ATOMS AND MOLECULES REVISON NOTES

 

ATOMS AND MOLECULES

What is an Atom?

• An atom is the smallest particle of an element.

📚 Dalton’s Atomic Theory -Jhon Dalton gave atomic theory.

1. Matter is made of tiny particles called atoms.
2. Atoms of same element are same.
3. Atoms of different elements are different.
4. Atoms combine in simple whole numbers.
5. Atoms cannot be created or destroyed in a chemical reaction.

What is the Law of Conservation of Mass?

👨‍🔬 Given by: Antoine Lavoisier

📖 Statement

👉 Mass can neither be created nor destroyed in a chemical reaction.

✏️ Easy Meaning

• In a chemical reaction,
  Total mass of reactants = Total mass of products

OR

👉 Nothing disappears, nothing new is created — it only changes form.

🧠 Why Does This Happen?

• In a reaction, atoms only rearrange.
• Atoms are not destroyed.
• Atoms are not created.
• So total mass remains same.

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🧪 Example

Suppose:

10 g Hydrogen + 80 g Oxygen → Water

Total mass before reaction = 10 + 80 = 90 g

Mass of water formed = 90 g

✔ Mass before = Mass after

Law of Constant Proportions

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👨‍🔬 Given by: Joseph Proust

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📖 Statement

👉 In a chemical compound, the elements are always present in a fixed proportion by mass, irrespective of its source or method of preparation.

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✏️ Easy Meaning (Very Simple Words)

• A compound always has the same elements.
• These elements are always present in the same fixed ratio by mass.
• It does not matter:
• Where the compound comes from.
• How it is made.

👉 The ratio never changes.

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🧪 Example 1: Water (H₂O)

Water contains:Hydrogen (H) & Oxygen (O)

By mass:

• Hydrogen = 1 part
• Oxygen = 8 parts

So, ratio = 1 : 8

Even if water is:From river 💧,From sea 🌊,From rain ☔,Made in laboratory 🧪

👉 The ratio of H and O will always be 1 : 8

It never changes.

DIFFERENCE BETWEEN ATOMS AND MOLECULES

ATOMS	MOLECULES
1.Atoms are the smallest particles that can exist independently	1.Molecules are made up of atoms
2.atoms are composed of neutrons,protons and electrons.	2.Molecules are composed of atoms.
3.Atoms are more reactive.	3.molecules are less reactive.
4.Atoms cannot divide into their particles by chemical methods .	4.molecules can be divide in to their atoms by chemical methods.
5.Example. oxygen (O)	Water (H2O)

ATOMICITY- No of atoms in a molecule is known as atomicity.

Atomicity	Example
Monoatomic (1)	He
Diatomic (2)	O₂
Triatomic (3)	O₃
Polyatomic (more than 3)	P₄

Ions-charged particles are called ions.

They are of two types:-

Cations:-the positively charged ions are called cations. Examples: Na⁺, Ca²⁺

2.      Anion: A negatively charged ion. Examples: F⁻, Cl⁻.

Molecular Mass

• Definition: The total of the atomic masses of all atoms in a molecule is called the molecular mass.

• Calculation: Multiply the atomic mass of each element by the number of its atoms in the molecule, then add these values.

Formula Unit Mass

• The sum of all atoms' atomic masses in an ionic compound's formula unit.

• Calculation: Add the atomic masses of the elements in the formula unit. For example, for Sodium Chloride (NaCl): 

(1×23) + (1×35.5) = 58.5 u

• Valency: The combining capacity of an element, showing how atoms mix with others. For example, hydrogen (H⁺) has a valency of 1, and oxygen (O²⁻) has a valency of 2.

·        Polyatomic Ions: If more than one polyatomic ion is present, place them in brackets. For example, calcium nitrate is written as Ca(NO₃)₂.

Valency table

Some important chemical formulas

1.      sodium oxide – Na₂O

2.       aluminium chloride – AlCl₃

3.       sodium sulphide – Na₂S

4.       magnesium hydroxide – Mg (OH)₂

5.       Al₂(SO₄)₃ – Aluminium sulphate

6.       CaCl₂ – Calcium chloride

7.       K₂SO₄ – Potassium sulphate

8.       KNO₃ – Potassium nitrate

9.       CaCO₃ – Calcium carbonate(lime stone)

10.  Magnesium chloride – MgCl₂

11.   Calcium oxide – CaO

12.   Copper nitrate – Cu(NO₃)₂

Give the names of the elements present in the following compounds.

