Chapter 1 — Matter and its Composition

Meaning and everyday view: Matter is anything that occupies space and has mass. This definition includes solids like wood and stone, liquids like water and oil, and gases like air. Even substances that seem invisible, such as the air we breathe, are matter because they take up space and have weight. Learning to identify matter helps children connect school science with the world around them: toys, food, clothes and even sunlight (not matter) can be compared.

How we test for matter: Two simple tests show whether something is matter: it takes up space (volume) and it has mass. Volume can be measured directly for liquids using a measuring cylinder, or found by displacement for irregular solids. Mass is measured with a balance. An object that passes both tests is matter.

States of matter: At normal classroom temperatures most matter appears in three states — solid, liquid and gas. Solids have definite shape and volume because their particles are closely packed. Liquids have definite volume but take the shape of their container because their particles can move past each other. Gases have neither definite shape nor volume and expand to fill any container. It helps to visualise particles: solids tightly packed, liquids loosely touching and gases widely spaced and moving fast.

Why this matters: Understanding what matter is prepares students for later ideas such as atoms and molecules, mixtures and chemical reactions. It also explains everyday phenomena: why ice melts, why perfume spreads through a room, and why balloons expand in warm weather. By recognising matter in different forms, students develop observational skills useful for experiments and practical life.

Measuring mass: Mass measures how much matter an object contains. In the classroom we use a beam balance or an electronic balance. A beam balance works by comparing the object with standard masses until the two pans balance. The electronic balance gives a direct digital reading. When using any balance, place the object carefully, set the scale to zero before starting, and record the reading to the correct unit. Mass stays the same whether the object is on Earth or moved slightly; it changes only when matter is physically removed or added.

Measuring volume of liquids: For liquids, a measuring cylinder or a measuring cup is used. Always place the cylinder on a flat surface and read the level at eye height. For clear liquids, read the bottom of the meniscus (the curved surface). Record the volume in millilitres (mL) or litres (L) using the marks on the cylinder. Small errors come from not reading at eye level or from parallax.

Measuring volume of solids: Regular solids such as cubes and cuboids use simple formulas: volume of a cuboid = length × breadth × height. Measure each side in centimetres (cm) and multiply to get cm3. A cube has volume side3. For irregular-shaped solids, use the displacement method: fill a graduated cylinder with a known volume of water, note the initial reading, immerse the solid fully and read the new level. The rise equals the solid’s volume. For very small objects, use a pipette and small measuring cylinder for more accuracy.

Connection to density and practice tips: Density links mass and volume: density = mass ÷ volume. You will use this formula later to compare materials. In experiments, always record units, repeat measurements for accuracy and average results. Keep balances level and clean, and handle glassware carefully to avoid breakage when measuring volumes.

Physical properties explained: Physical properties are features you can observe or measure without changing the substance into another. Colour, odour, melting point, boiling point, hardness, solubility and density are all physical properties. For example, melting point tells at what temperature a solid becomes a liquid. Solubility tells how much of a substance dissolves in a given amount of solvent. These properties help us identify materials and choose suitable uses — for instance, a hard material for cutting tools or a soluble material for medicines.

Physical changes: When a substance undergoes a physical change, its form may change but its chemical identity remains the same. Examples: melting ice, breaking glass, or dissolving sugar in water. Most physical changes are reversible: freeze the melted water and it becomes ice again; evaporate the water and recover dissolved sugar.

Chemical properties and chemical changes: Chemical properties describe how a substance reacts with others to form new substances. Flammability (ability to burn), reactivity with acids, ability to rust, and tendency to decompose are chemical properties. A chemical change occurs when new substances form; particles rearrange and bonds break or form. Indicators of chemical change include evolution of gas (bubbles), unexpected colour change, formation of a precipitate (solid forming from two solutions), and temperature change. Chemical changes are often not easily reversible — burning paper produces ash and gases that cannot be turned back into paper by simple means.

