RBSE Solutions Class 9 Science Chapter 2 Structure of Matter and Molecule

Structure Of Matter And Molecule Textbook Questions Solved

Objective Type Questions

 

Question 1. When did Maharishi Kanad give the idea of Parmanu?
(A) 500 BC
(B) 100 BC
(C) 460 BC
(D) 1808 BC
Answer: (A) 500 BC
In simple words: Maharishi Kanad first talked about 'Parmanu' (atom) around 500 BC. This was an early thought about the smallest particles of matter.

🎯 Exam Tip: Remember key historical figures and their contributions to science, along with approximate dates.

 

Question 2. What is the atomicity of oxygen in the ozone?
(A) 1
(B) 2
(C) 3
(D) 4
Answer: (C) 3
In simple words: Ozone is a special form of oxygen where each molecule has three oxygen atoms. Therefore, the atomicity of oxygen in ozone is three.

🎯 Exam Tip: Atomicity refers to the number of atoms in a molecule; for ozone (\( O_3 \)), it's three.

 

Question 3. Which of the following substance is not a solid at room temperature?
(A) Salt
(B) Alum
(C) Oxygen
(D) Sodium
Answer: (C) Oxygen
In simple words: At normal room temperature, oxygen exists as a gas, unlike salt, alum, and sodium which are all solids. Oxygen is a gaseous element essential for life.

🎯 Exam Tip: Understand the common states of matter for familiar substances at standard room temperature.

 

Question 4. The temperature at which matter from liquid state converts into the solid state is known as:
(A) Melting point
(C) Freezing point
Answer: (C) Freezing point
In simple words: The specific temperature where a liquid changes into a solid is called its freezing point. This is the reverse process of melting.

🎯 Exam Tip: Distinguish between melting point (solid to liquid) and freezing point (liquid to solid).

Structure Of Matter And Molecule Very Short Answer Type Questions

 

Question 6. Who first gave the knowledge of Parmanu?
Answer: The first person to explain the idea of 'Parmanu', which means atom, was the Indian philosopher Maharishi Kanad. He introduced this concept centuries ago.
In simple words: Maharishi Kanad, an Indian philosopher, first taught about 'Parmanu' or atoms.

🎯 Exam Tip: Recognize Maharishi Kanad for his foundational concept of 'Parmanu' in ancient Indian science.

 

Question 7. What is the approximate size of Parmanu?
Answer: The approximate size of a 'Parmanu' (atom) is incredibly small, measured to be about \( 10^{-10} \) meters. This tiny size shows how microscopic atoms are.
In simple words: A 'Parmanu' (atom) is about \( 10^{-10} \) meters in size.

🎯 Exam Tip: Remember the scale of atomic sizes, which are typically in the order of \( 10^{-10} \) meters.

 

Question 8. Write the molecular formula of water.
Answer: The molecular formula for water is \( H_2O \). This means each water molecule contains two hydrogen atoms and one oxygen atom.
In simple words: Water's molecular formula is \( H_2O \).

🎯 Exam Tip: Know the chemical formulas for common substances like water.

 

Question 9. A mixture of kerosene oil and water is what type of mixture?
Answer: A combination of kerosene oil and water forms a heterogeneous mixture. In such a mixture, the parts do not fully blend and remain separate, often forming visible layers.
In simple words: Kerosene oil and water make a heterogeneous mixture because they don't mix evenly.

🎯 Exam Tip: Understand that heterogeneous mixtures have visibly distinct parts, while homogeneous mixtures look uniform.

 

Question 10. By which method would we separate the water from sandy water?
Answer: To separate water from sandy water, the most suitable method is filtration. This process involves passing the mixture through a filter, which traps the sand particles while allowing the water to pass through.
In simple words: We use filtration to separate sand from water.

🎯 Exam Tip: Filtration is effective for separating insoluble solids from liquids.

 

Question 12. Give an example of a monoatomic molecule.
Answer: An example of a monoatomic molecule is Argon. Monoatomic means it exists as single atoms, not bonded with other atoms of the same element, like many noble gases.
In simple words: Argon is an example of a monoatomic molecule.

🎯 Exam Tip: Noble gases are typically monoatomic because they are stable as single atoms.

 

Question 13. Name the state of matter in which shape and volume are definite.
Answer: The state of matter that has both a definite shape and a definite volume is the solid state. Particles in a solid are closely packed and vibrate in fixed positions, giving them these properties.
In simple words: Solids have a fixed shape and volume.

🎯 Exam Tip: Remember the key properties (shape, volume, compressibility) of solids, liquids, and gases.

 

Question 14. Write the full form of CNG.
Answer: The full form of CNG is Compressed Natural Gas. It is a fossil fuel alternative that is cleaner than petrol or diesel and used in vehicles.
In simple words: CNG means Compressed Natural Gas.

🎯 Exam Tip: Know the full forms of common scientific abbreviations.

 

Question 15. Name the process in which liquid is converted into vapours.
Answer: The process where a liquid changes into its gaseous state (vapours) is called evaporation. This can happen slowly at any temperature below the boiling point.
In simple words: Evaporation is when a liquid turns into vapour.

🎯 Exam Tip: Evaporation is a surface phenomenon, while boiling occurs throughout the liquid.

