Chapter 6 — Substances in Daily Use

A substance is anything that has mass and occupies space. In daily life we meet many substances: water in a glass, air around us, sugar in a packet, the metal of a spoon, cloth in our clothes and wood in furniture. Some substances are simple and contain only one kind of particle — for example, distilled water or pure salt — while others are mixtures of different substances, like milk or lemonade. It is useful to recognise whether what we see is a pure substance or a mixture because this affects how it behaves when we heat, cool, or try to separate it.

When we study substances we look at properties that help us identify and use them. Observable properties include colour, smell and taste. Measurable properties include melting point, boiling point, density and conductivity. Practical properties that matter at home are solubility (does it dissolve in water?), hardness (will it scratch or break?), and whether it conducts electricity or heat. For example, metal spoons conduct heat and electricity; a rubber handle does not.

Classifying substances helps in daily decisions: whether to store a material in a glass jar or a plastic box, whether it will dissolve in water, or whether it can be safely heated. It also prepares us for experiments: before mixing two materials, we think about what might change visibly or chemically. Learning simple tests—like dissolving, filtering, or heating—helps students confirm what kind of substance they have. Clear understanding of the meaning of ‘substance’ and its properties is the first step in studying mixtures, changes and materials in later chapters.

Matter exists commonly in three states: solid, liquid and gas. Solids have a fixed shape and fixed volume — they do not flow easily. Particles in a solid are closely packed and only vibrate in place. Examples include a wooden block, a stone, or a pencil. Liquids have a fixed volume but take the shape of their container; their particles are less tightly packed than a solid and can move past each other. Examples are water, oil and milk. Gases have no fixed shape or fixed volume; they expand to fill their container. Gas particles are far apart and move freely; examples include air, oxygen and carbon dioxide.

Temperature affects state. Heating a solid may make it melt into a liquid; cooling a liquid may freeze it into a solid. Heating a liquid may make it boil into a gas; cooling a gas can condense it into a liquid. These changes are called physical changes because the substance remains the same chemically (for example, water remains H2O whether solid, liquid or gas). Observing melting ice, boiling water and condensing steam helps students connect states with particle ideas.

Properties such as compressibility and density also differ. Gases are easily compressible and least dense, liquids are almost incompressible and have intermediate density, and solids are least compressible and often most dense. These differences explain everyday phenomena like why balloons can be squashed but a jar of water cannot. Simple experiments — feeling a frozen ice-cream (solid), pouring syrup (viscous liquid), or inflating a balloon (gas) — make these ideas clear. Understanding states of matter helps when we cook, store food, and use household materials safely.

A mixture is formed when two or more substances are combined without chemically bonding to each other. Mixtures are common at home: tea made by adding sugar to hot water, air as a mixture of gases, and cereal mixed with milk. Mixtures may be homogeneous or heterogeneous. In a homogeneous mixture the composition is the same throughout; such a mixture is called a solution. For example, when sugar or salt disappears completely in water and the liquid looks uniform, we call it a solution. In a heterogeneous mixture the different parts are visible, like sand mixed with iron filings or fruit salad where each piece is seen separately.

In a solution there are two parts: the solute and the solvent. The solute is the substance that dissolves (for example, sugar) and the solvent is the liquid that does the dissolving (often water). Solubility tells us how much solute can dissolve in a given amount of solvent at a given temperature. Many solids dissolve better in warm water than cold — that is why tea brews faster with hot water. Besides true solutions, there are suspensions and colloids. Suspensions have large particles that eventually settle, like muddy water. Colloids have tiny particles that stay dispersed and do not settle easily, such as milk or starch in water.

Knowing the difference between solution and suspension helps in choosing separation methods: filtration can separate suspended solids but not dissolved solutes; evaporation can recover solute from a solution. Everyday testing — like placing a few drops of liquid on a watch-glass and observing after some time, or passing light through the liquid to see if it scatters (Tyndall effect) — helps classify the mixture. These simple observations and tests connect classroom ideas with routine activities like cooking, cleaning and storing food.

When a substance changes, the change may be physical or chemical. A physical change alters the form, shape or state of a material but does not form a new substance. Examples are melting, freezing, breaking, mixing and dissolving. If you dissolve salt in water and later evaporate the water, you recover the same salt — this shows the change was physical. Physical changes are usually reversible by simple means: freezing melted ice gives ice again; cutting cloth does not change its identity.

