Matter and its Composition | ICSE Class 7 Chemistry Notes

This note covers the meaning and composition of matter, mass and volume, demonstrations with air and water, atoms and molecules, and particle explanations of the solid, liquid and gaseous states.

What is matter, and how can we recognise it?

Definition: Matter is anything that has mass and occupies space. Mass describes the quantity of matter. Volume is the space occupied by matter.

Stones, water, soil, oil, sugar and air are examples of matter. They differ in appearance and behaviour, but all have mass and occupy space. Being visible is not part of the definition: air is matter even though we cannot see it.

A state of matter is a physical form of matter. A solid has a definite shape and volume. A liquid has definite volume but takes its container's shape. A gas has neither a fixed shape nor a fixed volume.

What do matter and material mean?

A material is a substance used to make an object. Wood, glass, metal, plastic and clay are materials. Objects can be made from different materials, and the same material can be used to make different objects.

For example, a pen may contain plastic, metal and ink. The pen is the object; plastic, metal and ink are its materials. Each material is matter because it has mass and occupies space.

Composition means what something is made of. Studying matter therefore involves asking both what materials make up an object and what tiny units make up those materials. These questions connect familiar objects with an explanation of their internal structure.

Which properties distinguish materials?

A property is a characteristic used to describe a material. Shape, colour and texture are properties; texture describes whether a surface feels rough or smooth. Materials can differ in these properties while still sharing mass and volume.

Do not use colour, hardness or visibility as the defining test for matter. Those features help describe particular materials. The common test is whether something has mass and occupies space, including materials that are not solid.

How do mass and volume describe matter?

Mass and volume describe different properties. Mass tells us the quantity of matter, whereas volume tells us how much space it occupies. Keeping these meanings separate helps us interpret measurements instead of treating every measurement as a measure of size.

How can mass be compared?

A balance is an instrument used to measure mass by weighing. A comparison can be made using identical paper cups partly filled with water, sand and pebbles. The readings provide evidence for which filled cup has more mass.

The units gram and kilogram are written using the symbols g and kg respectively. A unit is an agreed quantity used for measurement. Write a space between a numerical value and its unit symbol.

The International System of Units, abbreviated SI, provides standard units. The SI unit of mass is the kilogram. A mass of seven kilograms is written as 7 kg, with lower-case letters and no plural ending added to the symbol.

How can volume be compared?

Water partly fills a tumbler because it occupies space inside it. In identical tumblers, a higher water level indicates a larger volume of water. The capacity of a container is the amount it can hold.

The units litre and millilitre are written L and mL respectively. The SI unit of volume is the cubic metre, written m³. Here the raised 3 forms part of the symbol for cubic metre.

Write the quantity as well as the measurement. For example, 500 mL describes a volume of water, not its mass. A number without a unit does not communicate the complete measurement.

How does a balloon show that air occupies space?

Air is matter even though it is invisible. It has mass and occupies space. Inflating a balloon provides a simple way to observe the space occupied by air without needing to see the air itself.

What should we observe?

1. Observe a balloon before inflating it, noting its size and shape.
2. Blow air into the balloon and observe how its size changes.
3. Hold the opening closed so that the added air remains inside.
4. Compare the inflated balloon with its earlier appearance and relate the increase in size to the space occupied by air.

The observation, meaning what is noticed during the activity, is that the balloon expands as air enters it. The conclusion, meaning the explanation drawn from the observation, is that the air inside occupies space.

The rubber boundary makes the occupied space easier to recognise. The activity does not require us to see individual air particles, the tiny units making up the air. It uses a visible change in the balloon to investigate an invisible material.

What does this demonstration establish?

The change in the balloon's size demonstrates space occupation. It is not, by itself, a measurement of mass. Mass and volume remain distinct properties even when both belong to the same sample of matter.

Note: Invisible does not mean absent. Air inside a balloon is matter. Use the balloon's expansion as evidence that air occupies space, rather than describing the balloon as filled with nothing.