Structure Of Matter And Molecule Short Answer Type Questions

 

Question 16. What is the melting point? Define it.
Answer: The melting point is the specific temperature at which a solid transforms into a liquid, while staying at normal atmospheric pressure. At this point, the solid absorbs heat without its temperature rising until it has completely melted.
In simple words: Melting point is the temperature when a solid turns into a liquid at normal air pressure.

🎯 Exam Tip: Emphasize "atmospheric pressure" and "constant temperature during phase change" when defining melting point.

 

Question 17. Define the latent heat of vaporisation.
Answer: Latent heat of vaporisation is the amount of heat energy needed to change one kilogram of a liquid into a gas at its boiling point, while maintaining standard atmospheric pressure. This hidden heat is used to break the bonds between liquid molecules.
In simple words: Latent heat of vaporisation is the heat needed to turn 1 kg of a liquid into a gas at its boiling point and normal air pressure.

🎯 Exam Tip: Highlight "1 kg of a liquid," "at its boiling point," and "at atmospheric pressure" as key phrases in the definition.

Compound: It is a substance made up of two or more elements chemically combined with one another in a fixed ratio.

 

Question 19. What is a mixture? Explain it with an example.
Answer: A mixture is formed when two or more pure substances are combined physically, but not chemically. Unlike pure substances where all particles are identical, mixtures contain different types of particles. For instance, milk is a complex mixture of water, fats, and proteins. Many everyday things like seawater, air, and soil are also mixtures, and they can be uniform (homogeneous) or have different visible parts (heterogeneous).
In simple words: A mixture combines different things physically, like milk (water, fat, protein). They can be uniform or not.

🎯 Exam Tip: Emphasize that in a mixture, substances are physically combined, not chemically, and retain their original properties.

 

Question 20. Write four characteristics of the liquid state.
Answer: Here are four characteristics of the liquid state:
1. **No Fixed Shape, Fixed Volume:** Liquids do not have their own fixed shape; they take the shape of their container. However, they do have a definite volume because the particles are held together by forces that are strong enough to keep them within a certain space.
2. **Lower Density:** Liquids generally have a lower density compared to solids. This is because their particles are not as tightly packed as in solids due to weaker attractive forces, so there are fewer particles in the same amount of space.
3. **Ability to Diffuse:** Liquids can diffuse, meaning their particles can move and spread out into other liquids. This happens because the forces between liquid particles are not strong enough to hold them in fixed positions, allowing movement.
4. **Slightly Compressible:** Liquids are only slightly compressible. While their particles can move, there isn't much empty space between them compared to gases, so their volume doesn't change much under pressure.
In simple words: Liquids take the shape of their container but keep their volume. They are less dense than solids, can mix with other liquids (diffuse), and are only slightly squishable.

🎯 Exam Tip: Focus on how particle arrangement and intermolecular forces determine each characteristic of liquids.

These changes are mostly reversible in nature i.e. the original substance can be further obtained by reversing the reaction conditions.

 

Question 22. Explain the effect of pressure on gases when liquified.
Answer: Changing the pressure can alter a substance's physical state. When pressure on a gas increases, its particles are forced closer together. This closeness makes the attractive forces between particles stronger, causing the gas to turn into a liquid. For example, applying high pressure and lowering the temperature can easily liquefy a gas. This shows that both pressure and temperature are key factors in determining whether a substance is a solid, liquid, or gas.
In simple words: More pressure pushes gas particles closer, making them stick together and turn into a liquid. Lowering temperature also helps. So, pressure and temperature decide if something is a solid, liquid, or gas.

🎯 Exam Tip: Remember that both high pressure and low temperature are crucial for liquefying gases, as they bring particles closer and reduce their kinetic energy.

 

Question 23. Write any three characteristics of matter.
Answer: The general characteristics of matter particles are as follows:
1. **Particles have spaces between them:** There is always some empty space, no matter how small, between the particles that make up any substance.
2. **Particles are constantly moving:** All particles of matter are in continuous, random motion, even in solids where they just vibrate. This movement increases with temperature.
3. **Particles attract each other:** There are forces of attraction between particles, holding them together. These forces vary in strength among different states of matter.
In simple words: Matter particles have spaces, they always move, and they pull towards each other.

🎯 Exam Tip: These three fundamental characteristics explain many properties of solids, liquids, and gases.

Immiscible liquids in a mixture form different layers depending on their densities when allowed to stand undisturbed for some time. Each layer can be separately taken out by using a separating funnel. We can see from the figure, how kerosene oil and water can be separated by using a separating funnel.

 

Question 25. Define compound with an example.
Answer: A compound is a type of pure substance made up of two or more different elements that are chemically joined together in a specific, fixed ratio. For example, water (\( H_2O \)) is a compound of hydrogen and oxygen, and methane (\( CH_4 \)) is a compound of carbon and hydrogen. The way a compound is made stays the same everywhere, and its parts can only be separated using chemical processes, not simple physical ones. Interestingly, the compound's properties are usually very different from the elements it's made from.
In simple words: A compound is a pure substance where different elements are chemically stuck together in an exact way, like water or methane. Its properties are unique.

🎯 Exam Tip: Key terms for compounds are "chemically combined," "fixed proportion," and "properties differ from constituent elements."

