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Chapter: 7th Science : Term 1 Unit 3 : Matter Around Us

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7th Science : Term 1 Unit 3 : Matter Around Us : Book Back Questions Answers, Solution

Evaluation

 

I. Choose the appropriate answer.

 

1. Which of the following is an example of a metal?

a. Iron

b. Oxygen

c. Helium

d. Water

Answer : a. Iron

 

2. Oxygen, hydrogen, and sulphur are examples of which of the following?

a. Metalsb.

b. Non-metals

c. Metalloidsd.

d. Inert gases

Answer : b. Non-metals

 

3. Which of the following is a short and scientific way of representing one molecule of an element or compound?

a. Mathematical formula

b. Chemical formula

c. Mathematical symbol

d. Chemical symbol

Answer : b. Chemical formula

 

4. The metals which is a liquid at room temperature

a. Chlorine

b. Sulphur

c. Mercury

d. Silver

Answer : c. Mercury

 

5. An element which is always lustrous, malleable and ductile

a. non-metal

b. metal

c. metalloid

d. gas

Answer : b. metal

 

II. Fill in the blanks.

 

1. The smallest particle of matter that can exist by itself atom. 

2. A compound containing one atom of carbon and two atoms of oxygen is Carbon-di-oxide.

3. Carbon is the only non-metal conducts electricity.

4. Elements are made up of same kinds of atoms.

5. Symbol of some elements are derived from Latin or Greek names of the elements.

6. There are 118 number of known elements.

7. Elements are the simplest form of   pure substances .

8. The first letter of an element always written in Capital letter

 9. Molecule containing more than three atoms are known as Polyatomic.

10. Nitrogen is the most abundant gas in the atmosphere.

 

III. Fill in the Blanks.

 

1. Mercury: liquid at room temperature Oxygen: gas (at room temperature)

2. Non metal conducting electricity: Carbon Metal conducting electricity Copper

3. Elements: combine to form compounds:: Compounds: can be split into elements

4. Atoms: fundamental particle of an element:: Molecules :fundamental particles of a compound.

 

IV. True of False. If False, give the correct statement.

 

1. Two different elements may have similar atoms. [False]

Two different element may have different atoms.

2. Compounds and elements are pure substance. [True]

3. Atoms cannot exist alone; they can only exist as groups called molecules. [False]

Atom can exist alone; They can also exist as groups called molecules.

4. NaCl represents one molecule of sodium chloride. [True]

5. Argon is mono atomic gas. [True]

 

V. Answer in brief.

 

1. Write the chemical formula and name the elements present in the following compounds:

a. Sodium chloride

b. Potassium hydroxide

c. Carbon-di-oxide

d. Calcium oxide

e. Sulphur dioxide

a. Sodium chloride - NaCl  :   Sodium & Chlorine

b. Potassium hydroxide - KOH  : Pottassium, Oxygen & Hydrogen

c. Carbon-di-oxide  - CO2   :   Carbon & Oxygen

e. Sulphur dioxide - CaO  :   Calcium & Oxygen

d. Calcium oxide - SO2   :    Sulphur & Oxygen

2. Classify the following molecules as the molecules of element or compound


3. What do you understand by chemical formula of a compound? What is its significance?

A chemical formula is a symbolic representation of one molecule of an element or a compound. It provides information about the elements present in the molecule and the number of atoms of each element.

4. Define the following terms with an example of each:

a. Element

b. Compound

c. Metal

d. Non-metal

e. Metalloid

a) Element: Matter in its simplest form is called an element.

e.g: Hydrogen, Oxygen, Copper, lead, Iron

b) Compound : A compound is a pure substance that is formed when the atoms of two or more element combine chemically in definite proportions. e.g: Water, Sodium Chloride, Sugar. 

c) Metal: Elements that are malleable (a material may be flattened into thin sheets or various shapes) is called as metals. Metals are generally hard and shiny elements. e.g: Copper, Lead, Iron, Zinc, Nickal

d) Non-metal : Non-metals are generally dull and soft. Non-metals are poor conductors of heat and electricity. e.g: Oxygen, Hydrogen, Sulphur, Phosphorous, Bromine.

e) Metalloid : Metalloids exhibit the properties of both metals and non metals. e.g: Silicon, Orsenic, Antimony, Boron.

