CLASS. VIII

SUBJECT: CHEMISTRY

CHAPTER: 3. ELEMENTS ,COMPOUNDS AND MIXTURES

All the materials are made of matter which can neither be created nordestroyed.The matter is classified into pure substances and mixtures.Apure substance is composed of same kind of matter and it is classifiedinto elements and compounds.

ATOMS

An atom is the smallest particle of an element that exhibits allthe properties of an element.

CHARACTERISTICS:

1.atoms of most elements are very reactive and do not exist asfree atoms.

2. Atoms of one element are alike and they do not differ inproperties such as shape, size and mass.

3. An atom maintains its properties through all physical andchemical changes.

MORE TO KNOW ABOUT ATOMS:

Atoms are made up of negatively charged particles calledelectrons, positively charged particles called protons and neutralcharged particle called neutrons.protons and neutrons lie sidethe nucleus of an atom and electrons keep revolving in the shellaround the nucleus

https://youtu.be/R1RMV5qhwyE

MOLECULES:

Molecules are the smallest particles of pure substances that canexist independently and are made up of two or more atomscombined chemically in fixed proportion by weight and exhibit allthe properties of that pure substances.

HOMOATOMIC MOLECULES:

Homoatomic molecules are formed by atoms belonging to thesame element.

Examples:H2,O2,N2

HETEROATOMIC MOLECULHyder atomic molecules are formedby atoms belonging to different elements.

Examples:H2O, NH3

ELEMENTS:substance made up of same kind of atoms is calledan element. Atoms cannot be broken down into simplersubstances.

so far about 120 elements are known out of which 92 are naturallyoccurring in the earth crust .The remaining elements have beenmade by scientist in laboratories

CHARACTERISTICS OF ELEMENTS

1. An element is a pure substance

2. It is made up of only one kind of particles.

3. Elements may occur naturally or may be prepared ar tificially.

4. Element exist in all the three physical States.

CLASSIFICATION OF ELEMENTS

Noble or inert gases:

These are those gaseous element that do not react chemicallywith other elements and are stable.

SYMBOLS OF ELEMENTS:

The abbreviation of the name of an element to its initial letter orletters is called symbol of an element.

Chemical symbols are form of shorthand .the symbol of anelement consists of one or two letters of the elements name thesymbols are used all over the world and easier to write instead oftheir whole names.

SIGNIFICANCE OF SYMBOLS:

1. The name of the element

2. One atom of the element

https://youtu.be/DvL3DoyPR8E

ATOMICITY

Atomicity is defined as the number of atoms present in onemolecule of an element.

https://youtu.be/eyzVBiapXtM

VALENCY

It is defined as the number of electrons donated or accepted by anatom when it forms a chemical compound.

The atom that donates electron is said to have electropositivevalency.

The atom that accepts electron has electronegative valency.

The element with valency 1 are called monovalent elements.

The elements with valency 2 are called divalent elements.

The element with valency 3 are called trivalent elements.

The element with valency 4 are called tetravalent element

https://youtu.be/lVSF2lP4oBA

COMPOUNDS

When atoms of two or more different elements combined chemicalin a fixed proportion, they form compounds.

CHARACTERISTICS OF COMPOUNDS

1. A compound is a pure substance.

2. A compound has uniform composition.

3.A compound has distinct properties as the original componentlose their individual properties.

4. Elements in a compound cannot be separated by physical methodbut can be separated by chemical method..

CHEMICAL FORMULA OF COMPOUNDS

The symbolic representation of a molecule ofcompound is called the chemical formula ormolecular formula of that compound.

SIGNIFICANCE OF FORMULA

1. It represents the name of the compound.

2. It represents the molecule of a compound.33. It gives the names of the atom of all the elementpresent in a molecule

4. It gives the number of atoms of each elementpresent in the moleculehttps://youtu.be/olNaHLbd8zg

MIXTURES

If two or more substances mixed together in any proportion do notundergo any chemical change but retain their own properties theresulting mass is called mixture.

https://youtu.be/4mLst4JqyhU

HOMOGENEOUS MIXTURE

It is defined as a mixture in which the composition is the samethroughout the mixture.ex:sugar solution,petrol etc

HETEROGENEOUS MIXTURES

Heterogeneous mixture is a mixture that consists of physicallydistinct parts each with different properties.ex:mixture of sandand salt,mixture of iron and sulphur.