Testing and interpreting: In the lab, students learn to observe carefully and list indicators to decide whether a change is physical or chemical. Sometimes both occur together (some dissolutions produce heat), so ask questions: did the composition change? If yes, it is chemical. Record observations clearly, mention safety precautions when dealing with chemical reactions, and repeat observations when possible for reliable conclusions.

Definition and nature of elements: An element is a substance made of only one kind of atom and cannot be broken down into simpler substances by ordinary chemical methods. Elements are the simplest building blocks of matter and each element has characteristic properties such as colour, density and reactivity. Metals like iron and non-metals like oxygen are both elements, though they behave differently.

Atoms and symbols: Each element is made of atoms of that element. Scientists use short symbols to represent elements — often one or two letters — to make writing easier: H for hydrogen, O for oxygen, Fe for iron. Learning these symbols helps in writing simple formulas and word equations. For class 7, focus on common element names and symbols so you can recognise them in experiments and examples.

Occurrence and extraction: Some elements occur freely in nature (gold, silver) while others are found combined in minerals (iron in iron ores). Extracting elements from ores requires chemical processes and heating; these industrial methods are studied later. Elements combine in many ways to form the compounds that make up the materials we use daily.

Why elements matter: Understanding elements helps explain why substances behave as they do. For example, metals conduct electricity and heat because of how their atoms are arranged; non-metals like oxygen are essential for respiration. Recognising elements also prepares students to study the periodic table later, which arranges elements in useful patterns and predicts properties. At this stage, practice naming common elements and identifying simple examples in daily life.

What is a compound? A compound is a pure substance formed when two or more elements chemically combine in fixed proportions. In a compound, atoms of different elements join together by chemical bonds to make a new substance with properties different from the separate elements. For example, sodium is a soft metal and chlorine is a poisonous gas, but together they form sodium chloride (table salt), a solid which is useful for seasoning food.

Fixed composition and properties: A key property of a compound is that it has a fixed composition by mass — water always has two hydrogen atoms for each oxygen atom (H2O). Because of this fixed ratio, compounds have uniform properties and can be represented by chemical formulas. Compounds may form by reactions that release or absorb energy; decomposing them back to elements requires chemical change, such as electrolysis for water.

Formation and separation: Compounds are produced by chemical reactions and cannot be separated into elements by physical methods. For example, heating a mixture of sodium and chlorine will not make sodium chloride unless they react chemically under controlled conditions. To separate compounds into their elements or simpler compounds one must use chemical processes (decomposition, electrolysis, etc.).

Everyday examples and importance: Water (H2O), carbon dioxide (CO2), sugar (C12H22O11) and baking soda (sodium hydrogen carbonate) are common compounds. Knowing what compounds are helps explain why materials behave differently from their constituent elements and is essential for understanding nutrition, medicines, fuels and materials used in daily life. Basic lab activities such as mixing vinegar and baking soda show compound formation and gas evolution, illustrating chemical change.

Definition and general features: A mixture contains two or more substances physically combined, where each component keeps its own properties. Unlike compounds, mixtures do not have fixed composition — the amounts of each substance can vary. Mixtures can involve solids, liquids and gases in any combination, for example, air (gases), salt solution (liquid + dissolved solid) or a trail mix (different solids).

Homogeneous vs heterogeneous: Homogeneous mixtures, also called solutions, are uniform throughout; we cannot see separate parts with the naked eye. Examples include sugar dissolved in water and air (as a mixture of gases). Heterogeneous mixtures show distinct parts or phases, such as oil floating on water, sands in water, or a salad where components are clearly visible. The type of mixture determines how we will separate its components.

Properties and behaviour: Components in a mixture can be separated by physical methods because they are not chemically bonded. Solubility, particle size and magnetic properties help choose a separation method. Mixtures may be temporary (suspensions where particles settle) or stable (solutions where particles remain dissolved). Temperature, stirring and solvents affect how well components mix or separate.