Chemical changes produce one or more new substances with properties different from the original materials. They are often indicated by a colour change, release of gas, change in temperature, formation of a precipitate (a solid that appears when two solutions react), or an odour. Burning paper changes it into ash and gases; rusting of iron forms iron oxide, a new compound. Chemical changes are often not easily reversed by simple physical methods. For instance, you cannot recover burnt paper back into original paper by heating or cooling.

Simple classroom experiments clarify these ideas. Heat a small piece of wax to melt it — melting is physical. Burn a little candle wick (with care) and observe smoke and ash — that is chemical. Mix vinegar and baking soda to see bubbling (gas formation) — chemical reaction producing carbon dioxide. When testing changes, students should record observations and ask whether new substances appeared or previous substances can be recovered. These steps help decide the nature of the change and build a habit of careful scientific observation.

Separating the parts of a mixture is often possible using physical methods that take advantage of differences in properties such as particle size, solubility and boiling point. Filtration separates an insoluble solid from a liquid by passing the mixture through a porous medium (filter paper, cloth, or sand). The solid remains as residue on the filter and the clear liquid (filtrate) passes through. This method is used to remove leaves and dust from water or to separate sand from water.

Evaporation is used when a solute is dissolved in a solvent. By heating the solution, the solvent evaporates and leaves the solute behind as crystals; this is how salt is obtained from sea water. Distillation is a more refined method that takes advantage of different boiling points: the mixture is heated to boil the component with the lower boiling point, the vapour is condensed and collected. Distillation is used to obtain pure water from salty water or to separate components of crude oil in industries.

Other useful methods include decantation, where a liquid is poured off gently from a settled solid; sedimentation, where heavy particles settle on standing; and sieving, where a mesh separates large particles from smaller ones. Chromatography separates substances based on different rates of movement through a medium and is useful for separating colours in ink. Magnetic separation can remove iron filings from sand by using a magnet. Choosing the right method depends on the mixture: whether the components are soluble or insoluble, particle sizes, and boiling points. Practising these methods with safe household materials builds skill in planning experiments and interpreting results.

Water is one of the most important substances for life and daily activities. It is used for drinking, cooking, cleaning, bathing, farming and industry. Water has special properties: it dissolves many substances (so it is called a universal solvent), it absorbs heat and changes temperature slowly (high specific heat), and it exists naturally in three states — solid (ice), liquid (water) and gas (vapour). These properties make water central to weather, climate and living systems.

Because water dissolves many substances, it often contains dissolved salts and impurities. Some water sources also have suspended particles or harmful microorganisms. Clean drinking water should be free from disease-causing microbes and excessive dissolved salts. Common household methods to improve water quality include filtration (to remove suspended solids), boiling (to kill microbes), and simple sedimentation followed by careful decanting. For serious purification, techniques like chlorination, distillation or reverse osmosis are used in treatment plants. It is important to store drinking water in clean, covered containers to prevent recontamination.

Water conservation is vital because fresh water is limited. Simple ways to save water include fixing leaks, using buckets for washing instead of running taps, reusing water where safe (like using rinse water for plants), and collecting rainwater for household use. Understanding how to test water quality with simple observations — clarity, odour, presence of sediment, and taste — and taking small steps to conserve and purify water helps protect health and conserve this precious resource for future generations.

Hard water contains dissolved minerals, mainly compounds of calcium and magnesium. These minerals come from rocks such as limestone when rainwater passes through soil and rock. Hard water affects daily life: it reduces the ability of soap to produce lather, forms scum with soap, and leaves white deposits or scale on kettles, pipes and heating elements. Scale reduces the efficiency of heating equipment and may block pipes over time. Soft water, on the other hand, contains very few dissolved minerals and makes lather readily and cleans well.

Hardness may be temporary or permanent. Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium; boiling such water removes hardness by causing carbonate salts to precipitate as a solid which can be filtered out. Permanent hardness is caused by chlorides and sulfates of calcium and magnesium; boiling does not remove it. Other methods to soften water include adding washing soda (sodium carbonate) which precipitates calcium, using ion-exchange resins or water softener units which replace calcium and magnesium ions with sodium ions, and using reverse osmosis for low-scale domestic treatment.