Structure Of Matter And Molecule Long Answer Type Questions

 

Question 26. Explain the effect of temperature on the state of matter.
Answer: Temperature greatly affects the state of matter. When heat is added, particles gain kinetic energy, moving faster and vibrating more. This increased energy helps them overcome the attractive forces, leading to a change in state. For instance, a solid changes to a liquid (melting or fusion) at its melting point, and a liquid changes to a gas (vaporization or boiling) at its boiling point. Ice melts at \( 0^\circ C \) (or \( 273K \)), and water boils at \( 100^\circ C \) (or \( 373 K \)). During these changes, even with continuous heat supply, the temperature remains constant. This is because the absorbed heat (called latent heat) is used to break the bonds between particles, rather than increasing their kinetic energy. Conversely, decreasing temperature reverses these changes: gas turns to liquid (condensation) and liquid turns to solid (freezing or solidification).
In simple words: Heating makes particles move faster, turning solids to liquids, then gases. Cooling does the opposite. These changes happen at fixed temperatures, where 'hidden heat' helps particles change state without getting hotter.

🎯 Exam Tip: Distinguish between changes that increase and decrease temperature and how latent heat plays a role in phase transitions.

 

Question 27. Explain sublimation with an example.
Answer: Sublimation is a special process where certain solids, when heated, change directly into a gas or vapor without first becoming a liquid. The reverse also happens: the gas cools and turns back into a solid, skipping the liquid phase entirely. This method is useful for separating a mixture where one component sublimes and the other does not. Common examples of solids that sublime include camphor, ammonium chloride, and naphthalene. For instance, to separate a mixture of common salt and ammonium chloride, you can heat them. The ammonium chloride will turn directly into a gas, which then cools and solidifies on a cool surface, leaving the common salt behind.
In simple words: Sublimation is when a solid turns straight into a gas (or vice versa) without becoming a liquid. Camphor or ammonium chloride do this. It's used to separate mixtures.

🎯 Exam Tip: Remember key examples of substances that sublime and how this property is used in separation techniques.

 

Question 28. Write the three differences between the states of solid, liquid and gas.
Answer: The three main states of matter—solid, liquid, and gas—have distinct differences in their properties due to how their particles are arranged and interact. Solids have a fixed shape and volume because their particles are very tightly packed with strong attractive forces. Liquids do not have a fixed shape but maintain a fixed volume, as their particles are less tightly packed and have weaker forces, allowing them to flow. Gases have neither a fixed shape nor a fixed volume, as their particles are far apart with very weak attractive forces, letting them move freely and fill any container.

In simple words: Solids have fixed shape and volume, liquids have fixed volume but no fixed shape, and gases have neither. This is because particles are tightly packed in solids, looser in liquids, and very far apart in gases.

🎯 Exam Tip: Create a clear comparison table to quickly recall and differentiate the properties of solids, liquids, and gases based on particle arrangement and forces.

 

Question 29. Write the two differences between the physical and chemical change?
Answer: Here are two key differences between physical and chemical changes:

In simple words: Physical changes just alter how something looks (no new thing), while chemical changes make totally new things and change what the molecules are made of.

🎯 Exam Tip: Remember that the key to a chemical change is the formation of new substances, while physical changes are typically reversible and don't change identity.

 

Question 30. How do we purify the substance with distillation? Explain with a diagram.
Answer: Distillation is a method used to purify substances, especially to separate two liquids with significantly different boiling points (a difference of more than \( 25^\circ C \)) without them breaking down or reacting. Here's how simple distillation works: First, the liquid mixture is placed in a distillation flask and heated slowly. The liquid with the lower boiling point will vaporize first. These vapors then rise and enter a condenser, which is a tube surrounded by cold water. Inside the condenser, the hot vapors cool down and turn back into a liquid (condense). This purified liquid, called the distillate, is collected in a separate beaker. Meanwhile, the liquid with the higher boiling point remains in the distillation flask. For instance, a mixture of acetone and water can be separated this way, as acetone boils at a lower temperature than water.
In simple words: Distillation separates liquids if their boiling points are very different. You heat the mixture, one liquid turns to vapor, cools in a condenser, and turns back into a pure liquid, leaving the other liquid behind.

🎯 Exam Tip: Remember that distillation relies on differences in boiling points and is used for separating miscible liquids or a soluble solid from a liquid.

Structure Of Matter And Molecule Additional Questions Solved

I. Multiple Choice Questions (Mcqs)

 

Question 1. Which of the following is not a matter?
(A) Book
(B) Air
(C) Idea
(D) Cold drink
Answer: (C) Idea
In simple words: Among the given choices, an idea is not considered matter because it does not have mass or occupy space, which are the fundamental properties of matter.

🎯 Exam Tip: Matter is anything that has mass and occupies space; abstract concepts like ideas or emotions are not matter.

 

Question 2. A few substances are arranged in the increasing order of forces of attraction' between their particles. Which one of the following represents a correct arrangement?
(A) Water, Air, Wind
(B) Air, Sugar, Oil
(C) Oxygen, Water, Sugar
(D) Salt, Juice, Air
Answer: (C) Oxygen, Water, Sugar
In simple words: The correct arrangement showing increasing forces of attraction between particles is Oxygen (gas, weakest forces), then Water (liquid, moderate forces), and finally Sugar (solid, strongest forces).

🎯 Exam Tip: Remember the general trend: gases have the weakest intermolecular forces, liquids have moderate, and solids have the strongest.