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate

Solution:

The following are the names of the elements present in the following compounds:

(a) Quick lime – Calcium and oxygen (CaO)

(b) Hydrogen bromide – Hydrogen and bromine (HBr)

(c) Baking powder – Sodium, Carbon, Hydrogen, Oxygen (NaHCO₃)

(d) Potassium sulphate – Sulphur, Oxygen, Potassium (K₂SO₄)

 

 

CELL THE FUNDAMENTAL UNIT OF LIFE

 

CELL THE FUNDAMENTAL UNIT OF LIFE

What is a Cell?

• A cell is the smallest living unit of life.
• Cell was discovered by Robert Hooke (1665).

👉 Easy line: Cell = building block of life.

 

Cell Theory

The "Cell theory" was formulated by two biologists, M.J.Schleiden (1838), and T.Schwann (1839.

Cell theory states that

• Bodies of the living beings are made up of cells. The cell is the basic unit of life. 

• Activities of an organism are the sum total of activities of its cells. 

• Every new cell arises from pre-existing cells.
• Basic structure of cell

Cell Wall

• It is the outermost rigid, freely permeable layer found outside the plasma membrane in all the plant cells, bacteria, blue-green algae, some protists and all fungi. 

• It is absent in animal cells. 

• Cell walls protects the plant from the outside environment. It also provides a definite shape to the cell. 

• Cell wall of plant cells is formed of cellulose 

• It is formed of chitin in most of the fungi. 

Nucleus (Headquarter of the cell)

• Discovered by – Robert Brown (1831) "Nucleus is a double membrane bound dense protoplasmic body, which controls all cellular functions . Nucleus is considered as controller or director of the cell. 

• Structure : It is made up of following four components

• Nuclear membrane,Nucleoplasm ,nucleolus and chromatin threads.

Functions of nucleus 

• It controls all the metabolic activities of the cell. 

• It regulates cell cycle. 

• It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another.

 

Endoplasmic Reticulum

• Endoplasmic reticulum is a complex network of membrane bound channels or sheets, tubules and vesicles. It does not work alone. 

• The ER works closely with the Golgi apparatus and ribosomes. It creates a network of membranes found through the whole cell. 

• Two types of Endoplasmic Reticulum ; 

• Smooth and Rough Smooth ER/Agranular ER : It has smooth membranes which do not bear ribosomes. It acts as a storage organelle. It is important in the synthesis and storage of steroids. It is mainly made up of vesicles and tubules. IT HELPS in detoxifying harmful drugs.

• Rough ER/Granular ER : It has a rough membrane because a number of ribosomes are attached to its outer surface. RER is engaged in protein synthesis. It is mainly made up of cisternae. Tubules are very few.

Golgi complex (Traffic police of cell)

• Discovered by Camillo Golgi (1898) in nerve cells of owls. Golgi carried out a revolutionary method of staining individual nerve and cell structures. 

• Position : It is located near the nucleus.

• It is involved in cell secretion and acts as storage, modification and condensation or packaging membrane. 

• It forms the lysosomes and secretory vesicles. 

• It is the site of formation of glycolipids and glycoproteins.  

• Synthesis of cell wall material (polysaccharide synthesis). 

• Cell plate formation on during plant cell division.

 

 

 

 

 

 

Lysosomes (Little enzyme packages) 

• Lysosomes are spherical bag-like structures covered by a single membrane. 

• Lysosomes hold enzymes that were created by the cell. 

• The purpose of the lysosome is to keep the cell clean by digesting any foreign materials as well as worn-out cell organelles. 

• They might be used to digest food or break down the cell when it dies. 

Mitochondria (Powerhouse of cell) 

• Mitochondria are double membrane bound organelles of eukaryotic cells.

• Mitochondria are semi autonomous organelles bound by an envelope of two unit membranes and filled with a fluid matrix. 

• The outer membrane is smooth and has porous proteins which form channels for the passage of molecules through it. 

• The inner membrane is semipermeable. 

• It usually produces numerous infolds called cristae. 

• The cristae greatly increases the inner surface area of the mitochondria to hold a variety of enzymes. 

• Cristae bears minute, regularly spaced tennis racket shaped particles known as F1 particles or oxysomes. 

• Oxysomes are concerned with ATP synthesis. Matrix contain various respiratory enzymes. 

