Matter in Our Surroundings — NCERT Solutions | Class 9 Science | Academia Aeternum
Ch 1  ·  Q–
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Class 9 Science Exercise NCERT Solutions Olympiad Board Exam
Chapter 1

Matter in Our Surroundings

Step-by-step NCERT solutions with stress–strain analysis and exam-oriented hints for Boards, JEE & NEET.

9 Questions
20–30 min Ideal time
Q1 Now at
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Q1
NUMERIC3 marks

Convert the following temperatures to the Celsius scale.

(a) 293 K
(b) 470 K

📘 Concept & Theory Concept Used: Temperature Scales ›
p> Temperature can be expressed using different scales. The two most commonly used scales in science are the Kelvin scale (K) and the Celsius scale (°C).

The Kelvin scale is the SI unit of temperature and is widely used in scientific calculations. To convert a temperature from Kelvin to Celsius, we subtract 273 from the Kelvin value.

The relationship between the two scales is:

\[ \text{Temperature in Celsius} = \text{Temperature in Kelvin} - 273 \]

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Write the given temperature in Kelvin.

  2. Use the conversion formula: \[ ^\circ C = K - 273 \]

  3. Substitute the given Kelvin value into the formula.

  4. Perform the subtraction carefully.

  5. Write the answer with the unit °C.

📊 Graph / Figure Graph / Figure ›
K Kelvin Scale Subtract 273 °C Celsius Scale
Fig. 1 — Converting Kelvin to Celsius
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  8 steps
  1. (a) Convert 293 K into Celsius.
  2. Using the formula:\[^\circ C = K - 273\]
  3. Substituting the given value:\[^\circ C = 293 - 273\]
  4. Calculating:\[^\circ C = 20\]
  5. Therefore,\[293\;K = 20^\circ C\]
  6. (b) Convert 470 K into Celsius.
  7. Using the formula:\[ ^\circ C = K - 273\]
  8. Substituting the given value:\[ ^\circ C = 470 - 273\]
  9. Calculating:\[ ^\circ C = 197\]
  10. Therefore,\[470\;K = 197^\circ C\]
💡 Answer Final Answer ›
  1. (1)(a) 293 K = 20 °C
  2. (2)(b) 470 K = 197 °C
🎯 Exam Significance Exam Significance ›

This question tests the basic understanding of temperature scales and unit conversion, a fundamental concept in the chapter "Matter in Our Surroundings".

Questions based on Kelvin–Celsius conversion are frequently asked in:

  • CBSE Class 9 examinations
  • School periodic tests and annual examinations
  • NTSE foundation-level science questions
  • Olympiad examinations
  • NEET and JEE foundation courses

A clear understanding of temperature conversion is essential because melting point, boiling point, latent heat, and state-change calculations are often expressed using the Kelvin scale in science.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  6 points

  1. Kelvin is the SI unit of temperature.

  2. To convert Kelvin to Celsius, subtract 273.

  3. \[^\circ C = K - 273\]

  4. \[293\;K = 20^\circ C\]

  5. \[470\;K = 197^\circ C\]

  6. Temperature-scale conversion is a frequently tested concept in school and competitive examinations.
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1 / 9  ·  11%
Q2 →
Q2
NUMERIC3 marks

Convert the following temperatures to the Kelvin scale.

(a) 250°C
(b) 373°C

📘 Concept & Theory Concept Used: Temperature Scales ›

The Kelvin scale is the SI unit of temperature and is extensively used in scientific studies. Since the Kelvin scale begins at absolute zero, its readings are always 273 units higher than the corresponding Celsius temperatures.

To convert a temperature from Celsius to Kelvin, we add 273 to the Celsius value.

The conversion formula is:

\[ \text{Temperature in Kelvin} = \text{Temperature in Celsius} + 273 \]

Or simply,

\[ K = ^\circ C + 273 \]

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Write the given temperature in degrees Celsius.