 

Question 4. Which of the following is not correct regarding gases?
(A) Gases exert pressure.
(B) Gases have a very weak tendency to diffuse
(C) Gases are more compressible than liquids.
(D) Gases have weak intermolecular forces of attraction.
Answer: (B) Gases have a very weak tendency to diffuse
In simple words: The statement that gases have a very weak tendency to diffuse is incorrect. Gases actually diffuse very rapidly and easily because their particles are in constant, fast, random motion and have large spaces between them.

🎯 Exam Tip: Gases diffuse easily due to high kinetic energy and large spaces between particles.

 

Question 5. Which of the following has the highest intermolecular forces of attraction?
(A) Liquid water
(B) Liquid ethyl alcohol
(C) Gaseous \( CO_2 \)
(D) Solid \( CO_2 \)
Answer: (D) Solid \( CO_2 \)
In simple words: Solid \( CO_2 \) (dry ice) has the strongest intermolecular forces of attraction among the options. Solids, by their nature, have the strongest forces holding their particles together.

🎯 Exam Tip: Solids always exhibit stronger intermolecular forces compared to liquids and gases of the same substance.

 

Question 6. Which of the following correctly describes the color inside the gas jars, fifteen minutes after the glass plate is removed?
(A) Dark brown / Colourless
(B) Light brown / Dark brown
(C) Colourless / Dark brown
(D) Light brown / Light brown
Answer: (D) Light brown / Light brown
In simple words: When the glass plate is removed, the red-brown gas will spread upwards into the top jar, and the air will move downwards. After about fifteen minutes, both jars will appear light brown because the gases have mixed evenly throughout.

🎯 Exam Tip: Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, resulting in uniform distribution over time.

 

Question 8. Which one of the following sets of phenomena would increase on raising the temperature?
(A) Diffusion, evaporation, compression of gases
(B) Evaporation, compression of gas, solubility
(C) Evaporation, diffusion, expansion of gases
(D) Evaporation, solubility, diffusion, compression of gases
Answer: (C) Evaporation, diffusion, expansion of gases
In simple words: Raising the temperature increases the kinetic energy of particles. This boosts evaporation (liquids turn to gas faster), diffusion (particles spread out faster), and the expansion of gases (gas volume increases).

🎯 Exam Tip: Higher temperature means higher particle kinetic energy, which generally increases rates of processes involving particle movement and separation.

 

Question 9. Which of the following is a heterogeneous mixture?
(A) A solution of a solid in a liquid
(B) A solution of a solid in another solid
(C) A mixture of a gas in a liquid
(D) A mixture of a solid in another solid
Answer: (D) A mixture of a solid in another solid
In simple words: A heterogeneous mixture is one where the components are not uniformly distributed and can often be seen as separate phases. A mixture of a solid in another solid would typically be heterogeneous.

🎯 Exam Tip: Homogeneous mixtures (solutions) appear uniform, while heterogeneous mixtures have visible distinct components.

 

Question 10. Which of the following is not heterogeneous?
(A) Air
(B) Milk
(C) Steel
(D) Smoke
Answer: (C) Steel
In simple words: Steel is an alloy, which is a solid solution of metals, making it a homogeneous mixture. You cannot see its different components, unlike in milk or smoke.

🎯 Exam Tip: Understand that alloys (like steel) are homogeneous mixtures, while colloids (like milk and smoke) are heterogeneous, even if they sometimes appear uniform.

 

Question 11. Which of the following is a homogeneous system that is not a chemical compound?
(A) Common salt
(B) Oil
(C) Milk
(D) Solution
Answer: (D) Solution
In simple words: A "Solution" is a homogeneous mixture, meaning it looks the same throughout but its parts are not chemically joined, unlike a compound.

🎯 Exam Tip: Differentiate between pure substances (elements, compounds) and mixtures (homogeneous, heterogeneous). Solutions are the best example of homogeneous mixtures.

 

Question 13. A saturated solution of a salt in water can be made unsaturated by:
(A) heating
(B) cooling
(C) evaporation
(D) distillation
Answer: (A) heating
In simple words: A saturated solution contains the maximum amount of solute at a given temperature. To make it unsaturated, you can either add more solvent or increase the temperature, allowing more salt to dissolve.

🎯 Exam Tip: Remember that increasing temperature usually increases the solubility of solid solutes in a liquid, making a saturated solution unsaturated.

 

Question 14. Alum is used for making the finer clay particles in water to settle down faster. This process is known as:
(A) filtration
(B) sedimentation
(C) loading
(D) crystallization
Answer: (C) loading
In simple words: The process of adding alum to water to help fine clay particles settle faster is called loading. Alum helps the tiny clay particles clump together, making them heavier so they sink more quickly.

🎯 Exam Tip: Loading (or coagulation/flocculation) is a technique used in water purification to enhance the sedimentation of fine suspended impurities.

 

Question 15. A mixture consists of a soluble solid in a liquid. Which method is most suitable for obtaining both the components?
(A) Fractional distillation
(B) Sublimation
(C) Distillation
(D) Evaporation
Answer: (C) Distillation
In simple words: To obtain both a soluble solid and the liquid from their mixture, distillation is the most suitable method. Evaporation would only recover the solid.

🎯 Exam Tip: Use distillation if you want to recover both the solvent and the solute from a solution where the solute is non-volatile. Evaporation is for recovering only the solute.