This also explains why recognising matter requires more than looking for a visible object. Stones and water are easy to see; air needs a demonstration. The definition of matter applies to all of them.

What evidence shows that matter contains tiny particles?

A constituent particle is a basic unit making up a larger piece of a material. Matter consists of extremely small constituent particles. A visible grain of a material is not necessarily one constituent particle; it can contain very many of them.

What does grinding chalk show?

A stick of chalk can be broken into smaller pieces and ground into fine powder. Each visible speck remains chalk. Grinding is a physical change, a change in which no new substance forms: the pieces become smaller without becoming a different substance.

1. Start with a stick of chalk and break it into smaller pieces.
2. Continue until breaking the pieces by hand becomes difficult.
3. Grind the small pieces using a mortar and pestle, a bowl and grinding tool.
4. Observe the powder with a magnifying glass and recognise that each visible speck is still chalk.

The drawings show a chalk stick, two broken pieces, a heap of powder and the powder viewed through a magnifying glass. They show smaller pieces of chalk, not individual constituent particles.

See Fig. 7.1 in your NCERT textbook

What does dissolving sugar show?

Dissolving is the process in which a substance mixes into a liquid so that its separate grains are no longer visible. Stirring sugar into water produces a solution, a mixture in which the dissolved substance is distributed through the liquid.

The sugar remains present after its visible grains disappear. In the drinking-water demonstration, sweetness provides evidence of its presence. The tiny sugar particles separate and occupy available spaces between water particles. They have not stopped being matter.

Perform any tasting demonstration only with drinking water and food sugar under adult supervision, and only when instructed. Do not taste laboratory materials. The important conclusion is that disappearance from view does not establish disappearance of a substance.

Particle size matters when interpreting these observations. Constituent particles are so small that they cannot be seen even through an ordinary microscope. A magnifying glass reveals chalk specks, not the basic particles making up each speck.

What are atoms and molecules?

The tiny particles making up matter include atoms and molecules. An atom is a constituent particle of an element. An element is a substance that cannot be broken down into simpler substances; atoms of different elements differ from one another.

Iron consists of iron atoms, and gold consists of gold atoms. These examples describe composition at the particle level. A piece of iron is not one large atom: the visible piece contains a large number of its constituent particles.

How do atoms form molecules?

The atoms of most elements cannot exist independently. Two or more such atoms combine to form a stable particle called a molecule. A molecule can contain atoms of the same element or atoms of different elements.

Two hydrogen atoms combine to form a hydrogen molecule. Two oxygen atoms combine to form an oxygen molecule. In each case, the molecule contains atoms of one element, even though more than one atom is present.

A water molecule contains two hydrogen atoms and one oxygen atom. It therefore contains atoms of different elements. Counting the atoms in a molecule and identifying their kinds are separate parts of describing its composition.

Do not replace the word most with “all” when describing atoms that cannot exist independently. Likewise, do not describe every material as containing identical molecules. Use the appropriate particle description for the substance being discussed.

The term “particle” is useful when comparing states of matter without specifying the exact kind of atom or molecule. It does not mean a visible grain of powder, which itself contains many constituent particles.

What are the spaces between particles?

Interparticle spaces are the spaces between the constituent particles of matter. Particles are packed differently in different states. A state of matter is a physical form of matter, such as a solid, liquid or gas.

Solids have closely packed particles. Liquids have particles that are a little more loosely packed. Gas particles are widely separated. These differences help explain why the states behave differently when their shape or occupied space changes.

Does close packing mean no space?

Even in a solid, some space remains between the particles. Close packing means that the particles are near one another; it does not mean that every gap has disappeared. The interparticle spaces are not filled with air.

Generally, particles in a liquid are somewhat farther apart than in the solid state. Ice is an exception: its particles are farther apart than those in liquid water. This qualification prevents a general comparison from becoming an incorrect universal rule.