  2. Use the conversion formula: \[ K = ^\circ C + 273 \]

  3. Substitute the given Celsius value into the formula.

  4. Add 273 carefully.

  5. Express the final answer in Kelvin (K).

📊 Graph / Figure Graph / Figure ›
°C Celsius Scale Add 273 K Kelvin Scale
Fig. 1 — Temperature Conversion
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  8 steps
  1. (a) Convert 250 °C into Kelvin.
  2. Using the formula:\[K = ^\circ C + 273\]
  3. Substituting the given value:\[K = 250 + 273\]
  4. Calculating:\[K = 523\]
  5. Therefore,\[250^\circ C = 523\;K\]
  6. (b) Convert 373 °C into Kelvin.
  7. Using the formula:\[K = ^\circ C + 273\]
  8. Substituting the given value:\[K = 373 + 273\]
  9. Calculating:\[K = 646\]
  10. Therefore,\[373^\circ C = 646\;K\]
💡 Answer Final Answer ›
  1. (1)(a) 250 °C = 523 K
  2. (2)(b) 373 °C = 646 K
🎯 Exam Significance Exam Significance ›

Temperature conversion between Celsius and Kelvin is one of the most important foundational concepts in this chapter. Many numerical questions involving melting point, boiling point, latent heat, evaporation, condensation, and change of state use the Kelvin scale.

This concept is frequently tested in:

  • CBSE Class 9 school examinations
  • Unit tests and annual examinations
  • NTSE foundation-level science questions
  • Science Olympiads
  • NEET and JEE foundation programmes

Students should memorize both conversion formulas because they are repeatedly used throughout the chapter and in higher classes.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  5 points

  1. Kelvin is the SI unit of temperature.

  2. The conversion formula is: K = °C + 273.

  3. The freezing point of water is 273.15 K.

  4. The boiling point of water is 373.15 K.

  5. Kelvin is used in scientific calculations and measurements.
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2 / 9  ·  22%
Q3 →
Q3
NUMERIC3 marks

Give a reason for the following observations.

(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.

📘 Concept & Theory Concept Used: Sublimation and Diffusion ›

Matter is made up of tiny particles that are continuously moving. Depending on the nature of the substance and the surrounding conditions, these particles may change their state or spread from one place to another.

Two important concepts involved in this question are:

  1. Sublimation: The process in which a solid changes directly into vapour without passing through the liquid state.
  2. Diffusion: The spontaneous intermixing of particles of two substances due to the random motion of their particles.

Naphthalene, camphor, and ammonium chloride are common substances that undergo sublimation. Perfume demonstrates diffusion because its vapour particles spread rapidly through air.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Identify the phenomenon described in each observation.

  2. Relate the observation to the properties of particles of matter.

  3. Use the appropriate scientific term such as sublimation or diffusion.

  4. Explain the observation using particle movement.

📊 Graph / Figure Graph / Figure ›
Sublimation of Naphthalene Diffusion of Perfume Vapours
Sublimation and Diffusion: Two Key Processes in Matter
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  10 steps
  1. (a) Naphthalene balls disappear with time without leaving any solid.
  2. Naphthalene is a substance that undergoes sublimation.
  3. During sublimation, naphthalene changes directly from the solid state into vapour without first becoming a liquid.
  4. The particles of naphthalene gain enough energy to break free from the solid structure and disperse into the air.
  5. As this process continues, the entire solid eventually disappears without leaving any residue.
  6. Therefore, naphthalene balls disappear with time because they undergo sublimation.
  7. (b) We can get the smell of perfume sitting several metres away.
  8. Perfume contains volatile substances that easily evaporate at room temperature.
  9. When perfume is sprayed, its particles gain energy and escape into the air as vapour.
  10. These vapour particles then spread out and mix with the surrounding air through the process of diffusion.
  11. The random motion of the perfume particles causes them to move in all directions, allowing the scent to reach our nose even when we are several metres away.
  12. Therefore, we can smell perfume from a distance because its particles undergo diffusion through the air.
💡 Answer Final Answer ›

(a) Naphthalene balls disappear with time because naphthalene undergoes sublimation, changing directly from solid to vapour without passing through the liquid state.