 

Question 16. Identify the chemical change in the following:
(A) Freezing of water
(B) Rusting of iron
Answer: (B) Rusting of iron
In simple words: Rusting of iron is a chemical change because a new substance, iron oxide (rust), is formed when iron reacts with oxygen and water. Freezing of water is a physical change.

🎯 Exam Tip: A chemical change always produces new substances with different chemical properties.

 

Question 18. Which of the following mixtures can be best separated by sublimation?
(A) common salt and sodium nitrate
(B) benzene and water
(C) iodine and sand
(D) sodium sulphate and sand
Answer: (C) iodine and sand
In simple words: Sublimation is ideal for separating mixtures where one component sublimes (changes directly from solid to gas) and the other does not. Iodine is known to sublime, while sand does not.

🎯 Exam Tip: Identify substances that sublime (e.g., iodine, camphor, ammonium chloride) to correctly apply this separation technique.

 

Question 19. Which of the following mixtures cannot be separated by using a separating funnel?
(A) Kerosene oil and water
(B) Ethyl alcohol and water
(C) Turpentine oil and water
(D) Benzene and water
Answer: (B) Ethyl alcohol and water
In simple words: A separating funnel is used to separate immiscible liquids, which are liquids that do not mix. Ethyl alcohol and water are miscible, meaning they mix completely, and cannot be separated this way.

🎯 Exam Tip: Separating funnels are specifically for immiscible liquids that form separate layers.

 

Question 20. Which of the following is not a chemical change?
(A) Baking of cake
(B) Ripening of fruits
(C) Formation of clouds
(D) Rancidity of butter
Answer: (C) Formation of clouds
In simple words: The formation of clouds is a physical change, as it involves water changing state from vapor to liquid droplets, but it remains water. Baking, ripening fruits, and rancidity are all chemical changes.

🎯 Exam Tip: Remember that phase changes (like condensation in cloud formation) are physical changes, while processes that create new chemical substances are chemical changes.

 

Question 2. What is the matter?
Answer: Matter is anything that takes up space and has weight. All objects around us are made of matter.
In simple words: Matter is anything that has mass and takes up space.

🎯 Exam Tip: Remember that 'mass' refers to the amount of material in an object, and 'volume' refers to the space it occupies.

 

Question 3. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at \(273\) K cools more effectively than water at the same temperature because ice absorbs extra heat (called latent heat of fusion) from its surroundings as it melts. This absorbed heat is used to change its state from solid to liquid, leading to a stronger cooling effect.
In simple words: Ice cools things better because it takes more heat to melt it into water, making the surroundings colder.

🎯 Exam Tip: The 'latent heat of fusion' is the hidden heat energy absorbed during melting without a temperature rise, making ice a powerful coolant.

 

Question 4. What are the two ways in which the physical state of matter can be changed?
Answer: The physical state of matter can be changed in two main ways: (a) By changing the pressure. (b) By changing the temperature. Understanding these changes helps us explain everyday phenomena like boiling and freezing.
In simple words: We can change matter's state by changing its pressure or its temperature.

🎯 Exam Tip: Always remember that both temperature and pressure play a role, not just one, in determining a substance's state.

 

Question 5. What is the melting point of ice?
Answer: The melting point of ice is \(0^\circ\text{C}\), which is the same as \(273\) Kelvin. This is the exact temperature at which ice turns into liquid water.
In simple words: Ice melts at \(0^\circ\text{C}\) or \(273\) K.

🎯 Exam Tip: It's important to know both Celsius and Kelvin values for melting and boiling points, as they are often used interchangeably in physics and chemistry.

 

Question 6. Define element.
Answer: An element is a very basic type of matter that cannot be broken down into simpler substances using ordinary chemical methods. For example, gold and oxygen are elements.
In simple words: An element is a pure substance that cannot be split into anything simpler by chemical reactions.

🎯 Exam Tip: Think of elements as the fundamental building blocks from which all other matter is made.

 

Question 7. What is meant by pure substance?
Answer: A pure substance is made up of only one kind of particle, and all these particles are exactly the same in their chemical properties. Water is a good example of a pure substance.
In simple words: A pure substance has only one type of particle, and all its parts are chemically identical.

🎯 Exam Tip: Remember that compounds (like water) are also pure substances, as they consist of identical molecules.

 

Question 9. Name the apparatus used for separating a mixture of immiscible liquid.
Answer: A separating funnel is the apparatus used to separate a mixture of two liquids that do not mix with each other (immiscible liquids). This method works because the liquids have different densities.
In simple words: A separating funnel is used to separate liquids that don't mix.

🎯 Exam Tip: Immiscible liquids always form distinct layers, with the denser liquid at the bottom.

 

Question 10. Define mixture.
Answer: A mixture is a material made from two or more different substances that are mixed together but not chemically joined. The amounts of these substances can vary. For example, air is a mixture of several gases.
In simple words: A mixture is when different things are put together but don't chemically combine.

🎯 Exam Tip: The components of a mixture keep their individual properties and can often be separated by physical means.

 

Question 11. Define the term atom.
Answer: An atom is the smallest possible particle of an element that can take part in a chemical reaction. Atoms can sometimes exist alone, or they might join with other atoms.
In simple words: An atom is the tiniest part of an element that can join in a chemical reaction.

🎯 Exam Tip: Although atoms are the smallest unit of an element, they are composed of even smaller subatomic particles like protons, neutrons, and electrons.