Simple home tests help recognise hardness: add soap solution to a sample of water and shake; if a stable lather forms, the water is soft; if scum forms and little lather appears, the water is hard. Observing white deposits on a kettle after boiling water for some days is another sign. Knowing whether water is hard or soft helps in choosing detergents and care methods for appliances. In places with very hard water, using appropriate softening methods reduces soap use, saves energy by preventing scale, and extends the life of household appliances.

Soaps and detergents are substances used for cleaning. Both help remove dirt, oil and grease from surfaces or clothes by reducing the surface tension of water and surrounding oily particles. A soap molecule has a long chain with two ends: a hydrophobic tail that is attracted to oil and grease, and a hydrophilic head that is attracted to water. In water, soap molecules arrange around oil droplets to form structures called micelles; the oil is trapped inside and the micelle can be washed away. This is how soap cleans dishes or clothes.

Soaps are usually made by reacting fats or oils with a strong alkali (eg, sodium hydroxide) in a process called saponification. Detergents are synthetic cleaning agents made from petrochemicals and designed to perform well even in hard water. Unlike soap, many detergents do not form scum with calcium and magnesium ions, so they are effective in hard water. However, some detergents may contain chemicals that harm aquatic life or are slow to biodegrade, so choosing biodegradable and eco-friendly options is better for the environment.

Appropriate use is important: use the right quantity for cleaning — too little soap gives poor cleaning, too much wastes water and pollutes. For heavily soiled clothes, soak first to loosen dirt; for oily dishes, use warm water to help dissolve grease. Understanding how soaps and detergents work also explains why soap works well with warm water and why some products are labelled for specific uses such as laundry, dishwashing, or surface cleaning. Teaching simple experiments — washing an oily dish with water alone and then with soap — shows the cleaning action and connects chemistry with daily chores.

Some common household substances are classified as acids or bases (alkalis). Acids often taste sour and can react with metals and substances such as baking soda to produce gas; examples are lemon juice (citric acid), tamarind and vinegar (acetic acid). Bases taste bitter and feel slippery when dilute; soap solution and baking soda are mild bases. In school we often use indicators to test whether a substance is acidic, basic or neutral. Litmus paper is a simple indicator: blue litmus turns red in acid, red litmus turns blue in base. Natural indicators can be made from red cabbage, turmeric or beetroot. These natural extracts show different colours in acid and base and are safe for classroom use.

Indicators change colour because acids and bases change the structure of the indicator molecules. For example, red cabbage extract contains pigments that respond differently to hydrogen ion concentration. Another important idea is neutralisation: when an acid reacts with a base, they form a salt and water, generally reducing acidity. This principle is used practically in medicines (antacids neutralise excess stomach acid) and in agriculture (lime is added to acidic soils to neutralise them).

Simple experiments help students see these effects: mix lemon juice with baking soda and observe bubbling due to carbon dioxide gas — this is an acid-base reaction. Test household liquids with red cabbage extract to observe colour changes: acids give pink/red shades, neutrals purple and bases greenish to yellow. Remember safety: strong acids and bases can harm skin and eyes, so always handle with care under teacher supervision and use small quantities for classroom tests.

Foods contain several types of nutrients that our body needs. The three major groups we study here are carbohydrates, proteins and fats. Carbohydrates are the primary source of energy and are found in rice, wheat, potatoes, sugar and fruits. Starch is a common carbohydrate in foods like bread and potatoes. Proteins are essential for growth, repair and making enzymes; they are present in pulses, milk, eggs, fish and meat. Fats provide concentrated energy, help store vitamins and insulate the body; they are found in oils, butter, ghee and nuts.

Simple tests in the school lab can show the presence of these nutrients in food. Starch gives a blue-black colour when tested with iodine solution — this is a quick test for carbohydrate starch. Proteins give a violet or purple colour in the biuret test, which uses a mixture of copper sulfate and sodium hydroxide; this should be done with teacher supervision. Fats can be detected by rubbing a small amount of food on paper — if the paper becomes translucent or leaves a grease spot, fat is present. Another method for fat detection is the emulsification test: mixing the food with ethanol and then with water produces a milky emulsion if fat is present.