(b) We can smell perfume from several metres away because perfume vapours spread through air by the process of diffusion.

🎯 Exam Significance Exam Significance ›

This question tests the understanding of two fundamental concepts of the chapter: sublimation and diffusion.

Such reasoning-based questions are frequently asked in:

  • CBSE Class 9 examinations
  • School unit tests and annual examinations
  • Science Olympiads
  • NTSE foundation-level tests
  • NEET and JEE foundation programmes

Students should be able to connect everyday observations with scientific concepts because competency-based and application-oriented questions often follow this pattern.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  7 points

  1. Sublimation is the direct conversion of a solid into vapour.

  2. Naphthalene is a sublimable substance.

  3. Diffusion is the intermixing of particles due to their continuous motion.

  4. Perfume vapours spread through air by diffusion.

  5. Gases diffuse much faster than liquids and solids.

  6. The motion of particles of matter explains many everyday phenomena.

  7. Understanding sublimation and diffusion is essential for board examinations and competitive entrance preparation.
← Q2
3 / 9  ·  33%
Q4 →
Q4
NUMERIC3 marks

Arrange the following substances in increasing order of forces of attraction between the particles:

Water, Sugar, Oxygen

📘 Concept & Theory Concept and Theory: Intermolecular Forces and States of Matter ›

The particles of matter attract one another. The strength of this attraction determines whether a substance exists as a solid, liquid, or gas.

  • In gases, particles are far apart and the force of attraction between them is very weak.
  • In liquids, particles are closer together and the force of attraction is intermediate.
  • In solids, particles are tightly packed and the force of attraction is very strong.

Therefore, the general trend is:

Gas < Liquid < Solid

In the given question:

  • Oxygen is a gas.
  • Water is a liquid.
  • Sugar is a solid.
🗺️ Solution Roadmap Step-by-step Plan ›

  1. Identify the physical state of each substance.

  2. Recall the relative strength of intermolecular forces in solids, liquids, and gases.

  3. Arrange the substances from the weakest force of attraction to the strongest force of attraction.

  4. Write the final order

📊 Graph / Figure Graph / Figure ›
Force of Attraction Between Particles INCREASING FORCE Oxygen (Gas) Water (Liquid) Sugar (Solid)
Forces of Attraction in Different States of Matter
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  7 steps
  1. Identify the physical state of each substance.
    • Oxygen → Gas
    • Water → Liquid
    • Sugar → Solid
  3. Recall the relative strength of intermolecular forces in solids, liquids, and gases.
  4. In gases, particles are very far apart, so the force of attraction is the weakest.
  5. In liquids, particles are closer together, so the force of attraction is greater than in gases.
  6. In solids, particles are tightly packed, so the force of attraction is the strongest.
  7. Therefore:
    Oxygen has the weakest force of attraction.
    Water has a moderate force of attraction.
    Sugar has the strongest force of attraction.
  8. Arrange in increasing order.
  9. Increasing order means arranging from the weakest force of attraction to the strongest force of attraction.
  10. Hence,\[\text{Oxygen} < \text{Water} < \text{Sugar}\]
💡 Answer Final Answer ›

The substances arranged in increasing order of forces of attraction between the particles are:

Oxygen < Water < Sugar.

🎯 Exam Significance Exam Significance ›

This question is directly based on one of the most important concepts of the chapter: the relationship between the state of matter and the force of attraction between particles.