 

Question 12. Atoms of most elements are not able to exist independently. 'Name two atoms which exist as independent atoms.
Answer: While most atoms cannot exist independently, noble gases like Argon (\(Ar\)) and Helium (\(He\)) are exceptions. These atoms are very stable and do not need to combine with other atoms.
In simple words: Argon and Helium are two atoms that can exist on their own without joining others.

🎯 Exam Tip: Noble gases are characterized by their full outer electron shells, which makes them chemically unreactive and allows their atoms to be stable on their own.

 

Question 13. Define atomicity.
Answer: Atomicity refers to the total number of atoms found in a single molecule of an element or a compound. For example, an oxygen gas molecule (\(O_2\)) has an atomicity of two.
In simple words: Atomicity is how many atoms are in one molecule.

🎯 Exam Tip: Knowing the atomicity helps determine the molecular formula and chemical properties of a substance.

 

Question 14. From the following list choose elements, compounds, and mixture:
(a) Air
(b) Graphite
(c) Water
(d) Mercury
Answer: Let's classify the given substances: (a) Air: This is a **mixture** because it contains different gases like nitrogen and oxygen that are not chemically bonded. (b) Graphite: This is an **element**, as it is a form of pure carbon. (c) Water: This is a **compound**, made from hydrogen and oxygen chemically joined (\(H_2O\)). (d) Mercury: This is an **element**, a pure substance that cannot be broken down further. Knowing the difference helps in understanding how matter behaves.
In simple words: Air is a mixture, graphite and mercury are elements, and water is a compound.

🎯 Exam Tip: Elements and compounds are pure substances, while mixtures contain multiple substances that are not chemically bonded.

Structure of Matter and Molecule Solutions Short Answer Type Questions

 

Question 1. At room temperature why oxygen is a gas while water is a liquid?
Answer: At room temperature, oxygen exists as a gas, while water is a liquid. This difference occurs because the forces of attraction between oxygen molecules are much weaker compared to those between water molecules. Water molecules are strongly held together, keeping water in a liquid state. In contrast, oxygen molecules are not strongly attracted, allowing them to move freely as a gas. This shows how intermolecular forces influence the state of matter.
In simple words: Oxygen is a gas because its molecules don't stick together strongly, but water is a liquid because its molecules pull on each other more firmly.

🎯 Exam Tip: The strength of intermolecular forces determines a substance's physical state at a given temperature and pressure.

 

Question 2. Liquids generally have a lower density as compared to solids but you must have observed that ice floats on water. Find out why.
Answer: Generally, liquids are less dense than solids. However, ice (solid water) floats on liquid water. This happens because ice has a unique open, cage-like structure. When water freezes, its molecules arrange themselves in a way that creates more empty spaces between them. This increases the volume of the ice compared to the same mass of liquid water, making ice less dense and causing it to float. This property is vital for aquatic life in cold climates.
In simple words: Ice floats because when water freezes, its molecules spread out and create more empty space, making the ice lighter than the same amount of liquid water.

🎯 Exam Tip: Ice being less dense than water is an anomalous property of water, crucial for life on Earth as it prevents water bodies from freezing solid from the bottom up.

 

Question 3. What is the importance of the melting point of a solid or boiling point of a liquid?
Answer: The melting point of a solid and the boiling point of a liquid are important for two reasons: (a) They tell us about the strength of the attractive forces holding the particles together in a substance. Higher melting or boiling points mean stronger forces. (b) These points act as a test for the purity of a substance. Every pure substance has its own specific melting and boiling points, so if a substance melts or boils at a different temperature, it indicates impurities. These fixed points help scientists identify and confirm substances.
In simple words: Melting and boiling points show how strong the particle attractions are and help us check if a substance is pure.

🎯 Exam Tip: Pure substances have sharp and distinct melting/boiling points, while impure substances melt/boil over a range of temperatures.

 

Question 5. Classify the following into physical changes and chemical changes:
(a) Rusting of an iron nail.
(b) Melting of butter
(c) Burning of wax candle
(d) Glowing of an electric lamp.
Answer: Here's how to classify each change: (a) Rusting of an iron nail: This is a **chemical change** because new substances (iron oxides) are formed. (b) Melting of butter: This is a **physical change** as the butter changes form but remains butter. (c) Burning of wax candle: This is a **chemical change** because the wax reacts with oxygen to form new substances like carbon dioxide and water vapor. (d) Glowing of an electric lamp: This is a **physical change** as the filament gets hot and glows, but its chemical composition does not change. Understanding these differences helps us recognize if new materials are being created.
In simple words: Rusting and burning are chemical changes (new stuff made), while melting and glowing a lamp are physical changes (same stuff, different look).

🎯 Exam Tip: A key indicator of a chemical change is the formation of new substances, which is not reversible by simple physical methods.

 

Question 6. How is melting point related to intermolecular forces of attraction? Why is latent heat of vaporisation higher than latent heat of fusion?
Answer: The melting point of a substance is directly linked to its intermolecular forces of attraction; stronger forces mean a higher melting point, as more energy is needed to break these bonds. Latent heat of vaporisation is higher than latent heat of fusion because changing a liquid to a gas requires much more energy to completely overcome the strong attractions between particles and move them far apart. In contrast, melting a solid to a liquid only requires enough energy to weaken these attractions slightly, allowing particles to slide past each other. This difference in energy explains why boiling takes more heat than melting.
In simple words: Stronger forces between particles mean a higher melting point. Turning liquid into gas needs more heat than turning solid into liquid because particles in gas are much further apart.

