 2---T, 3---F, 4--- T, 5 ---F, 6 ---- T.

H. Write the formula of each of the following compounds

Water ---H2O, Calcium oxide --- CaO, Carbon dioxide ---Co2, Carbon monoxide ---CO, Magnesium oxide ---MgO,

Hydrogen chloride ---HCl, Sodium chloride---NaCl, Potassium chloride --- KCl , Magnesium chloride ---

MgCl2, Zinc chloride---ZnCl2, Nitric acid --- HNO3, Sodium nitrate ---- NaNO3, Sodium carbonate --- Na2CO3, Potasium carbonate ---K2CO3,

Calcium carbonate --- CaCO3.

NOTES:-

Elements : A chemical element is a pure chemical substance consisting of a single type of atom distinguished by its atomic no. which is the no. of protons in its atomic nucleus. Elements may be metal, metalloids, or non- metals.

Eg.:- elements- carbon, nitrogen, oxygen, silicon, aluminum, iron, copper, gold mercury, lead.etc

Atom: - The smallest particle of an element which can take part in chemical reaction. Atoms usually do not exist independently.

An atom has a nucleus which contains proton (+ charge) and neutron (no charge). Electrons (e- charge) moves around the nucleus in different shells.

Molecule: - Atoms combine together to form molecules. www.rsmaths99.com

Eg. : O2 -- oxygen molecule it has 2 atoms. H2 O- it consist of 2 hydrogen atoms and 1 oxygen atom.

Compound: - A compound is a pure substance (not mixture) formed by the combination of two or more elements...

Eg. :- NaCl ( sodium chloride, where Na & Cl is an element).

Contents of our universe is matter and energy ( radiation ) . Matter can be divided into two parts mixture & pure substance. Pure substance can be divided into element & compound.

Radicals :- Look at that SO4 and NO3 are group of elements that seem to be combining with various other elements. It cannot exist independently. Eg. Compound sodium

sulphate ( Na2 SO4 ), sodium nitrate (NaNO3 )

Such group of atoms of different elements which combines as single units has a

valency, but cannot exist independently are known as radicals. Some common

radicals :- carbonates (CO3 ), Phosphate (PO4 ), choloride (Cl)

When symbol of Helium is He than formulae is He. Symbol is Cl than formulae is Cl2

similarly P/P4, S/ S8

Atomic no.

Element

Symbol

Orbit / Shell

 

Valency

 

 

 

K

L

M

N

O

 

 

1

Hydrogen

H

1

 

 

 

 

 

1

2

Helium

He

2

 

 

 

 

Stable gas

0

6

Carbon

C

2

4

 

 

 

Up to 4 in outer most orbit

4

7

Nitrogen

N

2

5

 

 

 

8 5 = 3

3

10

Neon

Ne

2

8

 

 

 

Stable gas

0

8

Oxygen

O

2

6

 

 

 

8 -6 =2

2

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2. Acids, Bases and Salts

A.     Answer in 20 words www.rsmaths99.com

Q1. Classify the following substances into acidic and base substances.

Tomato juice ----- Acidic

Soap solution -----Base

Toothpaste -------Base

Lemon juice ----- Acidic

Vinegar -----Acidic

Q.2. Name three mineral acids and give their formulae.

Ans.:- Sulphuric acid ( H2 SO4 ), Nitric acid (NO3 ), hydrolic acid (HCl ),

Phosphoric acid (H3PO4 )

Q.3. Define acids.

Ans.:- Acids are substance that sour in taste turn blue litmus red and corrode metals, soluble in water they react with base and neutralize it.

Q.4. Define Bases:-

Ans. :- bases are substances which taste bitter and are soapy to touch , turn red litmus

blue and neutralize acids and form salt and water on reaction with water.

Q.5.:- What are soluble bases called? Give two examples.

Ans.:- Soluble bases are called alkalis. Two eg. Sodium hydroxide ( NaOH), Potassium

hydroxide ( KOH ).

Q.5. Define neutral substances.

Ans. :- A substance that has neither acidic nor alkaline character is called neutral

substance.

A. Name an acidic gas which is discharged into the atmosphere on the burning of fuels like coal and natural gas. How is this gas formed?