Such questions frequently appear in:

  • CBSE Class 9 board-pattern examinations
  • School unit tests and term examinations
  • Competency-based and assertion-reason questions
  • Science Olympiads
  • NTSE foundation-level examinations
  • NEET and JEE foundation programmes

Students should remember that many questions ask them to compare substances based on their physical states, melting points, boiling points, compressibility, and intermolecular forces.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  7 points

  1. Particles of matter attract one another.

  2. Gases have the weakest intermolecular forces.

  3. Liquids have intermediate intermolecular forces.

  4. Solids have the strongest intermolecular forces.

  5. General order: \[ \text{Gas} < \text{Liquid} < \text{Solid} \]

  6. Oxygen (gas) < Water (liquid) < Sugar (solid) in terms of force of attraction.

  7. Understanding the relationship between states of matter and intermolecular forces is crucial for board examinations and competitive entrance preparation.
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4 / 9  ·  44%
Q5 →
Q5
NUMERIC3 marks

What is the physical state of water at—

(a) 25°C
(b) 0°C
(c) 100°C

📘 Concept & Theory Concept & THeory: Physical States of Water and Temperature ›

Matter can exist in different physical states such as solid, liquid, and gas. The physical state of a substance depends on temperature and pressure.

Water is a unique substance because it can exist in more than one state at certain temperatures. Under normal atmospheric pressure:

  • 0°C is the melting point of ice.
  • 100°C is the boiling point of water.
  • At temperatures between 0°C and 100°C, water generally exists in the liquid state.

During melting and boiling, two physical states can coexist because the heat supplied is used as latent heat to change the state rather than increase the temperature.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Recall the melting point of water (0°C).

  2. Recall the boiling point of water (100°C).

  3. Compare each given temperature with these reference temperatures.

  4. Determine the physical state present at each temperature.

  5. Write the final answer with proper reasoning.

📊 Graph / Figure Graph / Figure ›
Physical States of Water 0°C Solid (Ice) + Liquid 25°C Liquid Water 100°C Liquid + Gas (Steam)
Physical States of Water at Different Temperatures
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  9 steps
  1. (a) Physical state of water at 25°C
  2. The temperature 25°C lies between the melting point (0°C) and boiling point (100°C) of water.
  3. Therefore, water exists in the liquid state at 25°C.
  4. Hence, \[\text{Water at } 25^\circ C \text{ is a liquid.}\]
  5. (b) Physical state of water at 0°C
  6. The temperature 0°C corresponds to the melting point of water.
  7. Therefore, water exists in the solid state at 0°C.
  8. Hence, \[\text{Water at } 0^\circ C \text{ is a solid.}\]
  9. (c) Physical state of water at 100°C
  10. The temperature 100°C corresponds to the boiling point of water.
  11. Therefore, water exists in the gaseous state at 100°C.
  12. Hence, \[\text{Water at } 100^\circ C \text{ is a gas (steam).}\]
💡 Answer Final Answer ›
  • (a) \[ 25^\circ C \rightarrow \text{Liquid} \]
  • (b) \[ 0^\circ C \rightarrow \text{Solid (ice) and Liquid (water)} \]
  • (c) \[ 100^\circ C \rightarrow \text{Liquid (water) and Gas (steam)} \]
🎯 Exam Significance Exam Significance ›

This question is important because it tests the concepts of melting point, boiling point, latent heat, and change of state, which are central topics in the chapter "Matter in Our Surroundings."

Similar questions are frequently asked in:

  • CBSE Class 9 examinations
  • School unit tests and annual examinations
  • Competency-based and case-study questions
  • Science Olympiads
  • NTSE foundation examinations
  • NEET and JEE foundation programmes

Understanding why two states coexist at melting and boiling points is essential for solving advanced questions related to latent heat and phase transitions.

Exam Insight

Students often incorrectly write only one state at 0°C and 100°C. Remember that at the melting point and boiling point, two states can coexist because the supplied heat is used for changing the state of matter.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  6 points

  1. Water melts at: \[ 0^\circ C \]

  2. Water boils at: \[ 100^\circ C \]

  3. Latent heat is involved when a substance changes state at a constant temperature.

  4. Melting and boiling are examples of change of state.

  5. At 100°C, water and steam can coexist.

  6. At 0°C, ice and water can coexist.

← Q4
5 / 9  ·  56%
Q6 →
Q6
NUMERIC3 marks

Give two reasons to justify:

(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.