🎯 Exam Tip: Remember that latent heat is absorbed or released during a phase change without a change in temperature, making it "hidden" heat.

 

Question 7. How will you determine the boiling point of water, experimentally? Draw a neat and labeled diagram.
Answer: To experimentally determine the boiling point of water, follow these steps: 1. Obtain a boiling tube and fit it with a cork that has a single hole. 2. Carefully insert a thermometer through the hole in the cork, making sure its bulb is above the water level, not touching the bottom or sides of the tube. 3. Fill the boiling tube with water. 4. Begin heating the boiling tube gently. 5. Continuously observe the thermometer. The temperature will rise until the water starts boiling vigorously and turning into vapor. At this point, the thermometer reading will become stable and remain constant, even if heating continues. This stable temperature is the boiling point of water. **Conclusion:** Water boils at \(100^\circ\text{C}\) (or \(373\) K), and its temperature stays constant during the entire process of changing into vapor. The added heat becomes latent heat of vaporisation.
In simple words: To find water's boiling point, heat water with a thermometer in it. When it boils, the temperature stops rising and stays steady at \(100^\circ\text{C}\), which is its boiling point.

🎯 Exam Tip: Ensure the thermometer bulb is in the steam, not the boiling water, for an accurate boiling point reading.

 

Question 8. Name A, B, C, D, E and F in the following diagram showing a change in its state: Increase heat and decrease pressure
Answer: In the given diagram illustrating changes in the states of matter: (A) **Melting (fusion)**: This is the process where a solid changes into a liquid. (B) **Vaporisation**: This is when a liquid changes into a gas. (C) **Condensation**: This is the process of a gas turning into a liquid. (D) **Freezing (Solidification)**: This refers to a liquid changing into a solid. (E) **Sublimation**: This is a direct change from solid to gas. (F) **Solidification of vapours (Sublimation)**: This is the reverse of sublimation, where gas directly turns into solid, also known as deposition. These processes are essential for understanding the physical properties of matter.
In simple words: A is melting, B is turning to gas, C is turning to liquid, D is freezing, E is solid to gas, and F is gas to solid.

🎯 Exam Tip: Pay close attention to the direction of the arrows in phase change diagrams; they indicate whether heat is being absorbed or released.

 

Question 10. Why is crystallisation considered a better technique than evaporation?
Answer: Crystallisation is a better purification method than simple evaporation for a few key reasons:

• During crystallisation, any impurities that might still be dissolved in the solution are effectively removed, resulting in a purer solid.
• Some solid substances, such as sugar, can break down or burn (char) when all the liquid evaporates under strong heat. Crystallisation avoids this by allowing crystals to form without excessive heating.

This makes crystallisation a gentler and more thorough way to get pure solids.
In simple words: Crystallisation is better because it removes impurities more completely and prevents sensitive solids from burning or breaking down during drying.

🎯 Exam Tip: Crystallisation is preferred for heat-sensitive compounds or when impurities are volatile and would evaporate along with the solvent.

 

Question 11. State the condition for using the method of centrifugation to separate contents of a mixture. State the principle involved in this process.
Answer: Centrifugation is used to separate mixture components when their densities are significantly different. The main principle behind this process is that when a mixture is spun rapidly in a centrifuge machine, the denser particles are forced outwards to settle at the bottom of the container, while the lighter particles remain closer to the top. This high-speed rotation effectively magnifies the gravitational force, causing separation.
In simple words: Centrifugation works best for mixtures where parts have different weights. Spinning quickly pushes heavier parts to the bottom and lighter parts stay on top.

🎯 Exam Tip: Centrifugation is widely used in laboratories and industries, for example, to separate cream from milk or blood cells from plasma.

 

Question 12. Classify the following as elements and compounds.
(a) Silver
(b) Methane
(c) Water
(d) Mercury
Answer: Let's classify these substances:

• **Elements:** Silver, Mercury. These are pure substances made of only one type of atom.
• **Compounds:** Methane, Water. These are substances formed when two or more different elements are chemically combined in a fixed ratio.

This classification helps us understand the basic building blocks of matter.
In simple words: Silver and Mercury are elements. Methane and Water are compounds.

🎯 Exam Tip: Remember that elements are listed in the periodic table, while compounds are formed from elements joined chemically.

 

Question 14. What determines the state of a substance? Suggest a method to liquefy gases. Water droplets are observed on the outer surface of a glass tumbler containing ice cold water. Give reason.
Answer: The state of a substance (solid, liquid, or gas) is primarily determined by its **pressure** and **temperature**. To liquefy gases, one can apply high pressure and lower the temperature. This brings the gas particles closer together, allowing attractive forces to form. The water droplets seen on the outside of a cold glass are due to water vapor in the air condensing into liquid water when it comes into contact with the cold surface. This is the same process that forms clouds.
In simple words: Pressure and temperature decide a substance's state. You can turn gas into liquid with high pressure and low temperature. Water droplets form on a cold glass because warm, moist air touches it and turns into liquid.

🎯 Exam Tip: Condensation is a phase change from gas to liquid, requiring the gas to lose heat, which happens on a colder surface.