Plastids

• Plastids are major organelles found in the cells of plants and algae. 

• Plastids are the site of manufacture and storage of important chemical compounds used by the cell. 

• Plastids often contain pigments used in photosynthesis and the types of pigments present can change or determine the cell's color. 

• Plastids are responsible for photosynthesis, storage of products like starch.Chloroplasts are the sites of photosynthesis. 

• They contain enzymes and coenzymes necessary for the process of photosynthesis. 

Plastids are of three types- 

• Chromoplasts - For pigment synthesis and storage. Chromoplasts are red, yellow and orange in color and are found in petals of flowers and in fruits. Their color is due to two pigments, carotene and xanthophyll. 

• Leucoplasts - Leucoplasts are colorless or white plastid. They occur in plant cells not exposed to light, such as roots and seeds. they are also involved in the formation of oil and proteins. 

• Chloroplasts -  Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light. e.g. the palisade cells of leaves. The green color of chloroplasts is caused by the green pigment chlorophyll.

 How do substances like CO ₂ and water move in and out of the cell? Discuss.

Solution: CO ₂ moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO ₂ in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO ₂ to diffuse from a region of higher (within the cell) to a lower concentration. H ₂ O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.

4. Why is the plasma membrane called a selectively permeable membrane?

Solution: The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.

o  Why are lysosomes known as suicide bags?

Solution: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

o  Where are proteins synthesised inside the cell?

Solution: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

 Why are lysosomes known as suicide bags?

Solution: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

9. Where are proteins synthesised inside the cell?

Solution: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

1. Make a comparison and write down ways in which plant cells are different from animal cells.

Solution: The following table depicts the differences between plant cells and animal cells.

Characteristic	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Shape of Cell	With distinct edges, the shape is either rectangular or square-shaped.	Round and irregular shape
Nucleus	Present. It lies on one side of the cell	Present. It lies in the centre of the cell
Lysosomes	Rarely present	Always present
Plastids	Present	Absent
Structure of Vacuoles	Single or a few large vacuoles that are centrally located	Presence of numerous and small vacuoles

 

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally small (1-10 μm)	Generally large (5-100 μm)
Nuclear Region	Not well defined; nucleoid instead of nucleus	Well-defined; surrounded by a nuclear membrane
Chromosomes	Single chromosome	More than one chromosome
Membrane-bound Organelles	Absent	Present

 

o  What would happen if the plasma membrane ruptures or breaks down?

Solution: If the plasma membrane ruptures or breaks down, molecules of some substances will freely move in and out of the cell. Normally, the plasma membrane acts as a barrier that controls the exchange of materials through processes like osmosis and diffusion

o  What would happen to the life of a cell if there was no Golgi apparatus?

Without the Golgi apparatus, cells would be unable to properly package and transport materials essential for their functioning. This would impair processes such as secretion, cell signaling, and the maintenance of cellular structure.

 

NUTRITION IN AMOEBA-------

If ribosomes are removed from the cell no protein formation takes place.

 

 

 

SAMPLE PAPER 3 CLASS 9

 

KVS, Zonal Institute Education Training, Gwalior

Sample Question Paper (2025-26)

Class – 9

Subject – Science (086)

M.M. – 80                                                                                Time allowed – 3 hours

 

General Instructions:

General Instructions:

(i) This question paper consists of 39 questions in 3 sections. Section A is Biology, Section, B is Chemistry and Section C is Physics.

 (ii) All questions are compulsory. However, an internal choice is provided in some questions. A student is expected to attempt only one of these questions.

SECTION A

 

