Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 1 Matter And Its Composition

Exercise 1

Question 1:
Answer:
(a) Melting point: The constant temperature at which a solid changes into liquid state by absorbing heat energy is called melting point.
(b) Boiling point: It is the temperature at which a liquid changes into vapour under atmospheric pressure.
(c) Evaporation: The slow passing of molecules of a liquid into gaseous state at a temperature below its boiling point.
(d) Freezing: It is a process in which a liquid changes into solid state by giving out heat energy.
These terms describe the specific temperatures or processes where a substance moves from one physical form to another. Understanding these helps us predict how materials behave when we heat or cool them.
Teacher's Tip: Remember "M-B-E-F" for the four main changes: Melting, Boiling, Evaporation, and Freezing.
Exam Tip: Always mention "constant temperature" for melting and boiling points to get full marks.

Question 2:
Answer: Boiling point of a liquid can be raised by increasing the atmospheric pressure.
When you increase the pressure on a liquid, you make it harder for the molecules to escape into the air. This means you need more heat to make it boil, which is why pressure cookers work so well.
Teacher's Tip: High Pressure = High Boiling Point.
Exam Tip: Use the term "atmospheric pressure" when defining how boiling points change with environment.

Question 3:
Answer: On heating, solid wax melts into liquid wax, which on further heating, is converted into wax vapours. These changes can be seen in a burning candle. The candle is made up of a solid wax. When we light a candle, the wax near its wick melts. The molten wax rises up the wick and is converted into wax vapour. The wax vapour mixes with oxygen in the air and burns. In a lighted candle, you can see the solid and the liquid states of wax. The vapour of wax can be seen rising from the wick for some time after the candle is put out.
[Image description: Inter-conversion of state of wax showing (a) a burning candle with molten wax and (b) a candle just extinguished with rising wax vapour].
A burning candle is a perfect mini-lab because it shows matter changing from solid to liquid to gas all at once. The heat from the flame provides the energy needed for these physical state changes to happen.
Teacher's Tip: Follow the wax: Solid -> Liquid (melts) -> Gas (vapour) -> Flame (burns).
Exam Tip: Draw a small diagram of a candle and label the solid, liquid, and gas zones if asked about state inter-conversion.

Question 4:
Answer:
(a) Sublimation: The process by which a solid directly change to its vapour state (or gaseous state) without passing through liquid state and vice versa is called sublimation.
(b) Liquefaction: It is a process of change of state of a substance from gaseous state to liquid state at a particular temperature. It is also known as condensation.
(c) Melting: It is process of changing from solid state to a liquid state at a particular temperature.
(d) Boiling: The process by which a liquid rapidly changes into a gaseous state, by absorbing the heat energy is called boiling.
These definitions explain how energy changes the distance between molecules in a substance. For example, in sublimation, molecules get so much energy they jump straight from a tight solid pack to a loose gas.
Teacher's Tip: Sublimation is like a "skip-step" because it misses the liquid stage entirely.
Exam Tip: Use "state of matter" or "physical state" in your explanations to sound more scientific.

Question 5:
Answer:
(a) Differences between An atom and A molecule:
An atom: Atom is a smallest particle of an element.
A molecule: Molecule is a group of two or more atoms combined together so it is bigger.

An atom: Atom consists of nucleus (containing protons and neutrons) and electrons.
A molecule: Molecule consists of combination of two or more like or different atoms chemically bound together e.g. H₂, HCl, NaCl etc.

An atom: Atom can neither be seen through naked eye nor through magnifying microscope.
A molecule: Molecule is not visible to naked eye, while can be seen through highly magnifying microscope.

An atom: Atom cannot be further divided.
A molecule: Molecule can further be divided to give individual atoms.

An atom: Atoms may or may not have independent existence.
A molecule: Molecules are capable of having independent existence. For example, atom of oxygen (O) has no independent existence while its molecule exists as O₂  in nature.

(b) Differences between Boiling and Evaporation:
Boiling: Boiling is the process in which liquid gets converted into gaseous state.
Evaporation: Evaporation is a process in which the liquid gets converted into its gaseous form at any temperature below its boiling point.

Boiling: Boiling occurs at the entire mass of the liquid. That is, it is a bulk phenomenon.
Evaporation: Evaporation occurs on the surface of the liquid. That is, it is a surface phenomenon.

Boiling: Boiling occurs rapidly.
Evaporation: Evaporation is a slow process.