The schematic drawings link a solid, a liquid in a beaker and a gas in a balloon to enlarged particle patterns. The solid pattern is closely ordered, the liquid pattern less ordered and the gas pattern widely spaced.

See Fig. 7.12 in your NCERT textbook

A schematic drawing is a simplified representation. The circles in these drawings stand for constituent particles. They make spacing easier to compare; they are not photographs of the particles or a view available through an ordinary magnifying glass.

How does sugar provide supporting evidence?

When sugar is added to water, the level initially rises. After the sugar dissolves, it may decrease to some extent. Sugar particles occupy available spaces between water particles, so the solution's volume is less than the sum of the separate volumes.

Do not claim that the level must return exactly to its original mark. The observation is a possible decrease to some extent after dissolving, not a fixed numerical change. No particular final reading follows without an actual measurement.

What holds the particles of matter together?

Interparticle attraction is the attractive force between constituent particles. An attractive force tends to draw particles towards one another. When the particles concerned are molecules, the attraction between them is called intermolecular attraction.

The strength of attraction depends on the nature of the substance and the distance between its particles. Even a slight increase in separation decreases the interparticle forces drastically. Attraction helps explain why particles remain together rather than behaving as unrelated pieces.

How does attraction differ between states?

Vibration means movement to and fro about a position. Solid particles vibrate, while liquid and gas particles have greater freedom to move from place to place. These differences connect the strength of attraction with the kind of motion possible.

Negligible means so small that it can be disregarded in this explanation. Describing gas attractions as negligible is more precise than changing the statement into an absolute claim that no attraction exists.

In liquids, attraction is slightly weaker than in solids, but still strong enough to keep the particles close together. A liquid can therefore flow while retaining a definite volume. Flowing does not mean that all attraction has vanished.

How do attraction and motion work together?

Strong attraction in a solid restricts its particles to movement about fixed positions. Weaker attraction in a liquid permits movement within a limited space. Negligible attraction in a gas allows its particles to spread through the available space.

To explain a state's behaviour, connect packing, attraction and movement. Saying only that “the particles are small” is insufficient: constituent particles are extremely small in all three states, so their size alone does not distinguish a solid from a gas.

Why do solids have a definite shape and volume?

A solid is a state of matter with a definite shape and volume. A stone, an iron nail, a piece of wood and a key are examples. Their constituent particles are tightly packed and held together by very strong attractive forces.

How do particles move in a solid?

Solid particles occupy fixed positions. They cannot move freely past one another, but they can move to and fro about those positions. This movement is called vibration or oscillation.

Fixed position does not mean complete stillness. It describes the position about which the particle vibrates. The distinction explains how particles can be moving while the solid as a whole retains its shape.

Because the particles remain closely held, a solid does not spread through all the space inside a container. Its particle arrangement accounts for its definite shape and volume. A description should connect these observable properties to the restricted movement of its particles.

Are visible grains single particles?

Grinding chalk gives smaller solid pieces, not freely moving individual molecules. A speck of chalk and a grain of sand each contain many constituent particles. Their small visible size does not make them the smallest units of matter.

This distinction is useful when describing powders. A heap consists of many separate solid grains. Moving those grains changes the arrangement of the heap; it should not be confused with particles moving past one another within a liquid.

Note: “Particles vibrate about fixed positions” is different from “particles do not move”. Include both the fixed positions and the vibration when explaining the solid state.

Heating a solid makes its particles vibrate more vigorously. This reinforces the point that the solid state does not mean absence of motion. The basic particle description is tight packing, strong attraction and restricted movement about fixed positions.

Why do liquids change shape but keep a definite volume?

A liquid has a definite volume but no fixed shape. It takes the shape of the container holding it. Its particles can move past one another, yet remain close enough for the liquid to occupy a limited space.

What does transferring water demonstrate?

1. Take three clean, dry containers of different shapes and label them A, B and C. These letters identify the containers.
2. Mark the 200 mL level on each container and fill container A to that mark.
3. Transfer the water carefully from A to B without spilling, and observe its shape and volume.
4. Transfer the same water from B to C without spilling, and compare its shape and volume again.

