Class 9 Science · Original EZT practice
Matter in Our Surroundings (Class 9): original worked practice
A puddle shrinks, a cold drink warms and a smell travels through air. Use particle motion and energy transfer to explain the change, then say which conditions your explanation assumes.
8 original worked examples, 8 fresh self-check questions and 8 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 17 October 2026:
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Explain spreading without stirring
Choose the method: Diffusion is spreading driven by random particle motion. Bulk liquid or air movement can also carry material, so a classroom observation may involve both.
EZT practice question
A tiny spot of water-soluble dye slowly spreads through still water. What particle idea helps explain it?
Show the worked solution
- Dye and water particles move randomly.
- The dye spreads from a region of higher concentration into the surrounding water.
- Stirring is not needed for diffusion, although convection or other currents can speed visible mixing.
Answer: Random motion produces diffusion.
Check: A sealed, undisturbed sample can still mix over time.
Watch for this: One observation does not by itself prove every claim about particle structure.
Now try it yourself
Why does a smell spread faster when a fan is turned on?
Check your answer
Moving air carries the vapour.
Diffusion still occurs, but the fan adds bulk air movement. Do not call all transport diffusion.
Separate a material from its pores
Choose the method: A porous solid can change shape when its framework bends and air leaves its pores. That does not make it a liquid or gas.
EZT practice question
A dry cleaning sponge becomes smaller when pressed and regains shape when released. Why can a solid behave this way?
Show the worked solution
- The sponge has a connected solid framework around air-filled pores.
- Pressing bends the framework and reduces pore space, pushing air out.
- Releasing allows its elastic framework to recover.
Answer: Its porous solid structure deforms.
Check: The sponge does not flow to fill a container like a liquid.
Watch for this: Avoid saying the solid framework cannot deform at all.
Now try it yourself
Why is a solid rubber ball's change of shape not proof that it is liquid?
Check your answer
Solids can deform under force.
A state description does not mean an object is perfectly rigid.
Use the stated temperature convention
Choose the method: The exact relation is T(K) = t(°C) + 273.15. A question may explicitly ask for the school approximation 273.
EZT practice question
Convert 18°C to kelvin exactly, then convert 315 K to Celsius exactly.
Show the worked solution
- 18 + 273.15 = 291.15.
- 315 - 273.15 = 41.85.
Answer: 291.15 K and 41.85°C
Check: Subtract 273.15 from 291.15 to recover 18°C.
Watch for this: Kelvin uses K, not a degree symbol. State when you round.
Now try it yourself
Convert 32°C to kelvin using the approximation 273.
Check your answer
305 K
32 + 273 = 305. The exact conversion would be 305.15 K.
Compare cooling capacity, not speed
Choose the method: For equal masses starting at 0°C, melting ice absorbs extra energy before the resulting water warms. This is about total heat absorption under comparable conditions, not an unconditional rate claim.
EZT practice question
Equal masses of ice and liquid water at 0°C reach the same final temperature above 0°C in separate warm drinks. Which absorbs more heat?
Show the worked solution
- The ice first needs latent heat of fusion to melt at 0°C.
- After melting, that water warms from 0°C to the final temperature, just as the original liquid water does.
Answer: The ice absorbs more heat.
Check: The extra energy is the melting energy for the same mass, assuming pure water near ordinary atmospheric pressure.
Watch for this: Do not conclude that ice must cool every drink faster; heat-transfer rate depends on contact and conditions.
Now try it yourself
Why can a pure ice-water mixture remain near 0°C while heat enters?
Check your answer
Energy melts the ice.
At the melting point and fixed pressure, phase change can absorb heat without raising temperature until melting finishes.
Trace heat during evaporation
Choose the method: Evaporation removes liquid molecules into vapour. When it draws energy from skin, the skin loses heat.
EZT practice question
Why may a wet sleeve feel cooler in a gentle breeze than in still air?
Show the worked solution
- Water evaporates from the sleeve.
- Some energy for evaporation comes from the sleeve and skin.
- Airflow removes humid air near the surface and can increase evaporation when surrounding air is less humid.
Answer: Evaporation removes heat, and airflow can help it continue.
Check: The effect is weaker when the air is already very humid.
Watch for this: A breeze need not always make air drier; compare the actual surrounding conditions.
Now try it yourself
Two equally wet cloths hang in otherwise similar dry air. One has gentle airflow. Which usually dries sooner?
Check your answer
The cloth with airflow.
Moving air reduces vapour build-up beside its surface.
Change one evaporation factor
Choose the method: Compare samples with the same liquid, temperature, airflow and humidity before attributing a difference to surface area.