Practical importance and examples: Many everyday materials are mixtures: milk, soil, alloys, and many foods. Industrial processes start with mixtures (crude oil) and separate them into useful products (petrol, diesel). Understanding mixtures aids in cooking, cleaning and making medicines. Simple classroom tests like filtering sand-water or dissolving sugar in water demonstrate mixture behaviour and set the stage for separation techniques taught later.

Understanding filtration: Filtration is a method to separate an insoluble solid from a liquid by using a porous material like filter paper, cloth or a porous plate. The mixture is poured through the filter; the solid particles are too large to pass and remain as residue on the filter paper, while the liquid that passes through is called the filtrate. Filtration works because of differences in particle size and is widely used in daily life and laboratories — for example, filtering tea leaves from tea or separating sand from water.

Sedimentation and decantation explained: Sedimentation uses gravity to allow heavier solid particles in a suspension to settle to the bottom over time. Once settled, the clear liquid above can be gently poured off — this pouring is called decantation. Sedimentation is slow but useful when particles are much denser than the liquid and large enough to fall. Combining sedimentation with decantation provides a simple method to separate muddy water before filtration to achieve clearer results.

How to carry out the methods safely and accurately: For filtration use a clean funnel and fold filter paper correctly so the flow is even. Pour slowly to avoid overflow. For sedimentation allow sufficient time for particles to settle and decant carefully to avoid disturbing the deposit. Fine particles may not settle easily; in such cases, coagulation (adding a coagulant) or filtration can help. Always label all containers and dispose of residues according to teacher guidance.

Examples and limitations: Filtration is excellent for separating sand from water but cannot separate dissolved salts. Sedimentation is useful for removing larger suspended solids but not for colloidal or very fine particles. In many procedures both methods are used together — first sedimentation/decantation to remove bulk solids, then filtration to obtain a clearer liquid. Understanding when to use each method builds good practical problem-solving skills for experiments and everyday tasks like cleaning water.

Evaporation: Evaporation separates a dissolved solid from its solvent by heating the solution until the liquid evaporates, leaving the solid behind as residue. This method is simple and useful when the solid is the desired product and purity is not critical. For example, shallow pans of seawater are heated by the sun to evaporate water and leave salt. In the classroom, an evaporating dish on a water bath can safely remove water from a salt solution, but care is needed to avoid bumping and splattering.

Crystallisation: Crystallisation is a gentler method focused on obtaining pure solid crystals from a saturated solution. First dissolve as much solute as possible in hot solvent to make a saturated solution and then filter the hot solution to remove insoluble impurities. Allow the filtered solution to cool slowly; as temperature drops, the solute’s solubility decreases and pure crystals form. The crystals are collected by filtration and dried. Slow cooling and careful handling give larger, purer crystals compared to rapid evaporation.

Why crystallisation gives purer products: During slow cooling the solute arranges into an orderly crystal lattice, excluding many impurities which remain dissolved in the liquid (mother liquor). Rapid evaporation traps impurities inside small crystals and produces powdery residue. Because of this, industries and laboratories use crystallisation to purify substances such as sugar and many inorganic salts.

Practical tips and limitations: Choose the right solvent — it should dissolve the solute well when hot but poorly when cold. Avoid contaminating the solution and perform hot filtration to remove dirt. Crystallisation is time-consuming and needs patience but yields higher purity. Evaporation is faster and simpler but often gives impure solids, so the method choice depends on whether purity or speed is more important.

Magnetic separation: Magnetic separation uses a magnet to remove magnetic materials such as iron filings from a mixture containing non-magnetic substances like sand or powder. Gently moving a magnet over or through the mixture attracts and holds magnetic particles, which can then be transferred to another container or removed by hand. This method is simple, quick and useful in many contexts — from cleaning up spills of metal filings in workshops to recovering iron particles in recycling plants.

Sieving: Sieving separates particles of different sizes using a sieve or mesh screen. The mixture is passed through a sieve; small particles pass through the holes while larger ones are retained. Sieving is commonly used in kitchens (sifting flour to remove lumps), in construction to separate different sizes of sand and gravel, and in laboratories to classify particles by size. Multiple sieves with different mesh sizes can be stacked to separate a mixture into several size fractions.