5. Write the symbols for the following elements and classify them as solid, liquid and gas Aluminum, carbon, chlorine, mercury, hydrogen and helium

Elements                   Symbol                       Classify

Aluminum                  Al                                 Solid

Carbon                        C                                   Solid

Chlorine                      Cl                                  Gas

Mercury                      Hg                                 Liquid

Hydrogen                     H                                  Gas

Helium                         He                                Gas

6. Classify the following as metals, non-metals and metalloids

Sodium, Bismuth, Silver, Nitrogen, Silicon, carbon, chlorine, Iron, copper

Elements              -                  Classify

Sodium                   -                  metal

Bismuth                    -                 metal

Silver                         -                metal

Nitrogen                      -              non-metal

Silicon                           -            metalloid

Carbon                            -          non-metal

Chlorine                          -         non-metal

Iron                                  -        metal

copper                              -        metal

7. Classify the following as elements and compounds

Water, common salt, sugar, carbon dioxide, iodine and lithium

water                      -     compound

commson salt         -   compound

sugar                         -   compound

carbon dioxide          -  compound

iodine                        -  element

lithium                      - element

8. Write the chemical formula for the following elements

a. Hydrogenb.

b. Nitrogen

c. Ozoned.

d. Sulphur

a) Hydrogen - H2

b) Nitrogen - N2

c) Ozone - O3

d) Sulphur – S8

9. What are elements? What are they made of? Give two examples.

A molecule of an element consists of a fixed number of one types of atom chemically combined. E.g: oxygen, hydrogen, carbon.

10. Define molecule.

A molecule is made up of two or more atoms chemically combined.

11. What are compounds? Give two examples.

A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions. e.g: ammonia, hydrogen chloride, calcium oxide.

12. Give an example for the elements derived from their Latin names.

Gold - Au, Copper - Cu

13. What is atomicity of elements?

Atomicity of an element implies that the total number of atoms present in one molecule of that element.

e.g : Hydrogen - 2 (diatomic), Sodium - 1 (monoatomic)

14. Calculate the atomicity of H2SO4.

Molecule of sulphuric acid (H2SO4) contains two atoms of hydrogen, one atom of Sulphur and four atoms of oxygen, Hence the atomicity is 2 + 1 + 4 = 7. 

 

VI. Answer in detail.

 

1. Differentiate metals and non metals.



Metals

1. Metals are lustrous. They have a shiny surface.

2. Metals are generally hard.

3. Most metals can be bent, beaten into sheets and they can drawn into wires

4. Most metals are good conductors of electricity

5. Most metals are good conductors of heat.

6. Most metals are making ringing sound when struck. Hence, they are used to make objects like bells

e.g: Copper, Silver, Gold, Iron, Nichal

Non-Metals

1. Non metals are non lustrous. They have non-shiny surface.

2. Non-metals are generally soft.

3. Non-metals are non ductile

4. Non-metals are bad conductors of electricity

5. Non-metals are bad conductors of heat.

6. Non-metals does not make any sound when they struck.

e.g: Hydrogen, Oxygen, Chlorine, Bromine, Sulphur.

2. Explain the characteristics of compounds

A compound if formed only when the constituent elements combine in a fixed proportion.

The properties of a compound are different from those of its constituent elements.

A compound cannot be broken down by physical methods. This is because a compound is made up of different elements that are chemically combined. Sodium chloride cannot be separated by physical methods such as filtration.

A compound can be separated into its constituent elements by chemical methods only. 

3. Describe the different ways in which we can write the symbols of elements. Give appropriate examples.

A symbol is an abbreviation or short representation of a chemical element. These symbols are allocated by the International Union of Pure and Applied Chemistry (IUPAC).

Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. Berzelius suggested that the symbols of elements be made from one or two letters of the name of the element.

Symbols for some elements as proposed by Dalton.


The following rules are followed while assigning symbol to an elements:

Chemical symbols usually consist of one or two letters.

The symbols of most elements correspond to the first letter (which is capitalized) of their English name. For example, the symbol for oxygen is "O" and that for hydrogen is "H".

Element       Symbol       

Hydrogen      H

Fluorine         F

Oxygen          0

Carbon          C

Phosphorus           P

Sulphur                 S

Potassium             K

Uranium               U

When there is more than one element that begins with the same letter, their symbols take two letters. The first letter is capitalised while the second letter has a lower case. For example, the names of both hydrogen and helium begin with H. So, hydrogen is represented by the symbol H and Helium by He. Similarly, the symbol for carbon is C while the symbols for calcium, chlorine and chromium are Ca, Cl and Cr, respectively.

Elements represented by symbols of two letters.

Element          Symbol

Aluminium             Al

Argon                      Ar

Arsenic                   As

Barium                   Ba

Nickel                    Ni

Bromine                Br

Chromium     Cr

Cobalt            Co

Helium          He

Magnesium    Mg

Calcium         Ca

Chlorine          Cl

The symbols for some elements are derived from their Latin names. For example, the symbol for gold is Au after its Latin name Aurum. Similarly, the symbols for copper is Cu after its Latin name Cuprum.

Element            -        Latin name         :    Symbol

Copper              -          Cuprum              :       Cu

Lead                  -          Plumbum            :       Pb

Potassium          -          Kalium               :         K

Iron                    -         Ferrum                :          Fe

Mercury             -         Hydrargyrum      :         Hg

Sodium               -         Natrium                :         Hg

4. Differentiate between elements and compounds.


Elements

An element is the simplest substance.

Elements combine to form compounds.

Atoms are the fundamental partcicle of an element.

Compounds

A compound is a chemical substance formed by the combination of two or more elements.

Compounds can be split into elements.

Molecules are the fundamental particles of a compound.