Boiling: Boiling occurs at a specific temperature.
Evaporation: Evaporation occurs at any temperature.

(c) Differences between Melting and Boiling:
Melting: The process of changing from solid state to a liquid state at a particular temperature is called melting or fusion.
Boiling: The process of change of liquid to vapour form all parts of the liquid at a particular temperature is called boiling.

Melting: Melting refers to the phenomenon when a solid transforms into a liquid.
Boiling: Boiling refers to the phenomenon when liquid transform into a gas.

Melting Example: Melting of ice.
Boiling Example: Boiling of water.

(d) Differences between Gas and Vapour:
Gas: A substance exists as a gas at the room temperature and atmospheric pressure.
Vapour: A substance is a solid or liquid under ordinary condition but it is gaseous under specific conditions.

Gas: It is present at ordinary conditions of temperature.
Vapour: Its temperature is lower than the boiling point of its liquid state.

Gas e.g.: Nitrogen, oxygen.
Vapour e.g.: Iodine, Camphor.
Atoms are the basic building blocks, while molecules are what you get when those blocks are clicked together. A single Lego piece is like an atom, and a small structure made of two pieces is like a molecule.
Teacher's Tip: Boiling happens everywhere (bulk); Evaporation happens only at the top (surface).
Exam Tip: If asked for differences, always mention that boiling occurs at a specific temperature while evaporation happens at any temperature.

Question 6:
Answer:
(a) Water boils of 100°C under 1 atmosphere pressure.
(b) At high altitude water boils below 100°C.
(c) A liquid evaporates below its boiling point.
(d) When a substance is heated kinetic energy of the particles increases.
(e) Solids have the negligible inter-particle space.
(f) Gases have the negligible inter-particle forces.
These scientific facts show how pressure and heat affect the behavior of particles. In solids, particles are squeezed tight, while in gases, they are free to fly around.
Teacher's Tip: Kinetic energy is just a fancy word for "movement energy."
Exam Tip: Remember that at high altitudes, the lower pressure causes water to boil at a lower temperature.

Question 7:
Answer:
(a) Increase in atmospheric pressure
(b) Sulphur
(c) Inter-conversion of state of matter
Inter-conversion of matter means changing the physical state without changing the chemical identity of the substance. These changes are usually reversible by adding or removing heat.
Teacher's Tip: To change the state, just change the heat!
Exam Tip: Use specific technical terms like "Inter-conversion" to describe state changes.

Question 8:
Answer:
(a) Sublimation
(b) Melting
(c) Evaporation
(d) Vaporisation
Physical changes like melting and evaporation occur because of the gain of heat energy. Sublimation is a unique process where the liquid state is skipped entirely.
Teacher's Tip: Sublimation is like a "Shortcut" between solid and gas.
Exam Tip: Spell technical words like "Sublimation" correctly to ensure full credit.

Question 9:
Answer:
(a) Increase in temperature favours Evaporation. When evaporation occurs, remaining liquid becomes cooler. The particles of the liquid absorb heat energy from surroundings to regain energy lost during evaporation which makes the surroundings cold.
(b) Earthen pot has pores which help in evaporation. Some of the water continuously seeps out from these pores. This water absorbs heat of vaporization from the remaining water and gets evaporate. Thus, the remaining water loses heat and gets cooled.
(c) This happens because, when the petrol changes from liquid state to the vapour state, is absorbs heat energy from the palm. The palm thus loses heat and gets cooled.
(d) In humid weather wet clothes take longer time to dry up due to the slow evaporation of water from their surface.
(e) Evaporation is a surface phenomenon. With increase in surface area, evaporation increases. Hot tea in Saucer cools faster than in a cup and hence we can sip faster.
Evaporation causes cooling because the fastest, "hottest" molecules leave the liquid first, leaving the cooler ones behind. This is why we feel a chill when we step out of a swimming pool or when we use a fan to dry sweat.
Teacher's Tip: To speed up evaporation: Heat it up, Spread it out, or Blow it away!
Exam Tip: Mention "Latent heat of vaporisation" when explaining why evaporation causes cooling for higher marks.