Combining methods and practical use: Often magnetic separation and sieving are combined with other techniques. For example, to clean a mixed sample of gravel, sand and iron, sieve first to separate sizes and then use a magnet on the sand fraction to remove iron. For best results, spread the material thinly and work on a clean surface to see separation clearly. Remember that magnetic separation works only for magnetic materials; non-magnetic metals like aluminum cannot be removed by a magnet.

Limitations and safety: Sieving is not effective if particles stick together or if the mixture is wet and clogging occurs; drying or washing may be needed first. While magnetic separation is clean and safe, avoid putting strong magnets near electronic devices or credit cards. Teach students to handle sieves and magnets carefully to avoid injuries and ensure consistent, reliable separation results.

Chromatography (paper method): Chromatography separates components of a mixture based on how strongly they cling to a stationary phase (paper) versus how well they dissolve and move in a mobile phase (solvent). In simple paper chromatography, a small spot of ink or dye is placed near the bottom of a strip of chromatography paper. The paper’s end is dipped into a solvent that moves up the paper by capillary action. Different components travel at different speeds and form separate spots at different heights. This method is useful to separate and identify pigments in inks and plant extracts and to check purity.

How to interpret chromatography results: Measure the distance each spot travelled and the distance the solvent travelled to calculate Rf values later; for class 7 it is enough to note which colours separate and how far each moves. A pure dye will usually give one spot, while a mixture gives several spots. Chromatography is sensitive, requires small sample sizes and is widely used in science and forensics.

Simple distillation: Distillation separates liquids based on different boiling points. When a liquid mixture is heated, the component with the lower boiling point vaporises first; the vapour is then cooled in a condenser and collected as liquid. Simple distillation can separate a solvent from a dissolved solid (e.g., obtain pure water from salt solution) or separate two liquids with sufficiently different boiling points. Distillation needs careful heating and proper glassware to avoid accidents.

Safety and classroom practice: For paper chromatography, use non-toxic solvents such as water or alcohol under supervision and avoid inhaling fumes. For distillation, follow teacher instructions when heating and use clamps and heat sources correctly. Both techniques show how physical properties like solubility and boiling point help separate mixtures. Observing demonstrations or performing safe, small-scale experiments reinforces these separation principles.

Particle theory of matter: The particle idea explains matter as made of tiny discrete particles — atoms and molecules. An atom is the smallest unit of an element that retains its chemical identity. A molecule is a group of two or more atoms bonded together and represents the smallest unit of a compound that shows its properties. This simple model helps explain why materials behave differently in different states and during reactions.

How atoms form molecules: Atoms join together to form molecules. In some elements, atoms pair up to become stable — for example, oxygen exists as O2 where two oxygen atoms form a molecule. In compounds, atoms of different elements combine in fixed ratios: water is represented by H2O, meaning two hydrogen atoms combine with one oxygen atom to make each water molecule. Representing molecules with symbols and simple diagrams (balls connected by lines) helps visualise composition without complex atomic theory.

Explaining properties with particles: The particle model explains several observations: solids are rigid because particles are packed tightly and vibrate in place; liquids flow because particles can move around each other; gases expand because particles move freely and are far apart. Mixing and dissolving occur because particles of solute spread among solvent particles. During chemical reactions atoms are rearranged to form new molecules; atoms themselves are conserved, which leads to the conservation of mass idea.

Limitations and classroom focus: Detailed structure inside atoms (electrons, protons, neutrons) and bonding theories are studied later. For class 7, the emphasis is on visualising particles, writing simple formulas like H2, O2, H2O, and using particle ideas to predict state changes, mixing behaviour and basic reaction outcomes. Simple models and diagrams combined with hands-on experiments strengthen understanding of matter at the particle level.

Recap of types of change: A physical change alters the form or appearance of a substance without changing its chemical identity — examples include melting, freezing, cutting and dissolving. In contrast, a chemical change produces one or more new substances with different properties; burning, rusting and decomposition are common chemical changes. Recognising which type has occurred requires careful observation and reasoning.