The water changes shape to fit each container, while its volume remains 200 mL. The heights of the marks need not look alike because the containers have different shapes. Compare the marked volumes rather than judging volume from height alone.

The drawings show a bottle labelled A, a jug labelled B and a tumbler labelled C. Each contains water marked 200 mL. The water takes a different shape in each vessel.

See Fig. 7.5 in your NCERT textbook

If a container is not clean, some water may stick to its walls. The next container's reading may then be slightly less than 200 mL. That possible transfer loss should not be confused with a liquid lacking a definite volume.

How does particle attraction explain this?

The attractions in liquids are slightly weaker than in solids but still hold particles close together. Movement is possible within a limited space, allowing liquid particles to rearrange. This gives liquids their ability to flow.

Passing a finger through water temporarily displaces it. After the finger is removed, the water returns to occupy the space. The water has not been permanently cut like a solid object.

Shape and volume must therefore be stated separately. “A liquid changes shape” is correct. “A liquid changes its volume to fill every container” is incorrect: changing the boundary shape does not mean spreading throughout all available space as a gas does.

Why do gases spread and compress readily?

A gas has neither a fixed shape nor a fixed volume. Gas particles move freely in all directions, with negligible attraction between them. Their movement is random, meaning that they do not follow one shared, fixed direction.

Gases tend to occupy all the available space in a container and acquire its shape. Their particles have much more space between them than particles in solids and liquids. This explains both spreading and their response to compression.

What does a syringe demonstration show?

Compression means reducing the volume occupied by a sample. A syringe is a barrel with a movable plunger, the sliding part used to push material in or out. Use a syringe without a needle for this demonstration.

1. Pull the plunger outwards so that the syringe contains air.
2. Cover the open end with a thumb to prevent the air from escaping.
3. Push the plunger slowly and steadily inwards.
4. Observe that the trapped air occupies a smaller volume, showing that its particles can be brought closer together.

The reduction in volume results from reducing the spaces between particles. It does not mean that the particles themselves have disappeared. Keeping the opening closed matters because it prevents escape of air from explaining the reduced occupied space.

Repeating the demonstration with water shows that water is practically incompressible: its volume is very difficult to reduce in this way. Retain “practically”; it is not an absolute statement about every possible condition.

How can gas movement be recognised?

Fragrance from a burning incense stick spreads through a room as air particles hit and help spread fragrance particles. This provides evidence of particle motion. The fragrance travels through matter; it is not evidence that the room was empty of particles before it arrived.

How can we compare the three states using the particle model?

A particle model is a simplified explanation of matter in terms of its constituent particles. It links properties we can observe, such as shape and volume, with particle packing, movement and attraction.

Which comparisons belong together?

The packing comparison is general. Remember the exception that particles in ice are farther apart than those in liquid water. A comparison table should be read alongside this qualification rather than used to erase it.

Liquids and gases are called fluids, meaning substances that flow and do not retain a fixed shape. They share this property, but differ in volume: a liquid has a definite volume, whereas a gas fills the available space.

How can an explanation be checked?

Begin with the observed property. Next describe how the particles are arranged and how they move. Finally, connect their movement with the strength of attraction. This order makes the explanation more useful than a disconnected list of state names.

For example, water takes the shape of a jug because its particles can move past one another. It retains a definite volume because attraction still keeps the particles close together within a limited space.

Check each statement for its proper strength. Use slightly weaker for liquid attractions, negligible for gas attractions and practically incompressible for water in the syringe demonstration. These terms preserve the distinction between a useful model and an over-absolute claim.