Question 10:
Answer:
(a) Naphthalene balls become smaller day by day as they have very weak force of attraction operating between their particles, which break away from other particles from the surface of solid without heating.
(b) In gases the particles are far apart and there is enough space available for compression. Hence, gases can be compressed easily.
(c) Heat energy supplied increases the rate of vibration of the particles and decreases the inter-particle attraction.
(d) Light has no mass and it does it occupy space. Thus, it is not considered as matter.
(e) According to ‘Law of Conservation of Mass’, “Mass can neither be created nor destroyed in a chemical reaction.” However, it may change from one form to other.
Matter must have mass and take up space, which is why light is energy but air is matter. These laws ensure that even if things change shape or disappear from sight, the total amount of "stuff" in the universe stays the same.
Teacher's Tip: Light has no weight, so it is not matter!
Exam Tip: State the Law of Conservation of Mass exactly as written to ensure full credit.

Question 11:
Answer: In summers, we perspire more. Cotton being a good absorber of water helps in absorbing the sweat and exposes it to the atmosphere for evaporation. When sweat evaporates from our body, it takes heat from our body. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.
Our bodies use the cooling power of evaporation to stay safe in the heat. Cotton fabric pulls the moisture away from your skin so that it can meet the air and turn into vapour more easily.
Teacher's Tip: Cotton clothes are like sponges that help our sweat evaporate better.
Exam Tip: Clearly link the "absorption of heat from the body" to the feeling of coolness.

Question 12:
Answer: Balloon get heat from sun and on heating, the vibration of particles increases and the inter-particle force of attraction between them gets reduced, therefore, balloon bursts.
Heat makes the gas particles inside the balloon move much faster and push harder against the rubber walls. Eventually, the pressure becomes so great that the rubber stretches to its breaking point and snaps.
Teacher's Tip: Heat makes things expand; cold makes things contract.
Exam Tip: Use the term "vibration of particles" when explaining why substances expand when heated.

Question 13:
Answer: Law of conservation of mass: It states that mass can neither be created nor destroyed in a chemical reaction. During any change, physical or chemical, matter is neither created nor destroyed. However it may change from one form to another.

Experimental Verification of Law of Conservation of Mass
Requirements: H-shaped tube called Landolt’s tube, Sodium chloride solution, silver nitrate solution, etc.
Procedure: A specially designed H-shaped tube is taken. Sodium chloride solution is taken in one limb of the tube and silver nitrate solution in the other limb as shown in figure. Both the limbs are now sealed and weighed. Now the tubes is averted so that the solutions can mix up together and react chemically. The reaction takes place and a white precipitate of silver chloride is obtained.
AgNO₃ + NaCl → AgCl ↓ + NaNO₃
Silver nitrate Sodium chloride Silver chloride Sodium nitrate
The tube is weighed again. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube. Thus, this experiment clearly verifies the law of conservation of mass.
[Image description: Diagram of an H-shaped Landolt's tube containing NaCl solution in one side and AgNO₃ solution in the other].
This experiment proves that in a closed system, the mass before a chemical reaction is exactly the same as the mass after. Even when a solid white precipitate forms, no new atoms are created and none are lost.
Teacher's Tip: Think of a chemical reaction like a dance; the partners change, but the number of people on the dance floor stays the same.
Exam Tip: Identify "Landolt’s tube" by its H-shape to correctly describe this specific experiment.

Question 14:
Answer: Law of conservation of mass is applied to a burning candle. A candle is made of solid wax. When it is lighted, wax near its wick melts and changes into to liquid form. The molten wax rises up the wick and is converted into wax vapours. The wax vapours their mix with oxygen in the air.
Thus, in burning of candle the matter is neither created nor destroyed but one form is changed into the other form.
In a candle, some wax becomes liquid and some turns into gas that burns away. If you could trap all the smoke and gas produced, you would find they weigh exactly the same as the wax and oxygen used up.
Teacher's Tip: Burning doesn't destroy matter; it just turns solid wax into invisible gases.
Exam Tip: When discussing the Law of Conservation, always clarify that it applies to both physical and chemical changes.

Question 15:
Answer: The reaction is:
Ethanoic acid + Sodium Carbonate -> Sodium ethanoate + Carbon dioxide + Water
(Reactants) (Products)
Total mass of reactants = (6{ g} + 5.3{ g}) = 11.3{ g}
Total mass of products = (8.2 + 2.2 + 0.9){ g} = 11.3{ g}
As the total mass of reactants is equal to the total mass of products. Hence, the reaction follows Law of conservation of mass.
This calculation provides mathematical proof that the amount of matter stays constant during a reaction. By adding up the weights of the liquids and powders you start with, you find they match the weights of the new materials produced.
Teacher's Tip: Total Left Side (Reactants) must always equal Total Right Side (Products).
Exam Tip: Always show the addition for both sides clearly to prove your work.