Ans.:- On the burning the fuels like coal and natural gas in the presence of oxygen in air

form acid oxides like carbon dioxides (CO2 ), Nitrogen oxide (NO), and nitrogen dioxide(2NO2 ). They are formed by this reaction:

C(coal) + O2 CO2 ( Burns write on the arrow)

CH4 + 2O2 CO2 + 2H2 O www.rsmaths99.com

(natural gas)

S + O2 - SO2 ( sulphur dioxide )

 

2S2 + O2 - 2SO3 ( sulphur trioxide)

 

N2 + O2 - 2NO (nitric oxide)

2NO + O2 - 2NO2 (nitrogen dioxide)

 

Q2. What are the general properties of basic substances?

Ans. Basic substances have the following general properties:-

1.      They generally test bitter.

2.      They turn red litmus blue.

3.      They neutralize acids. www.rsmaths99.com

4.      Caustic alkalis corrode glass and some metals like aluminium, Zinc, tin and lead.

C Answer in 100 words

Q1. Write the properties of an acid.?

Ans. Properties of an acid :-

1.The have a sour taste.

2. They turn blue litmus red.

3. Acids are corrosive in nature. They damaged the surface of the metals because like sour food react with the metal container and form metal salts and hydrogen gas.

Zn + H2 SO4 ----- ZnSO4 + H2 www.rsmaths99.com

5.      Acid react with carbonates and bicarbonates to liberate (release) CO2 with effervescence.

Na2 CO3 + 2HCl ----- 2NaCl + H2 O + CO2

6.      Acid react with bases and get neutralized It and form salt and water.

7.      Acids are produced when oxides of non-metals react with water.

Q.2. Describe an activity to show the effect of an acid on carbonates and

hydrogencarbonates/ bicarbonates.

Ans.: Take some baking soda (sodium bicarbonate) and put one or two drops of lemon juice (citric acid) or vinegar( acetic acid) on it.If test liquid is acidic, CO2 will evolve with effervescence i.e. gas bubbles will be produced with a hissing sound.

Sodium hydrogen carbonate + citric acid (lemon juice) --- sodium citrate + water + carbon dioxide.

Q.3. What is acid rain ? How is it formed? Mention three bad effect of acid rain.

Ans. The acidic gases sulphur dioxide (SO2) and oxygen present in atmosphere this mixture becomes sulphuric acid and it is soluble in water. They are bought down by rain. Rain with a high acid content is called acid rain.

We can show the formation through these equation:-

S + O2 - SO2 ( sulphur dioxide ) www.rsmaths99.com

 

2S2 + O2 - 2SO3 ( sulphur trioxide)

Q4. Write a note on the uses of bases.

Ans. Bases are used for various purposes.

i] They are used for netutralising acids and acidic substances.

ii] Amonia is used for manufacturing nitrogenous fertilizers like urea, ammonium sulphate and ammonium nitrate.

iii] Caustic alkalis (NaOH and KOH) are used in making soap from oils and fats.

iv] Lime[Ca(OH)2], is used as whitewash.

v] Coloured oxides are used for making coloured glasses.

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3. CHANGES AND REACTIONS

A.

Q1. Define a physical change. www.rsmaths99.com

Ans. A change in which appearance can change but its chemical composition remains

same. No new substances are formed and which can generally be reversed by

reversing the conditions is called a physical changes.

Q2. Define a chemical change.

Ans. A change in which composition of a substance is changed. In which new

substances are formed and which cannot be reversed by reversing the conditions is called a chemical change.

Q3. Classify the following into physical and chemical changes.

a] The cooking of food - Chemical changes

b] The evaporation of a liquid. Physical change www.rsmaths99.com

c] The digestion of food - Chemical change

d] Burning - Chemical changes

e] The condensation of water vapour - Physical change

f] Rusting of iron - Chemical changes

Q4. What are the following called?

Ans. a] Combination reaction

b] Decomposition reaction

c] Displacement reaction

d] Neutralisation reaction

B

Q1. How would you decide whether a change is physical or chemical? Mention two

characteristic of each kind of change.

Ans. In physical changes no new substances are formed where as in chemical changes new substances are formed. Two characteristics are given below:-

Physical changes: i] No new substances are formed only physical state changes.

ii] Changes are reversible.