1.	The phenomenon where cytoplasm shrink in a hypertonic medium is called (a) Frontolyzes     (b) Plasmolysis               (c) Acidolysis                     (d) Electrolysis	1
2.	__________ are cemented to one another, forming an irregular layer. These form the outer protective layer of the skin in an animal’s body. (a)Connective tissue     (b) Muscular tissues   (c) Nervous tissues  (d) Compound epithelium cell	1
3.	An unripe green fruit changes colour when it ripens. The reason being (a)Chromoplasts changes to chlorophyll (b) Chromoplasts changes to chromosomes (c) Chromosomes changes to chromoplasts   (d) Chloroplast changes to chromoplasts	1
4.	The Nodes of Ranvier are found in: (a)Nerve cells            (b)Heart cells                (c) Liver cells                (d) All of the above	1
5.	The management and production of fish is called: (a)  Apiculture  (b) Pisciculture  (c) Sericulture (d) Aquaculture	1
6.	Rapid elongation of a bamboo stem is due to  (a)Lateral meristem      (b) Intercalary meristem      (c) Apical meristem          (d) Cambium	1
7.	Weeds affect the crop plants by: (a) Killing plants before they grow                 (b) Dominating the plants to grow (c) Competing for resources like nutrients, light, and water (d) All of the above	1
The following two questions consist of two statements – Assertion (A) and Reason (R) . Answer these questions by selecting the appropriate option given below. A.   Both Assertion (A) and Reason (R) are true and Reason (R) is the         correct explanation of Assertion (A). B.    Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).      C. Assertion (A) is true, but Reason (R) is false.      D. Assertion (A) is false, but Reason (R) is true.
8.	Assertion: Draught breeds of cows are used mainly as beasts of burden. Reason: Draught breeds of cow give less milk but are strong and sturdy.	1
9.	Assertion: Plasma membrane is selectively permeable. Reason: Plasma membrane allows some molecules to pass through it more easily than other	1
10.	Name the following. (a) Tissue that forms the inner lining of our mouth. (b) Tissue that connects muscle to bone in humans. (c) Tissue that transports food in plants. (d) Tissue that stores fat in our body.	2
11.	Classify the following into osmosis/diffusion: (a) Swelling up of a raisin on keeping in water. (b) Spreading of virus on sneezing. (c) Earthworm dying on coming in contact with common salt. (d) Shrinking of grapes kept in thick sugar syrup. OR Why are lysosomes called the suicidal bags of the cell?	2
12.	What are milch and drought animals?	2
13.	Diagrammatically show the difference between the three types of muscle fibers.	3
14.	In agriculture practices, higher input gives higher yield. Discuss how?	3
15.	Case study Based Questions Riya observed onion peel under a microscope during her biology lab. She noted that the cells were rectangular and arranged in rows. Each cell had a thick boundary and a dense, round structure in the centre. Later, she examined cheek cells and found them irregular in shape but also had a prominent central structure. (a) What is the dense, round structure Riya observed in both onion and cheek cells, and what is its primary function? (b) What is the difference between the outer boundary of onion cells and cheek cells, and why is there a difference? (c) Based on Riya’s observation, compare plant and animal cells in terms of shape, outer boundary, and presence of specific cell structures.	4
16.	Fill in the blanks: (a) Farming without the use of chemicals such as fertilizers, herbicides and pesticides is called __________ (b) Growing wheat and groundnut on the same field is called _________ (c) Planting soybean and maize in alternate rows in the same field is called ________ (d) Growing different crops on a piece of land in pre-planned succession is  known as _______ (e) Xanthium and Parthenium are commonly known as ________	5