EZT practice question
Equal small volumes of water sit in a wide tray and a narrow cup under the same conditions. Which should evaporate sooner?
Show the worked solution
- The tray exposes more liquid surface.
- More surface molecules can escape at once under otherwise similar conditions.
Answer: The wide tray should lose its water sooner.
Check: Keep volumes and surroundings comparable; otherwise another factor may dominate.
Watch for this: A fair comparison changes one factor at a time.
Now try it yourself
Why may puddles remain longer on a humid day than a dry day with similar temperature and wind?
Check your answer
High humidity slows net evaporation.
More water vapour in the nearby air reduces the net transfer from liquid to air.
Explain evaporation from fabric
Choose the method: Fabric affects how sweat spreads and how air reaches it. Evaporation can cool skin, but no fabric guarantees cooling in every environment.
EZT practice question
A light, loose cotton shirt absorbs sweat and spreads it over a larger area. How can that help cooling in dry moving air?
Show the worked solution
- Spreading sweat exposes more wet surface.
- Airflow helps remove water vapour.
- Evaporation can take heat from the fabric and skin.
Answer: The conditions can support evaporative cooling.
Check: In humid air, evaporation may be slow even with absorbent fabric.
Watch for this: Absorption alone is not cooling; the water must evaporate for this mechanism.
Now try it yourself
Why can a soaked, tightly clinging shirt feel uncomfortable in humid air?
Check your answer
Evaporation is limited.
High humidity and poor airflow can leave fabric wet rather than rapidly carrying heat away.
State the pressure condition
Choose the method: Sublimation changes solid directly to gas. The reverse change is deposition.
EZT practice question
A small dry-ice sample disappears at ordinary atmospheric pressure without a pool of liquid carbon dioxide. Explain.
Show the worked solution
- Dry ice is solid carbon dioxide.
- At ordinary atmospheric pressure it changes directly to gas rather than passing through a stable liquid phase.
Answer: It sublimes.
Check: The process and pressure explain the lack of a liquid pool.
Watch for this: Dry ice can cause cold burns and gas build-up. This is an explanation, not an instruction to handle it.
Now try it yourself
Frost forms directly from water vapour on a sufficiently cold surface. Name the change.
Check your answer
Deposition
Gas changes directly to solid, the reverse of sublimation.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Does pure water always boil at 100°C?
Show answer and reason
No.
Boiling temperature depends on pressure; 100°C is the usual rounded value near one atmosphere.
2. Convert 302 K exactly to Celsius.
Show answer and reason
28.85°C
302 - 273.15 = 28.85.
3. Why can a shallow wet plate dry faster than a deep narrow beaker with the same water volume?
Show answer and reason
More exposed surface area.
This comparison assumes other evaporation conditions are similar.
4. Why is a gas easier to compress than a liquid?
Show answer and reason
Gas particles are much farther apart.
Pressure can reduce the large spaces between particles; liquid particles are already close together.
5. During melting of a pure solid at fixed pressure, must temperature rise continuously?
Show answer and reason
No.
At its melting point, energy can drive phase change while temperature stays approximately constant.
6. Does a liquid have a fixed shape like its container?
Show answer and reason
No fixed shape of its own.
It takes the shape of the occupied part of a container while retaining an approximately fixed volume.
7. What is wrong with "all spreading in air is diffusion"?
Show answer and reason
Moving air can transport material too.
A draught or fan adds bulk motion; distinguish that from random molecular diffusion.
8. Same mass and temperature: why can condensing steam transfer more heat than hot liquid water cooling to the same lower temperature?
Show answer and reason
Condensation releases extra latent heat.
For comparable pure-water conditions, steam releases energy when it becomes liquid before the liquid cools. Avoid testing this on skin.
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Questions about this practice page
- Is this the current NCERT Science 9 Chapter 1?
- No. This page targets the older Matter in Our Surroundings chapter/topic. The current official Chapter 1 link checked for this page is Exploration: Entering the World of Secondary Science. Check your edition and school topic list.
- How do evaporation and boiling differ?
- Evaporation occurs at the liquid surface over a range of temperatures. Boiling forms vapour bubbles throughout a liquid when conditions allow, at a boiling temperature that depends on pressure and composition.
- Is 273 K exactly 0°C?
- No. Exactly 0°C is 273.15 K. School questions sometimes state the approximation 273; use the convention the question gives and show any rounding.
- Do these questions predict an exam?
- No. They practise explanations and calculations for this topic. They are not a frequency analysis, marks scheme or promise of assessed coverage.
- Are these copied textbook exercises?
- No. The scenarios and explanations are original topic practice. Official links establish topic and edition context, not an exercise-number answer key.