📘 Concept & Theory concept and Theory ›

The physical state of a substance depends on the arrangement of its particles, the force of attraction between them, and the temperature of the surroundings.

Solids and liquids have distinct characteristics:

  • Liquids have a definite volume but no definite shape. They can flow and take the shape of the container in which they are kept.
  • Solids have both definite shape and definite volume because their particles are tightly packed and held together by strong forces of attraction.

To justify the state of a substance, we examine its properties and compare the surrounding temperature with its melting point and boiling point.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Identify the physical state being discussed.

  2. Recall the characteristic properties of that state of matter.

  3. Relate the given substance to those properties.

  4. Provide two scientifically correct reasons to justify the answer.

📊 Graph / Figure Graph / Figure ›
Properties of Liquid and Solid States Water (Liquid) Fluid Shape • Fixed Volume Iron Almirah (Solid) Fixed Shape • Fixed Volume
Properties of Water and Iron Almirah at Room Temperature
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  12 steps
  1. (a) Water at room temperature is a liquid.
  2. Room temperature is approximately:\[25^\circ C\]
  3. Water melts at:\[0^\circ C\]
  4. and boils at:\[100^\circ C\]
  5. Since room temperature lies between 0°C and 100°C, water naturally exists in the liquid state.
  6. Therefore, water at room temperature is a liquid.
  7. The particles of water have moderate intermolecular attraction. They can move freely past one another, allowing water to flow and take the shape of its container while maintaining a definite volume.
  8. (b) An iron almirah is a solid at room temperature.
  9. Iron is a solid substance whose particles are closely packed and strongly attracted to one another.
  10. The melting point of iron is much higher than room temperature.
  11. Therefore, iron remains in the solid state under normal conditions.
  12. The two reasons are:
  13. An iron almirah has a definite shape and a definite volume. Its particles are closely packed and cannot move freely, which is a characteristic property of solids.
  14. Iron has a very high melting point, much higher than room temperature. Therefore, it does not melt and remains solid under ordinary conditions.
🎯 Exam Significance Exam Significance ›

This question tests the student's understanding of the characteristic properties of solids and liquids, one of the core concepts of the chapter.

Such reasoning-based questions are commonly asked in:

  • CBSE Class 9 examinations
  • School periodic and annual examinations
  • Competency-based assessment questions
  • Science Olympiads
  • NTSE foundation examinations
  • NEET and JEE foundation courses

Students should learn to justify answers using scientific properties rather than simply stating the state of matter.

Exam Insight

In "Give reasons" questions, always mention at least two scientific properties such as shape, volume, particle arrangement, force of attraction, melting point, or boiling point. This helps secure full marks in board examinations.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  7 points

  1. Water exists as a liquid between: \[ 0^\circ C \text{ and } 100^\circ C \]

  2. Liquids have a definite volume but no definite shape.

  3. Solids have both definite shape and definite volume.

  4. Iron particles are closely packed with strong intermolecular forces.

  5. Iron has a very high melting point and therefore remains solid at room temperature.
  6. Properties of matter are explained by the arrangement and motion of particles.
  7. Reasoning-based questions on solids, liquids, and gases are frequently asked in board and competitive examinations.
← Q5
6 / 9  ·  67%
Q7 →
Q7
NUMERIC3 marks
Why is ice at 273 K more effective in cooling than water at the same temperature?
📘 Concept & Theory Concept and Theory: Cooling Effectiveness of Ice vs Water at 273 K ›

When a substance changes its state, it absorbs or releases heat without any change in temperature. This hidden heat is known as latent heat.

The heat required to convert a solid into a liquid at its melting point without changing its temperature is called the latent heat of fusion.