Understanding these nutrients helps students choose a balanced diet: carbohydrates for energy, proteins for growth, and fats for energy storage and vitamin absorption. It also builds awareness that processed foods may have high fat or sugar, and that variety in meals ensures different nutrients are supplied. Practical food tests and simple comparisons between foods promote healthy eating habits and scientific thinking about what we eat every day.

Objects around us are made from materials chosen for certain properties: strength, hardness, flexibility, conductivity, transparency, absorbency and resistance to water or heat. Metals are strong, malleable (can be beaten into thin sheets), ductile (can be drawn into wires) and good conductors of heat and electricity; they are used for utensils, wires and tools. Plastics are lightweight, resist corrosion, can be moulded into many shapes and are often water-resistant — used for containers, toys and pipes. Wood is hard, can be cut and shaped, insulates against heat and electricity, and is used for furniture and building.

Textiles (cloth) may be made from natural fibres like cotton and wool or synthetic fibres such as nylon and polyester. Cotton is breathable and absorbent, making it good for summer clothes; wool keeps warm and is suitable for winter wear. Synthetic fibres dry quickly and may be more durable. Paper is light and easy to write on, and biodegradable in many cases. Each material has advantages and disadvantages; for example, metal utensils last long but may corrode, while plastic is light but may not be biodegradable.

Simple tests can show these properties: check conductivity by making a simple circuit with a battery and bulb; place a piece of material in the circuit and see if the bulb glows — metals allow current, wood and plastic do not. Test flexibility by bending strips of materials; test transparency by placing an object behind them and seeing visibility. These observations guide everyday choices: pick a metal pan for cooking because it conducts heat, choose cotton clothes in hot weather, or use a plastic container for wet foods but prefer glass if you want to avoid plastic contact with hot food. Understanding properties also supports recycling decisions and safe use of materials at home and school.

Polymers are large molecules made of repeating units called monomers joined together in long chains. Many common materials known as plastics are polymers. Natural polymers include rubber and cellulose (in plants), while synthetic polymers include polyethylene used in bags, polyvinyl chloride (PVC) used in pipes, and polystyrene used in foam packaging. The way monomers join and how chains are arranged determines properties: some polymers are flexible and elastic (rubber bands), others are rigid and strong (plastic chairs).

The useful properties of plastics — lightness, resistance to corrosion, and mouldability — make them widely used in packaging, toys, household items and construction. However, many synthetic plastics are not biodegradable, so they persist in the environment for a long time, causing litter and harming wildlife. Recycling reduces waste by converting used plastic into new products. Different plastics have recycling symbols and codes that help sort them for recycling. Some plastics can be melted and remoulded; others are thermosetting and cannot be remoulded after forming.

Because of environmental concerns, alternatives and behaviours are important: reduce single-use plastic, reuse items where safe, and recycle according to local rules. Biodegradable plastics and materials like cloth, jute or glass are better choices for many uses. In school, students can sort household items by plastic type, learn local recycling options, and carry reusable bags and bottles. Understanding the chemistry and environmental impact of polymers links material science with responsible habits for the community and the planet.

Substances used in homes can affect the environment if not handled properly. Detergents, paints, oils and plastics may reach rivers and lakes through drains and runoff, harming aquatic plants and animals. Used motor oil and chemicals can contaminate soil and groundwater. Burning mixed waste releases smoke and harmful gases that affect air quality. Pesticides and excess fertiliser from gardens can wash into water bodies and cause algal blooms, which reduce oxygen and harm fish. Even batteries contain metals that pollute soil when dumped in regular trash.

Small changes at home reduce environmental damage. Do not pour cooking oil or motor oil down drains; collect and recycle used oil where facilities exist. Use biodegradable soaps and limited amounts of detergents, and avoid pouring chemical cleaners into storm drains. Segregate waste into wet (compostable) and dry (recyclable) to make recycling easier. Composting kitchen waste returns nutrients to the soil and reduces landfill volume. Reduce single-use plastics by using cloth bags, refillable bottles, and reusable containers where possible. Learn local collection centres for electronic waste and batteries and use them instead of throwing items into household garbage.

Schools can lead community actions: organise waste audits to see what the family throws away, run plastic collection drives, or set up small composting units. Teaching these habits helps students connect science to civic responsibility: clean water and air protect health, and thoughtful disposal and recycling conserve resources for future generations. Understanding the environmental effects of household substances turns classroom learning into practical, everyday actions for a cleaner neighbourhood and planet.