SOME EXAMPLES OF MIXTURES

SOLID- SOLID MIXTURES: It contains two solids as components.

Ex:Brass(copper and zinc) homogeneous

Gun powdre(charcoal and sulphur) heterogeneous

SOLID -LIQUID MIXTURES: It contains one solid and another liquidas components.

Ex:sand and water(heterogeneous)

Sugar and water(homogeneous)

LIQUID- LIQUID MIXTURES: It contains two liquids as components.

EX:oil and water(homogeneous)

Mercury and water(heterogeneous)

SOLID- GAS MIXTURES: It contains a a solid mixed with a gaseoussubstance.

Ex:smoke(soot in air) heterogeneous.

Dust(fine sand in air) homogeneous.

LIQUID- GAS MIXTURES

It contains a liquid mixed with a gaseous substance.

Ex:ammonia and water (homogeneous)

GAS- GAS MIXTURE

It contains a gas mixed with another gaseous substance.

Ex:ammonia and hydrogen (homogeneous)

METHODS OF SEPARATION

EVAPORATION

https://youtu.be/YHb8NlrdWUk

CRYSTALLIZATION

https://youtu.be/pvilJbKVxmw

SUBLIMATION

https://youtu.be/lZTTzAkeSFQ

DISTILLATION

FRACTIONAL DISTILLATION

https://youtu.be/bkYqqJa5P8w

SEPERATING FUNNEL

https://youtu.be/L13QUwqKwlU

CHROMATOGRAPHY

SEPARATION OF CONSTITUENTS OF THE MIXTURE

1. TO SEPARATE THE CONSTITUENT FROM A MIXTURE OF SAND,SAWDUST AND COMMON SALT

sand can be separated out from the mixture by the sedimentationmethod. Then the remaining mixture is allowed to undergofiltration which separates saw dust from salt solution.Finally saltcan be separated from solution by evaporation of water from it.

2.TO SEPARATE THE CONSTITUENT OF MIXTURE OF IRONFILLING , SULPHUR AND COMMON SALT

First iron filings are removed from the mixture by the magneticseparation method.sulphur being insoluble in water can beseparated out by filtration method. Finally salt can be removedfrom the solution by evaporation of water from it.

https://youtu.be/sBCLafMPVt8

Worksheet 1

FILL IN THE BLANKS

1. Substance made up of just one kind of atom is called ________

2.__________is a metal in liquid form.

3.A solution of sugar or table salt in water is an exampleof__________ mixture.

4. Ozone is an example of ________molecule.

5.___________is the Latin name of gold.

PICK THE ODD ONE OUT GIVING SCIENTIFIC REASON

1. Carbon, chalk ,sodium, gold

2. Hydrogen ,mercury, nitrogen ,chlorine

3. Helium, neon, chlorine, argon

4. Distillation flask, funnel, thermometer, liebig condenser

5. Sand and water, oil and water,clay and water,salt and water.

ANSWER THE FOLLOWING QUESTIONS.

1.Name the smallest particle of substance that can exist freelyand retain the characteristic of that substance.

2. Name the method used to separate a mixture of naphthaleneand common salt.

3. Name the phases in the separating funnel.

4. Name the method used for separation of of insoluble solid byallowing the solid to settle.

5.Name the method used for separation of two solids, one ofwhich is magnetic in nature and other is non magnetic.

MATCH THE FOLLOWING

1. Fractional distillation.

2. Evaporation.

3. Chromatography.

4.Solder.

(a) adsorption

(b) colloid.

(C) ethanol and water

(d) common salt from sea water

(e)alloy5.Milk.

WORKSHEET 2

WRITE DOWN THE SYMBOL FOR EACH OF THESE ELEMENTS

1.Silver

2.Sodium

3. Aluminium.

4. Gold

WRITE DOWN THE MOLECULAR FORMULA FOR EACH OF THEFOLLOWING.

1. Chlorine

2. Calcium oxide

3. Lead sulphide

4. Hydrogen

WRITE DOWN THE NAME OF THE ELEMENTS OR COMPOUNDFOR EACH OF THESE SYMBOLS.

1.MgO.

2 .Sb.