Ice at 273 K and water at 273 K have the same temperature:

\[ 273\;K = 0^\circ C \]

However, ice possesses an additional cooling advantage because it must absorb latent heat from the surroundings before it can completely melt into water.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Identify the temperature of ice and water.

  2. Recall the concept of latent heat of fusion.

  3. Determine what happens when ice melts

  4. Compare the amount of heat absorbed by ice and water.

  5. Explain why ice produces greater cooling.

📊 Graph / Figure Graph / Figure ›
Cooling Effectiveness: Ice vs Water at 273 K Visualizing Latent Heat of Fusion in a Deep Sea Blue theme Why is Ice at 273 K more effective in cooling? ICE (273 K) EXTRA HEAT ABSORBED To overcome forces (334 J/g) Latent Heat of Fusion WATER (273 K) NORMAL HEAT TRANSFER Sensible heat for temp rise only Sensible Heat Only Ice absorbs +334 J/g more heat to melt than water does to warm up.
Cooling Effectiveness of Ice vs Water at 273 K
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  10 steps
  1. Compare the temperatures.
  2. Ice is at:\[273\;K\]
  3. Water is also at:\[273\;K\]
  4. Thus, both have the same temperature.
  5. Understand what happens to ice.
  6. Before ice can become water, it must first melt.
  7. During melting, ice absorbs heat from its surroundings without any rise in temperature.
  8. This heat absorbed is called the latent heat of fusion.
  9. Compare heat absorption.
  10. Water at 273 K can absorb heat only when its temperature starts increasing.
  11. Ice at 273 K absorbs:
    1. Latent heat of fusion to melt into water.
    2. Additional heat if the temperature of the resulting water rises.
  12. Therefore, ice absorbs more heat from the surroundings than water at the same temperature.
  13. Explain cooling effectiveness.
  14. Since ice absorbs more heat, it produces a greater cooling effect compared to water at 273 K.
🎯 Exam Significance Exam Significance ›

This is one of the most important conceptual questions from the chapter because it tests the understanding of latent heat of fusion, a frequently examined topic.

Similar questions are commonly asked in:

  • CBSE Class 9 examinations
  • School periodic and annual examinations
  • Competency-based and application-based questions
  • Science Olympiads
  • NTSE foundation examinations
  • NEET and JEE foundation courses

This concept also explains why ice is used in ice boxes, cold storage units, and cooling applications instead of water at the same temperature.

Exam Insight

Whenever a question compares ice and water at 273 K (0°C), always mention latent heat of fusion. Merely stating that ice absorbs heat is not sufficient for a complete answer. The keyword "latent heat of fusion" is essential for scoring full marks.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  8 points

  1. \[273\;K = 0^\circ C\]

  2. Latent heat of fusion is the heat required to convert a solid into a liquid at its melting point.

  3. Ice absorbs latent heat while melting.

  4. Water at 273 K does not need latent heat to change its state.

  5. Ice absorbs more heat from the surroundings than water at the same temperature.

  6. Greater heat absorption results in greater cooling.

  7. Ice is therefore more effective as a cooling agent than water at 273 K.

  8. Latent heat is one of the most important concepts for board and competitive examinations.
← Q6
7 / 9  ·  78%
Q8 →
Q8
NUMERIC3 marks
What produces more severe burns, boiling water or steam?
📘 Concept & Theory concept and Theory: Burn Severity of Boiling Water vs Steam at 373 K ›

Heat energy can exist in two forms:

  1. Sensible Heat – Heat that changes the temperature of a substance.
  2. Latent Heat – Heat absorbed or released during a change of state without any change in temperature.

When water boils at:

\[ 100^\circ C \]

it changes into steam. During this process, water absorbs a large amount of heat known as the latent heat of vaporization.

Therefore, steam contains not only the heat corresponding to its temperature but also the additional latent heat absorbed during vaporization.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Compare the temperatures of boiling water and steam.