Glossary

• Matter — Anything that has mass and occupies space, including solids, liquids and gases.
• Mass — The quantity of matter, measured in units such as grams and kilograms.
• Volume — The space occupied by matter, measured using units such as litres and cubic metres.
• Material — A substance used to make an object, such as wood, glass or metal.
• Constituent particle — A basic unit making up a larger piece of a substance or material.
• Atom — A constituent particle of an element, differing from atoms of other elements.
• Molecule — A stable particle formed when two or more atoms of the same or different elements combine, capable of independent existence.
• Interparticle space — A space between constituent particles, present even when those particles are closely packed.
• Intermolecular attraction — The attractive force acting between molecules and helping to hold them together.
• Vibration — The to-and-fro movement of a particle about its position in a solid.
• Solid — A state with definite shape and volume, whose particles vibrate about fixed positions.
• Liquid — A state with definite volume but no fixed shape, whose particles can move past one another.
• Gas — A state with no fixed shape or volume, whose particles move freely in all directions.
• Fluid — A substance that flows and does not retain a fixed shape, including liquids and gases.
• Compression — A reduction in the volume occupied by a sample, illustrated by trapped air in a syringe.

Common errors and misconceptions

• Misconception: Air is not matter because it cannot be seen. Correct: Air has mass and occupies space; an inflating balloon demonstrates its occupation of space.
• Misconception: Mass and volume are two names for size. Correct: Mass describes the quantity of matter, while volume describes the space that matter occupies.
• Misconception: A visible grain of chalk is one atom. Correct: The grain contains many constituent particles, too small to see through an ordinary microscope.
• Misconception: Solid particles are completely motionless. Correct: They vibrate about fixed positions, although they cannot move freely past one another.
• Misconception: There is no attraction between liquid particles because liquids flow. Correct: Attraction is slightly weaker than in solids but keeps liquid particles close together.
• Misconception: Gas particles shrink when air is compressed. Correct: In the syringe demonstration, particles come closer together and the spaces between them decrease.
• Misconception: Dissolving sugar must leave the water level exactly unchanged. Correct: After the initial rise on adding sugar, the level may decrease to some extent as it dissolves.
• Misconception: Particles in every solid are closer together than in its liquid. Correct: Generally liquid particles are somewhat farther apart, but ice is an exception to this comparison.

Exam-style questions with model answers

Q1. Define matter and state the meaning of its volume. [2 marks]

1. Matter is anything that has mass and occupies space.
2. The volume of matter is the space that it occupies.

Q2. A balloon expands when air is blown into it and its opening is held closed. State the conclusion about air and explain why this observation does not itself measure mass. [2 marks]

1. The balloon's expansion shows that the added air occupies space inside it.
2. The observation concerns occupied space, or volume; it gives no balance reading or other measurement of mass.

Q3. A sample of 200 mL of water is transferred without spilling or leaving water behind from a clean container A to differently shaped clean containers B and C. Predict its shape and volume, and explain the shape change using particles. [3 marks]

1. The water takes the shape of each new container, so its shape in B and C differs from its shape in A.
2. Its volume remains 200 mL in each container because the same sample is transferred without loss. A change of container shape does not mean a change of water volume.
3. Liquid particles can move past one another within a limited space, allowing the water to fit the new container.

Q4. Explain the solid state in four points: particle packing, attraction, motion, and resulting shape and volume. [4 marks]

1. The constituent particles of a solid are tightly packed, with only small spaces between neighbouring particles.
2. Very strong interparticle attraction holds those particles together and keeps them in their fixed positions.
3. The particles vibrate to and fro about those positions; they are not completely motionless and cannot move freely past one another.
4. This closely held arrangement gives a solid a definite shape and a definite volume.

Q5. A needle-free syringe contains air. Its opening is sealed, and pushing its plunger inwards reduces the occupied air volume. In a repeat with water, the plunger hardly moves. Explain these observations in five separate points using particles. [5 marks]

1. Sealing the opening prevents air from escaping, so the smaller occupied space shows compression of the trapped sample rather than loss of air.
2. Gas particles have large spaces between them. These spaces allow the particles to be brought closer together when the plunger is pushed.
3. As the particles come closer, their interparticle spaces decrease, reducing the volume occupied by the same trapped air.
4. This explanation involves changes in particle separation, not the disappearance of particles or a claim that each particle has become smaller.
5. Water is practically incompressible in this demonstration. Its particles are already close together, so its volume is very difficult to reduce in the same way.