Observations that distinguish changes: Indicators of chemical change include gas evolution (bubbles), permanent colour change, formation of a precipitate, emission or absorption of heat, light production, and changes that cannot be reversed easily. For physical changes, we usually see shape change, state change or mixing where no new substance forms and the process is often reversible.

Word equations: Word equations give a simple way to represent chemical reactions by naming reactants and products. They do not show amounts or atomic detail but clearly state what reacts and what forms. For example, rusting of iron is written as: iron + oxygen → iron oxide. The reaction of vinegar and baking soda can be written as: vinegar + baking soda → sodium acetate + water + carbon dioxide. Word equations are useful in class to describe reactions observed in demonstrations.

Class activities and safety: Students should practice predicting whether changes are physical or chemical from short descriptions and write appropriate word equations. Simple, safe demonstrations — like dissolving salt in water (physical) and mixing vinegar with baking soda (chemical, producing gas) — help reinforce learning. Emphasise safety: perform chemical observations under teacher supervision, wear goggles where necessary, and do not taste or smell unknown chemicals directly.

Basic idea of conservation: The conservation of matter means that during ordinary physical and chemical changes the total mass of the substances involved remains the same if the system is closed. This follows from the particle idea: atoms are neither created nor destroyed in normal chemical reactions, only rearranged to form new substances. For class-level experiments, showing conservation requires careful setup to prevent loss or addition of matter from outside.

Class demonstration and reasoning: A simple demonstration is to burn magnesium ribbon inside a closed container placed on a balance. After the reaction, if no gas escapes, the measured mass stays the same even though the appearance changes — magnesium ribbon becomes white magnesium oxide powder. If the burning is done in open air, mass might increase because oxygen from air joins magnesium, or it might appear to decrease if gases escape; this is why closed systems are important to show conservation clearly.

Everyday examples connected to conservation: During baking soda and vinegar reaction in a sealed bottle, the mass stays unchanged though gas is produced inside. When water freezes in a sealed container, the mass before and after freezing is the same. Understanding conservation helps when interpreting weighing results in experiments and explains puzzling changes in mass if gases escape or if material is added from the surroundings.

Practical tips for experiments: To show conservation use closed flasks or sealed systems and measure mass before and after. Record observations like gas formation and colour change, and explain mass changes by considering gases entering or leaving the system. Teach students to think about where each atom goes during a reaction and to use word equations to track reactants and products. This habit of careful measurement and accounting is essential for accurate experimental science.

Importance of safety: Working in a chemistry lab requires habits that protect you and others. Safety prevents accidents like burns, cuts and harmful exposures. Good laboratory behaviour also produces reliable results because careful handling of materials and equipment reduces errors in measurement and observations. As you learn practical chemistry, build a routine of safety and cleanliness.

Essential safety rules: Always wear safety goggles and an apron to protect eyes and clothing. Tie back long hair and avoid loose sleeves. Never taste or directly sniff chemicals; use wafting (wave a hand to carry a small smell toward your nose) only when teacher permits. Keep work area tidy, label all bottles and never mix unknown chemicals. Know the location of the fire extinguisher, first-aid kit and eye-wash station and how to alert the teacher in case of an accident.

Handling apparatus and materials: Inspect glassware for cracks before use and handle hot glass with tongs or heat-resistant gloves. Use clamps to hold flasks or test tubes when heating; point open test tubes away from people. Measure liquids accurately with measuring cylinders and pipettes, and note the meniscus when reading volumes. When heating substances, use a water bath rather than direct flame for delicate work. Dispose of wastes as instructed — solids, liquids and broken glass require different disposal methods.

Developing practical skills: Good practical technique includes careful measurement, clear note-taking, repeated trials and proper observations. Learn to set up apparatus neatly, follow step-by-step procedures and clean equipment after experiments. Practice communication by writing clear lab notes describing steps, observations and conclusions. These skills increase safety, improve experimental accuracy and prepare you for more advanced laboratory work in higher classes.