  2. Recall the concept of latent heat of vaporization.

  3. Determine what happens when steam comes in contact with skin.

  4. Compare the amount of heat released by boiling water and steam.

  5. Identify which causes more severe burns.

📊 Graph / Figure Graph / Figure ›
Burn Severity: Steam vs Boiling Water at 373 K Visualizing Latent Heat of Vaporization in a Deep Sea Blue theme Which produces more severe burns? BOILING WATER (373 K) SENSIBLE HEAT ONLY Heat released as water cools SEVERITY STEAM (373 K) LATENT HEAT RELEASED +2260 J/g released upon condensation CRITICAL Steam releases massive energy (2260 J/g) as it condenses back into boiling water.
Burn Severity: Steam vs Boiling Water at 373 K
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  13 steps
  1. Compare their temperatures.
  2. Boiling water exists at: 180°C (373 K)
  3. Steam formed from boiling water is also at: 180°C (373 K)
  4. Thus, both have the same temperature.
  5. Understand the formation of steam.
  6. To convert water into steam at 100°C, additional heat must be supplied.
  7. This heat does not increase the temperature. Instead, it is absorbed as latent heat of vaporization.
  8. Therefore, steam contains extra energy in addition to its temperature.
  9. Observe what happens when steam touches the skin.
  10. When steam comes in contact with the skin, it condenses into water.
  11. During condensation, steam releases the latent heat of vaporization that it had previously absorbed.
  12. This releases a large amount of heat energy onto the skin.
  13. Compare the effects.
  14. Boiling water can transfer only its thermal energy.
  15. Steam transfers:
    1. Its thermal energy at 100°C.
    2. Its latent heat of vaporization during condensation.
  16. Consequently, steam transfers more heat to the skin than boiling water.
  17. Hence, steam causes more severe burns.
🎯 Exam Significance Exam Significance ›

This is one of the most frequently asked conceptual questions from the chapter "Matter in Our Surroundings."

The question tests the understanding of:

  • Latent heat of vaporization
  • Change of state
  • Condensation
  • Heat transfer
  • Application of scientific concepts in daily life

Similar questions are regularly asked in:

  • CBSE Class 9 examinations
  • School periodic and annual examinations
  • Competency-based and assertion-reason questions
  • Science Olympiads
  • NTSE foundation examinations
  • NEET and JEE foundation programmes
Exam Insight

Students often write that steam causes more severe burns because it is "hotter" than boiling water. This is incorrect. Both steam and boiling water are at 100°C.

The correct explanation must include the term "latent heat of vaporization", which is the key marking point expected in examinations.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  8 points

  1. Water boils at: 100°

  2. Steam and boiling water can have the same temperature.

  3. Steam contains additional latent heat of vaporization.

  4. Latent heat is absorbed during vaporization and released during condensation.

  5. Steam releases more heat to the skin than boiling water.

  6. More heat transfer results in more severe burns.

  7. Therefore, steam is a more effective heating medium than boiling water.

  8. Latent heat of vaporization is a high-priority concept for board and competitive examinations.
← Q7
8 / 9  ·  89%
Q9 →
Q9
NUMERIC3 marks
Identify the changes of state in the following stages:
  1. Stage A
  2. Stage B
  3. Stage C
  4. Stage D
  5. Stage E & F
📘 Concept & Theory Concept and Theory ›

Matter exists in three common physical states:

  • Solid
  • Liquid
  • Gas

A substance can change from one state to another by gaining or losing heat energy. These transformations are known as changes of state or phase transitions.

The major changes of state are:

  • Melting (Fusion): \[ \text{Solid} \rightarrow \text{Liquid} \]
  • Vaporization (Boiling): \[ \text{Liquid} \rightarrow \text{Gas} \]
  • Condensation: \[ \text{Gas} \rightarrow \text{Liquid} \]
  • Freezing (Solidification): \[ \text{Liquid} \rightarrow \text{Solid} \]

During a change of state, the temperature remains constant because the supplied or released heat is used as latent heat.