Q6. Compare solids, liquids and gases under these five headings: shape, volume, particle packing, particle motion and strength of attraction. Include the ice exception when discussing packing. [5 marks]

1. Solids have a definite shape. Liquids and gases have no fixed shape and take the shape of their containers.
2. Solids and liquids have definite volumes. Gases have no fixed volume and tend to occupy all the available space.
3. Solid particles are tightly packed; liquid particles are generally somewhat farther apart, and gas particles widely separated. Ice is an exception because its particles are farther apart than those in liquid water.
4. Solid particles vibrate about fixed positions. Liquid particles move past one another within limited space. Gas particles move freely and randomly in all directions.
5. Attraction is very strong in solids, slightly weaker in liquids and negligible in gases. These differences help explain the different patterns of particle movement.

Q7. One hydrogen molecule contains two hydrogen atoms, one oxygen molecule contains two oxygen atoms, and one water molecule contains two hydrogen atoms and one oxygen atom. Compare their composition in three separate points. [3 marks]

1. A hydrogen molecule contains two atoms, both of hydrogen. It therefore contains atoms of one element, even though it has more than one atom.
2. An oxygen molecule also contains two atoms of one element, but those atoms are oxygen atoms rather than hydrogen atoms.
3. A water molecule contains three atoms in all: two hydrogen atoms and one oxygen atom. Unlike the other two molecules, it contains atoms of two different elements.

Q8. Adding sugar to water initially raises the level. After stirring, the sugar is no longer visible and the level may decrease to some extent. Explain the observations in three points without assuming an exact final level. [3 marks]

1. The sugar has dissolved rather than ceased to exist. Its constituent particles are still present, although the separate sugar grains are no longer visible.
2. The separated sugar particles occupy available spaces between the water particles, explaining why the volume of the solution is less than the sum of the separate volumes.
3. Adding the sugar first raises the level because the undissolved sugar occupies space in the water. The observations do not establish an exact final level after dissolving.

Key takeaways

• Matter has mass and occupies space; air satisfies this definition even though it cannot be seen.
• Mass describes the quantity of matter, while volume describes the space occupied by it.
• Matter contains extremely small constituent particles; a visible grain of powder contains many such particles.
• Atoms can combine into molecules; a water molecule contains two hydrogen atoms and one oxygen atom.
• Solid particles are tightly packed and vibrate about fixed positions under very strong attractive forces.
• Liquid particles remain close but move past one another, giving a definite volume without a fixed shape.
• Gas particles move freely in all directions, and their large separations allow gases to be compressed readily.
• Particle explanations must preserve qualifications, including slightly weaker liquid attraction, negligible gas attraction and practically incompressible water.

Test yourself

Which two properties define matter?

Matter has mass and occupies space. Both properties belong to solids, liquids and gases.

What does a balloon's expansion demonstrate about air?

Air occupies space inside the balloon. The expansion makes that occupied space easier to recognise.

Why is a visible chalk speck not one constituent particle?

The speck itself contains many constituent particles, which are too small to see through an ordinary microscope.

What is the difference between an atom and a molecule?

An atom is a constituent particle of an element. Atoms of most elements cannot exist independently, so two or more of them combine to form a stable particle called a molecule, for example two hydrogen atoms in a hydrogen molecule.

Are the particles in a solid stationary?

No. They vibrate about fixed positions but cannot move freely past one another.

Why can a liquid flow and still have a definite volume?

Its particles can move past one another, while attraction keeps them close together within a limited space.

What changes when trapped air is compressed?

The particles come closer together, reducing the spaces between them and the volume occupied by the air.

Which exception qualifies the general comparison of solid and liquid particle spacing?

Ice is an exception: its particles are farther apart than those in liquid water.