Structure of Matter and Molecule Long Answer Type Questions

 

Question 1. What are the different states of matter? Name the factors which determine the state of matter. Explain.
Answer: Matter commonly exists in three main states: **Solid**, **Liquid**, and **Gas**. The state of matter is determined by two crucial factors: 1. **Temperature**: Temperature controls the kinetic energy of the particles. Higher temperatures mean particles move faster, increasing their kinetic energy and leading to transitions from solid to liquid, or liquid to gas. 2. **Pressure**: Pressure affects how close particles are to each other, influencing the intermolecular forces of attraction. Higher pressure can force particles closer, enabling gases to turn into liquids or even solids. These two factors work together to decide whether a substance will be solid, liquid, or gas at any given moment.
In simple words: Matter is solid, liquid, or gas. Its state depends on temperature (how fast particles move) and pressure (how close particles are).

🎯 Exam Tip: Always remember that the balance between particle kinetic energy and intermolecular forces dictates the physical state of matter.

 

Question 2. Explain the following:
1. Ice floats over water.
2. Dry ice sublimes.
3. Two cubes of ice are pressed hard between the palm when the pressure is released the two cubes join together.
4. The temperature of the water does not change once the boiling starts, in spite of the heat being supplied.
Answer:1. **Ice floats over water:** Ice has a unique open, cage-like structure. When water freezes, its volume expands, making it less dense than liquid water. This lower density allows ice to float on top of the water. This property is crucial for aquatic life. 2. **Dry ice sublimes:** Dry ice, which is solid carbon dioxide, does not melt into a liquid. Instead, its vapor pressure becomes equal to the atmospheric pressure at a temperature well below its melting point, causing it to change directly from a solid to a gas. This process is called sublimation. 3. **Two cubes of ice pressed together join when released:** When pressure is applied to ice, its melting point temporarily lowers. This causes a thin layer of ice between the cubes to melt. When the pressure is released, the melting point returns to normal, and this thin layer of water refreezes, bonding the two cubes together. This phenomenon is known as regelation. 4. **Water temperature stays constant during boiling:** Once water reaches its boiling point, any additional heat energy supplied is used to overcome the forces of attraction between liquid particles, converting them into gas. This energy, known as latent heat of vaporisation, does not increase the temperature but rather changes the state of the substance.
In simple words:1. Ice floats because it's less dense than water due to its open structure. 2. Dry ice turns straight into gas because its vapor pressure matches the air pressure before it can melt. 3. Pressing ice cubes makes a bit of ice melt, then when released, it refreezes and joins the cubes. 4. Water's temperature doesn't rise during boiling because the heat goes into turning it into steam, not making it hotter.

🎯 Exam Tip: For each explanation, clearly state the underlying principle (e.g., density, vapor pressure, effect of pressure on melting point, latent heat) to score full marks.

 

Question 3. What is meant by fractional distillation? How is it differ from simple distillation? What part of the fractional distillation apparatus makes it more efficient and possess an advantage over simple distillation process? Explain using a diagram. Give one example of fractional distillation in industry.
Answer: Fractional distillation is a technique used to separate a mixture of two or more miscible liquids that have boiling points close to each other. It's similar to simple distillation, where a volatile liquid is vaporized and then condensed back into liquid form. **Difference from Simple Distillation:** Simple distillation separates liquids with widely different boiling points. Fractional distillation, however, uses a fractionating column filled with beads or packing material. This column provides a large surface area for repeated vaporization and condensation cycles. **Efficiency of the Fractionating Column:** The fractionating column is the key component that makes this process more efficient. Liquids with higher boiling points condense further down the column and drip back, while liquids with lower boiling points rise higher, allowing for better separation of components. The longer the column, the more efficient the separation. This advantage is crucial for separating liquids with small differences in boiling points. **Industrial Example:** A prime example of fractional distillation in industry is the separation of different components from crude petroleum in oil refineries, such as gasoline, diesel, and kerosene. This method is vital for many industrial processes.
In simple words: Fractional distillation separates liquids with close boiling points using a special column that makes it more efficient than simple distillation. It's used to get different fuels from crude oil.

🎯 Exam Tip: Always emphasize the role of the fractionating column in providing multiple theoretical distillation stages, enabling better separation of closely boiling liquids.

 

Question 4. A child worked to separate the mixture of dyes constituting a sample of ink. He marked a line by the ink on the filter paper and placed the filter paper in a glass containing water as shown in the figure. The filter paper was removed when the water moved near the top of the filter paper.
(a) What would you expect to see, if the ink contains three different coloured components?
(b) Name the technique used by the child.
(c) Suggest one more application of this technique.
Answer:(a) If the ink has three different colored components, you would observe three distinct colored spots at different heights on the filter paper. The most soluble color would travel highest up the paper, the moderately soluble one would be in the middle, and the least soluble color would remain closest to the starting line. This happens because each component moves at a different speed. (b) The technique used by the child is called **Chromatography**. (c) This technique is also used to separate and identify various components, such as amino acids found in blood after hydrolysis. It's a powerful tool for analysis in science.
In simple words:(a) If the ink has three colors, you'd see three color spots on the paper, each at a different height, based on how well it mixes with water. (b) The method is called chromatography. (c) It can also be used to find different amino acids in blood.

🎯 Exam Tip: In chromatography, the separation depends on the differing solubilities and adsorption properties of the components in the stationary and mobile phases.