5. Write any five characteristics of compound.

A compound if formed only when the constituent elements combine in a fixed proportion.

The properties of a compound are different from those of its constituent elements.

A compound cannot be broken down by physical methods. This is because a compound is made up of different elements that are chemically combined. Sodium chloride cannot be separated by physical methods such as filtration.

A compound can be separated into its constituent elements by chemical methods only.

"Molecules are the fundamental particles of a compound."

6. List comparative properties of metals and non-metals. Give three examples of each.


Metals

1 .Metals are lustrous.They have a shiny surface.

2. Metals are generally hard.

3. Most metals can be bent, beaten into sheets and they can drawn into wires  

4. Most metals are good conductors of electricity

5. Most metals are good conductors of heat.

6. Most metals are making ringing sound when struck. Hence, they are used to make objects like bells.

e.g: Copper, Silver, Gold, Iron, Nichal

Non-Metals

1 .Non metals are non lustrous. They have non-shiny surface.

2 .Non-metals are generally soft.

3. Non-metals are non ductile

4. Non-metals are bad conductors of electricity

5 .Non-metals are bad conductors of heat.

6 .Non-metals does not make any sound when they struck.

e.g: Hydrogen, Oxygen, Chlorine, Bromine, Sulphur.

7. Write down the properties of metalloids.

Metalloids exhibit the properties of both metals and non-metals:

1) Look shiny as metals.

2) Brittle as non-metals.

3) Conduct heat and electricity letter than non-metals but not as well as metals.

4) They are solids at room temperature as metals.

5) Their hardness is less than that of metals but more than that of non-metals.

 

VII. Rewrite the sentence in correct form

 

1. Elements contains two or more kinds of atoms and compounds contains only one kinds of atoms.

Elements contains only one kinds of atoms and compounds contains two or more kinds of atoms.

 

VIII. Higher Order Thinking questions

 

1. Lists the metals, non-metals and metalloids which you used in your house, schools. Compare their properties.


2. Aakash noticed that the metal latch on gate was difficult to open during hot sunny days. However, this same latch was not difficult to open at night. Aakash observed that the latch and the gate are exposed to the sun during the day.

a. Formulate a hypothesis based on the information provided.

b. Briefly state how you would test the hypothesis stated in (a).

a. Formulate a hypothesis based on the information provided.

During hot sunny days the gate and the latch are exposed to the sunlight. Due to the absorption of heat from the sun the gate and the latch expand. This makes no space between the latch and gate. Therefore it was difficult to open during hot sunny days. Absorbed heat is released at night. This regains the space between the latch and gate due to the contraction of them. So it was not difficult to open at night. 

b. Briefly state how you would test the hypothesis stated in (a). Experiment to test the above observation

Take an iron rod and measure the length. Now keep the rod over the stove and heat it. Again measure the length using a wooden scale without touching. You can observe the increase in the length of the rod.

Now take the hot rod using a tongs and cool it under a water tap. Again measure its length. Now you can observe that the rod regains its original length. (Caution: Don't touch the hot rod)

3. What changes take place in the movement and arrangement of particles during heating process?

i) Size and number of particles and mass of the matter do not change upon heating.

ii) Particles of matter gain energy, vibrate vigorously and overcome the strong forces of attractin between one another. 

iii) Then, particles move slightly further apart and break free from one another and move randanly.

iv) This results changes in its state. For Example, When solid ice is heated 0°c, it melts to become liquid water. In the same way, liquid water is heated to 100°c, it boils to become steam. Temperature at which solid changes into its liquid state is called Melting Point. Temperature at which liquid changes into its gaseous state is called boiling Point.

v) Density of gas decreases due to its expansion upon heating.

4. In the diagram below, the circle, square and triangle represent the atoms of different elements.


5. In the diagram above, identify all combinations that represent

a. A molecule of a compound

b. A molecule of an element consisting of two atoms

c. A molecule of an element consisting of three atoms

Answer: d) molecules of a mixture.

 

IX. Assertion-reason questions

Directions: Please refer to the following instructions:

 

1. Assertion : Oxygen is a compound.

      Reason : Oxygen cannot be broken down into anything simpler.

2. Assertion : Hydrogen is an element.

       Reason : Hydrogen cannot be broken down into anything simpler.

3. Assertion : Air is a compound.

        Reason : Air consists of carbon dioxide.

4. Assertion : Air is a mixture of elements only.

        Reason : Only nitrogen, oxygen and neon gases exist in air.

5. Assertion : Mercury is solid in room temperature.

        Reason : Mercury is a non-metal. 

A. Both statements are true and the 2nd statement is a correct explanation of the 1st statement.

B. Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.

C. The 1st statement is false while the 2nd statement is true.

D. Both statements are false.

Answer:

1. C. The 1st statement is false while the 2nd statement is true.

2. A. Both statements are true and the 2nd statement is a correct explanation of the 1st statement.

3. D. Both statements are false.

4. A. Both statements are true and the 2nd statement is a correct explanation of the 1st statement.

5. D. Both statements are false.

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