Chemical changes: i] new substances are formed. ii] Changes are not reversible.

Q2. Discuss why the dissolving of salt in water is a physical change.

Ans. We can get the salt again from the solution by evaporating the water. Thus we can say the dissolving of salt in water is a physical change. www.rsmaths99.com

Q3. Discuss an example of physical and chemical changes happening together.

Ans. The burning of a candle is an example of both physical and chemical changes.

The melting of wax and the solidification of molten wax are physical changes. The

burning of molten wax is a chemical change.

C.

Q1. Giving one example of each kind, show that a change in energy takes place when a

physical or a chemical change occurs. www.rsmaths99.com

Ans. An example of physical changes:- When an ice is converted into water, heat is

absorbed by the ice and vice- versa when water converted into ice heat is released by the water.

An example of chemical changes:- Cooking of food is a chemical change. In this process a lot of heat energy is absorbed by the food.

Q2. Describe an activity to find the conditions required for rusting.

Ans. Activity: Take two test tube, some unrusted nails , a cork and stand.

1.      Take a few unrusted iron nails in a test tube A, and cover them with

ordinary water. Leave the test tube on a stand. www.rsmaths99.com

Boil some distilled water in test tube B in order to remove the dissolved air. Drop a few unrusted nails into it. Put some oi over the water surface to cut it off from the air above. Cork the test tube and allow it to stand.

Observation: After a few days rusting occurs only in the nails placed in test tube A, not in B.

Conclusion: i] Only moist air causes rusting.

ii] Air- free water can not cause rusting. Presence of both water and oxygen are required to form rust on iron.

 

4. FIBRES FROM ANIMALS

A.

Q1. Name two animal fibres suitable for making fabrics. www.rsmaths99.com

Ans. silk and wool are animal fibers that are suitable for making fibers.

Q2. What is a larva? Give n example from the sources of animal fibres.

Ans. A lsrva refers to a phase of an animals lifr cycle that occurs before metamorphosis.

Eg.- a silkworm emerging from the egg laid by the female silk moth id in its leave stage.

Q3. What is cocoon? Of what is it formed? www.rsmaths99.com

Ans. A cocone is a soft covering of silk threads that is spun by on an insect to protect itself.

Q4. What do you mean by the throwing of silk?

Ans. throwing of silk (or silk throwing ) is the process of preparing thrown silk by twisting the raw silk.

Q5. What is sheep shearing? How many times is a sheep generally shorn in India?

Ans. sheep shearing is the process of cutting off the fleece from the body of sheep to

obtain wool. In India sheep are shorn twice a year.

Q6. Which is the best season for the mating of sheep? www.rsmaths99.com

Ans. The spring is the best mating season for the sheep.

Q7. What are the washed wool. Clipped wool and pulled wool?

Ans. Washed wol is obtained from sheep that are washed thoroughly to remove dirt from their wool. Clipped wool is the wool obtained from live sheep. Pulled wool is the wool obtained from dead sheep.

B

Q1. Discuss moulting in silkworms. www.rsmaths99.com

Ans. Moulting is the process of growth and maturation of a larva such as silkworm during moulting, a silkworm sleeps four time while in its feeding and sleeping periods, its skin cracks. When the larva awakes, it leaves the old skin and form a new skin.

Q2. What are raw silk and spun silk?

Ans. Raw- silk: it is stronger silk that is obtained from the filaments of cocoons. These

filaments are reeled together to yield raw silk.

Spun silk: it is an inferior quality of silk that is obtained from damaged cocoons.

Q3. Why is the crossbreeding of sheep necessary in India? Name three exotic breeds with which crossbreeding is done in our country.

Ans. cross breeding of sheep is necessary in India to improve the quality and quantity of wool. The native sheep are cross- bred with the exotic breeds to yield better wool.

Three exotic breeds used for cross- breeding in India are Merino, Ram and Dorset.

C

Q1. Discuss the chemical properties of wool. www.rsmaths99.com

Ans. following are the chemical properties of wool;

1. It is affected by heat and changes its colour at 100oC but does not catch fire

easily. It turns yellowish if left in hot and humid coalition for a long time.

2.      It gets dissolved in acids and bases.

3.      Its fibers can be bleached without any loss of strength.

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