SECTION B

17.	Which of the following correctly represents 360g of water? (a) 20 moles of H₂O                                                   (b) 2 moles of H₂O (c) 1.2044×10²⁵ molecules of water                           (d) 6.022 × 10²³ molecules of water	1
18.	Which of the following is an example of a heterogeneous mixture? (a) Lemonade            (b) Vinegar        (c) Air             (d) Orange juice with pulp	1
19.	When ice at 0°C is converted to water at 0°C, what happens?  (a) Heat is absorbed                                           (b) Heat is released (c) Temperature increases                                 (d) Temperature decreases	1
20.	The number of atoms present in 0.5 moles of Nitrogen atoms is the same as in: (a) 8 g of Oxygen                                                        (b) 12 g of Carbon (c) 24 g of Magnesium                                                (d) 32 g of Sulfur	1
21.	Which of the following is a pure substance? (a) Milk                    (b) Salt                  (c) Air                        (d) Soil	1
22.	What is the basic unit of matter? (a) Proton                (b) Neutron            (c) Atom                    (d) Molecule	1
23.	In which state of matter do particles have the highest kinetic energy? (a) Solid                  (b) Liquid               (c) Gas                      (d) Plasma	1
The following question consist of two statements – Assertion (A) and Reason (R) . Answer these questions by selecting the appropriate option given below. A.   Both Assertion (A) and Reason (R) are true and Reason (R) is the         correct explanation of Assertion (A). B.    Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).      C. Assertion (A) is true, but Reason (R) is false.      D. Assertion (A) is false, but Reason (R) is true.
24.	Assertion: Atoms always combine to form molecule and ions. Reason: Atoms of most element are not able to exist independently.	1
25.	21.    Give the chemical formulae for the following compounds (a) Ammonia  (b) Carbon monoxide (c) Hydrogen chloride  (d) Aluminium fluoride	2
26.	Give reason for the following: (i)              Butter is generally wrapped in wet clothes during summer if no refrigerator is available? (ii)            When sugar crystals dissolve in water, the level of water does not rise?	3
27.	Determine whether each of the following change is physical or chemical. Give reason.  (i) A balloon filled with hydrogen gas explodes upon contact with a spark. (ii) Copper turns green on exposure to air and water (i)              A metal surface becomes dull because of continued abrasion.	3
28.	Case study Based Questions (4 marks each) Rutherford (1871-1937) was known as the ‘Father’ of nuclear physics. Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for fast moving alpha (α)-particles were made to fall on a thin gold foil. On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features: There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom. Drawbacks of Rutherford’s model of the atom: The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable. (1) Which of the following scientist was known as the ‘Father of nuclear physics? (a) J.J. Thomson (b) John Dalton (c) E. Rutherford (d) Neil’s Bohr (2) Positively charged centre in an atom is termed as (a) Nucleus (b) Molecule (c) Atom (d) Protons (3) Identify the correct statement Statement 1 – Positively charged centre in an atom called the nucleus. Statement 2 – The electrons revolve around the nucleus in circular paths. Statement 3 – Nearly all the mass of an atom resides in the nucleus. Statement 4 – The size of the nucleus is very small as compared to the size of the atom. (a) Only 2 (b) Both 3 & 4 (c) Both 1 & 2 (d) All of the above (4) Write the features of Rutherford’s nuclear model of an atom?	4
29.	Name the process associated with the following: (a) A drop of ink placed on the water’s surface contained in a glass spread throughout the water. (b) Dry ice is kept at room temperature and at one atmospheric pressure. (c) An acetone bottle is left open and the bottle becomes empty. (d) Settling of sand when a mixture of sand and water is left undisturbed for some time. (e) A fine beam of light entering through a small hole in a dark room illuminates the particles in its paths.	5

SECTION C

30.	Which of the following is a scalar quantity?  (a) Displacement        (b) Velocity                   (c) Speed               (d) Acceleration	1
31.	A goalkeeper in a football game pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to  (a)increase the rate of change of momentum            (b) decrease the rate of change of momentum  (c) increase the force exerted by the balls on the hands   (d) exert larger force on the ball	1
The following question consist of two statements – Assertion (A) and Reason (R) . Answer these questions by selecting the appropriate option given below. A.   Both Assertion (A) and Reason (R) are true and Reason (R) is the         correct explanation of Assertion (A). B.    Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).      C. Assertion (A) is true, but Reason (R) is false.      D. Assertion (A) is false, but Reason (R) is true.
32.	·        Assertion (A): Energy can neither be created nor destroyed, only converted from one form to another. Reason (R): This is the law of conservation of energy, which states that the total energy in a closed system remains constant.	1
33.	A car starts from rest and attains a velocity of 20 m/s in 10 seconds. Find its acceleration.	2
34.	Explain why some of the leaves may get detached from a tree if we vigorously shake its branch. OR An 8000 kg engine pulls a train of 5 wagons, each of 2,000 kg along a horizontal track. If the engine exerts a force of 40,000 N and track offers a force of friction of 5,000 N, then calculate: (a) the net accelerating force and (b) the acceleration of the train	2
35.	Give one example of each of the following situations. (i) Uniformly accelerated motion. (ii) Accelerated motion with uniform magnitude of velocity. (iii)Motion in v-t graph is a horizontal line parallel to X- axis.	3
36.	What would happen, if a fielder stops the fast-moving ball suddenly? Justify your answer.	3
37.	Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also, define wavelengths and time period using this curve.	3
38.	Objects having density more than that of liquid in which they are immersed, sink on the surface of the liquid. A block of wood is floating on water while an iron nail sinks when placed in water. The teacher explains that this is due to the difference in their densities and buoyant force acting on them. a) What is the condition for an object to float in a liquid? b) What is buoyant force? c) Why does the iron nail sink in water? d) Which principle explains the floating and sinking of objects in fluids?	4
39.	(a) What is kinetic energy? Give one example (b) Give the formula to calculate kinetic energy. (c ) Calculate the kinetic energy of a bullet of mass 8 g which is fired with a velocity of 80 m/s.	5