Question 16:
Answer: The reaction will be as follows:
Methane + Oxygen → Carbon dioxide + Water
According to law of conservation of mass,
Total mass of reactants = Total mass of products
Mass of methane + mass of oxygen = Mass of carbon dioxide + Mass of Water
Mass of methane + 32{ g} = 22 + 18{ g}
Mass of methane = (40 - 32) = 8{ g}
8{g} of methane is required.
To find a missing mass, you simply treat the chemical equation like an algebra problem. Since the total mass on both sides must be equal, you can subtract the known parts from the total to find the unknown.
Teacher's Tip: Use the formula: Unknown = (Total{ Products}) - (Known{ Reactants}).
Exam Tip: Don't forget to write the unit 'g' for grams in your final answer.

Question 17:
Answer: Word equation for the reaction is:
Sodium + Chlorine → Sodium Chloride
According to law of conservation of mass,
Total mass of reactants = Total mass of products
Mass of sodium + Mass of chlorine = Mass of sodium chloride
23{ g} + {Mass of chlorine} = 58.5{ g}
Mass of chlorine = 58.5 - 23 = 35.5{ g}
35.5{ g} of chlorine is needed.
This problem shows how elements combine in specific weights to form a compound like table salt. By knowing the mass of the final salt and the mass of the sodium used, we can calculate the exact amount of chlorine gas consumed.
Teacher's Tip: The total mass of salt is always the sum of the metal and the gas that made it.
Exam Tip: If you get a decimal in your subtraction, keep it in the final answer for accuracy.

Question 18:
Answer: Word equation for the reaction is:
Magnesium + Oxygen → Magnesium oxide
According to law of conservation of mass,
Total mass of reactants = Total mass of products
Mass of magnesium + Mass of oxygen = Mass of magnesium oxide
4.8{g} + 3.2{g} = {Mass of magnesium oxide}
Mass of magnesium oxide = 8{ g}
When magnesium metal burns, it actually gains weight because it grabs oxygen atoms from the surrounding air. This result (8{g}) is the combined weight of the metal and the gas that reacted with it.
Teacher's Tip: Metals usually weigh MORE after burning because they "ate" some oxygen!
Exam Tip: Ensure your addition is correct (4.8 + 3.2 = 8.0) to get the final point.

Question 19:
Answer:
(a) (iv) No fixed shape and size highly compressible.
(b) (i) The solid starts melting.
(c) (i) evolved
(d) (iv) decreases with increasing pressure.
(e) (iii) Iodine
These multiple-choice questions review the fundamental characteristics of the three states of matter. For example, knowing that iodine can sublime helps distinguish it from other common solids.
Teacher's Tip: Gases are the only state of matter that you can easily squash (compress).
Exam Tip: Read all four options before picking the best one to avoid "trick" answers.

Question 20:
Answer:
(a) does not, melting, boiling
(b) liquid, solid
(c) temperature, temperature
(d) gaseous, sublimation
(e) high and negligible
Filling in these blanks reinforces the vocabulary needed to talk about chemistry like a pro. Each word describes a specific rule about how heat changes the way atoms stick together.
Teacher's Tip: Sublimation refers to the gas phase, while melting refers to the liquid phase.
Exam Tip: Be careful to distinguish between "melting" (solid to liquid) and "boiling" (liquid to gas).

Question 21:
Answer:
(a) freezing
(b) less
(c) sublimation
(d) remains constant
One-word answers test your ability to recall precise scientific definitions quickly. For instance, the fact that temperature stays the same during a phase change is a very important concept in thermodynamics.
Teacher's Tip: During a phase change, the thermometer stops moving even if you keep heating it!
Exam Tip: Make sure your spelling is perfect for one-word answer questions.

Question 22:
Answer:
Column A: (a) Constituent of matter | Column B: Molecules
Column A: (b) No compressibility | Column B: Solid
Column A: (c) Maximum expansion | Column B: Gas
Column A: (d) Conversion of a gas into liquid | Column B: Condensation
Matching columns help you link general concepts with their specific scientific names or properties. It shows how "no compressibility" is the defining feature of a solid compared to a gas.
Teacher's Tip: Solids don't like to be squeezed; gases love to spread out!
Exam Tip: Draw straight lines if matching, or rewrite the pairs clearly to avoid confusing the examiner.