🗺️ Solution Roadmap Step-by-step Plan ›

  1. Observe the initial and final states in each stage.

  2. Identify whether the substance gains or loses heat.

  3. Match the change with the correct scientific term.

  4. Explain the role of latent heat in phase transitions.

📊 Graph / Figure Graph / Figure ›
Phase Changes of Matter Horizontal diagram of Solid, Liquid, and Gas phase transitions with external influences Increase heat and decrease pressure SOLID LIQUID GAS A D B C E F Decrease heat and increase pressure
Phase Changes of Matter
✏️ Solution Complete Solution ›
Step-by-step Solution  ·  16 steps
  1. Stage A
  2. In this stage:
    Solid → Liquid
  3. A solid absorbs heat and changes into a liquid.
  4. This process is called Melting or Fusion.
  5. Stage B
  6. In this stage:
    Liquid → Gas
  7. A liquid absorbs heat and changes into a gas.
  8. This process is called Evaporation or Vaporization.
  9. Stage C
  10. In this stage:
    Gas → Liquid
  11. A gas releases heat and changes into a liquid.
  12. This process is called Condensation.
  13. Stage D
  14. In this stage:
    Liquid → Solid
  15. A liquid releases heat and changes into a solid.
  16. This process is called Freezing or Crystallization.
  17. Stage E
  18. During this stage, only the temperature changes while the substance remains in the same physical state.
  19. Therefore, no change of state occurs.
  20. Stage F
  21. During this stage, only the temperature changes while the substance remains in the same physical state.
  22. Therefore, no change of state occurs.
🎯 Exam Significance Exam Significance ›

This question is important because it covers all major changes of state discussed in the chapter. Understanding these transformations is essential for topics such as melting point, boiling point, latent heat, evaporation, condensation, and sublimation.

Similar questions frequently appear in:

  • CBSE Class 9 examinations
  • School periodic and annual examinations
  • Competency-based and diagram-based questions
  • Science Olympiads
  • NTSE foundation examinations
  • NEET and JEE foundation programmes

Students should be able to identify changes of state from diagrams, flowcharts, graphs, and real-life situations.

Exam Insight

Questions based on phase-change diagrams are very common. Remember that during a change of state, the temperature remains constant because the absorbed or released heat is used as latent heat.

🔑 Key Takeaways Key Takeaways ›
Key Takeaways  ·  9 points

  1. Melting: \[ \text{Solid} \rightarrow \text{Liquid} \]

  2. Evaporation: \[ \text{Liquid} \rightarrow \text{Gas} \]

  3. Condensation: \[ \text{Gas} \rightarrow \text{Liquid} \]

  4. Freezing: \[ \text{Liquid} \rightarrow \text{Solid} \]

  5. Sublimation: \[ \text{Solid} \rightarrow \text{Gas} \]

  6. During phase changes, latent heat is absorbed or released.

  7. Temperature remains constant during melting and boiling.

  8. Stages E and F involve only temperature changes and no change of physical state.

  9. Changes of state are among the most important concepts in this chapter for both board and competitive examinations.
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Is Matter Around Us Pure | Science Class 9 | Academia Aeternum
Is Matter Around Us Pure | Science Class 9 | Academia Aeternum — Complete Notes & Solutions · academia-aeternum.com
The world around us is made up of different kinds of substances, but are all of them pure? In this chapter, Is Matter Around Us Pure?, we will explore the difference between pure substances and mixtures, and learn how to identify them. You'll study homogeneous and heterogeneous mixtures, solutions, colloids, and suspensions, along with interesting phenomena like the Tyndall effect. The chapter also introduces important separation techniques such as filtration, evaporation, crystallization,…
🎓 Class 9 📐 Science 📖 NCERT ✅ Free Access 🏆 CBSE · JEE
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