3.CuS

4.He

CLASSIFY THE FOLLOWING AS PURE SUBSTANCES OR MIXTURES

1. Silver

2. Tap water.

3.sodium chloride

4.Milk.

A CLASSIFY THE FOLLOWING AS HOMOGENEOUS ORHETEROGENEOUS MIXTURE.

1.Brass

2.Air

3.Dust

4.Carbondioxide and Nitrogen

WRITE THE PROCESS USED IN EACH OF THE FOLLOWINGSEPARATION

1.Orange and blue eyes from black ink.

2.Water from salt solution.

3.Alchol from the mixture of alcohol and water.

ANSWER THE FOLLOWING QUESTIONS IN SHORTQUE 1.Could distillation be used to separate air intooxygen nitrogen carbon dioxide argon and so forth?explain.

Ans.No, because distillation is a process of convertingliquid into vapour by heating and then condensing thevapours. All the about substances are gases so already

boiled. However if they are cooled below their boilingpoint it could work.

Que 2.Which element are required to make sugar ?Also

give the chemical formula of sugar.Ans. Elements required to make sugar are carbohydrogenand oxygen.

The chemical formula of sugar isC12H22O11

Que3. "The properties of a compound are the same as theproperties of the elements that makes up the element ."Is thisstatement correct ?If not correct it.

Ans. The statement is not correct. The correct statement is,"Theproperties of a compound are different from its constituentelements that makes the compound."

Que4. What are the advantages of chromatography?

Ans. 1.A Very small amount of substance sufficient.

2.Quantitative estimation of a constituent is also possible.

Qué 5. Explain the method used to separate coconut oil and water?

Ans. Take the mixture of coconut oil and water in separatingfunnel .Allow it to stand for sometime. Two layers are observed.The upper layer is of coconut oil and the lower layer is of water.Open the stopcock and allow the lower layer of water to run off inthe beaker slowly. Then the upper layer of coconut oil is collectedin a separate beaker.

Que6. What do you understand by term 'atomicity '?Give theatomicity of sulphur.

Ans. Atomicity is defined as the number of atoms present in onemolecule of an element.

Atomicity of sulphur is eight.

Que 7. What is Liebig's condenser? write the function of Liebig'scondenser.

Ans. Condenser is a long glass tube surrounded by wider glasstube having an inlet and outlet for water.

The function of Liebig'scondenser is to convert the vapour intoliquid by cooling.

CHAPTER 1-MATTER

ANSWER THE FOLLOWING QUESTIONS IN SHORT

QUE 1.Explain the term interconversion of matter with an example.

Ans.Interconversion of matter is the change of matter from one state to anotherand back to its original state.

Example. Water can be turn into solid ice by cooling ,and into steam by heating.

Que 2. Why do solids have fixed shape but gases neither have a fixed shape nor afixed volume?

Ans.Because in solids molecules are closely packed ,the intermolecular forces arevery strong and there is no intermolecular space . whereas in gases the moleculesare loosely packed and have no intermolecular force between them.

Que3. Why do the three States of matter differ?

Ans.Due to difference in the arrangement of their atoms ,intermolecular force ofattraction and intermolecular space.

.

.Que 4. Complete the following table and thus compare theproperties of solid liquids and gases.

Ans

Sr no Physicalstate

Volume

definite

Definite

Shape Compressibi Force oflity attraction

1

2

3

Solid

Liquid

Gas

Definite Incompressi Strongble

Not definite Compressibl Weake to a smallextent

Not definite Not definite Highlycompressibl

No

e

Que5. Solid do not diffuse into one another .Explain on the basis ofkinetic theory of matter.

Ans. In solid intermolecular space is less and hence kinetic energyis very low. Therefore diffusion cannot occur in solids.

Que6. What happens to liquid ,if the intermolecular space in itdecrease ?How can you decrease the intermolecular space?

Ans. If the intermolecular space in it decreases liquid changes tosolid. Intermolecular space can be decreased by decreasing thetemperature. (Freezing).

Que 7 .Liquid as well as gases are called fluids .State two wayshow they differ from each other.

Ans.Liquids and gases are called fluids because they can be madeto flow,or move.

In liquid molecules are loosely packed whereas in gaseousmolecules are very loosely packed.

Liquid molecules have moderate force of attraction whereas forceof attraction in gases are negligible.