Materials Around Us

An object is a thing such as a notebook, tumbler or pen. A material is a substance used to make an object, such as paper, wood, glass, metal, plastic or clay. A pen may combine plastic, metal and ink; one material can also be used in many different objects.

Classification means arranging objects or materials into groups using a shared property. The same collection can be grouped by shape, colour, material or use. State the chosen property: a group of red objects and a group of metal objects answer different questions.

Activities 6.1 and 6.2 ask you to identify materials and then group objects. Inspect the relevant part of a mixed-material object rather than assigning every part the same property. Organising kitchen utensils or shop goods is a practical use of classification.

Materials in historical practice

The opening reference describes clay and iron as crucible materials. The pottery box describes selecting and preparing clay, shaping it, applying decorative coatings and firing it in a kiln; fired clay is terracotta. Harappan pots served cooking and storage needs, including grain, oil and ghee. This illustrates material choice and processing; no new archaeological date or “earliest” claim is added here.

The material must suit the intended use. A tumbler must retain water: glass, steel and suitable plastic can do this, while ordinary uncoated cloth allows water through. Different parts of an object may need different properties. Ordinary paper is not a suitable cooking vessel over a flame.

A tennis ball, cricket ball and hand-exercise ball are designed for different purposes. Size, texture, hardness and bounce can matter. You cannot infer that one is suitable for another sport merely because both are round.

1. 1

   Predict a comparison

   For Activity 6.4, compare the supplied balls and predict which will bounce higher. Record the samples and the property being tested.

2. 2

   Keep the drop comparable

   Release from the same height onto the same surface, without adding a push. Compare similar-sized samples where possible and repeat. Real balls can also differ in construction; the result need not be due to material alone.

3. 3

   Record the result, not an invented ranking

   Record high, medium or low bounce, or measured rebound heights if available. Explain the observed difference and the intended use. The chapter supplies an observation table, not a universal numerical bounce result.

Lustre describes a shiny surface. Fresh surfaces of metals such as iron, copper and aluminium are usually lustrous; air and moisture can make a metal look dull. Paper, unpolished wood, rubber and jute are common non-lustrous examples.

Shininess alone does not establish that a material is a metal. Polish, wax or a coating can make another material shiny. Colour and rough or smooth texture are additional observations, not substitutes for identifying the material.

The chapter compares hard and soft samples by pressing or scratching them. A sponge compresses more easily than rubber, and rubber more easily than iron. These are relative comparisons: name the samples and the test. Ease of bending, compression and scratching are not interchangeable measurements.

For Activity 6.5, record the object, its material and the observed resistance to pressing. A pillow can be soft while its fastener is hard. Do not damage useful objects or scratch glass or sharp fragments as a test.

A transparent sample lets you see an object clearly through it. A translucent sample lets light through but does not give a clear view of the object. An opaque sample prevents you from seeing the object through it.

Clear window glass, clean water and air are familiar transparent examples. Frosted glass and butter paper are translucent. A wooden board, cardboard and ordinary metal sheets are opaque. Test the actual sample: thickness, frosting, coatings and suspended material can affect what you see.

In the hide-and-seek picture, the wall and tree block the view, the frosted door gives a hazy view, and the clear window gives a clear view. For Activity 6.6, keep the object and lighting comparable while checking each sample. Clear appearance alone does not establish that water is pure or safe to drink.

In Activity 6.7, place small amounts of sugar, salt, chalk powder, sand and sawdust in separate labelled tumblers with comparable amounts of water. Predict, stir and allow time before recording observations. Keep stirring time, water temperature and sample amounts reasonably comparable; do not taste experimental mixtures.

Small amounts of sugar and salt dissolve in sufficient water: the separate grains are no longer visible. They are described as soluble in water. Sand, chalk powder and sawdust do not dissolve appreciably in this classroom test; particles can remain suspended, settle or float. Floating or sinking alone does not decide solubility.

The dissolved sugar is still present. If no material is added, spilled or lost, the combined mass of sugar and water stays the same after mixing. “Disappears” in the observation table means disappears from view, not from existence.

Solubility always concerns a material in a particular liquid under particular conditions. More stirring does not make every substance soluble. A soluble substance can also remain partly undissolved when more is added than that amount of water can dissolve.

Some liquids mix with water to give one uniform mixture; others separate into layers after standing. Vinegar mixes with water, while mustard oil separates. Honey’s sugars can dissolve in water with mixing. Wheat flour can form a cloudy mixture rather than a true solution; cloudiness is not proof of complete dissolving.

Gases can dissolve too. Dissolved oxygen supports aquatic life. This oxygen is not the same thing as the oxygen chemically bound within water molecules. Water’s ability to dissolve many materials also matters in the body.

ORS: purpose and preparation matter

The textbook uses oral rehydration solution as a real-world example of dissolved sugar and salts used for dehydration. Its household example uses six level teaspoons of sugar and half a level teaspoon of salt in one litre of safe water, consistent with WHO guidance. For packaged ORS, follow that packet’s exact water volume; some sachets are made for 200 mL, so “every packet needs one litre” is not a safe general rule. This is an educational example, not a replacement for medical care or a licence to drink laboratory mixtures.

Activity 6.8 half-fills identical cups with water, sand and pebbles, then compares them on a balance. Predict first and record measured results. Equal filled space does not guarantee equal mass; gaps between pebbles also form part of the filled space.

To compare the contents rather than the cups, use equal empty-cup masses or subtract each empty cup’s mass from its filled mass. Do not invent a fixed ranking when the samples and packing have not been measured.

Mass is introduced as the amount of matter in an object. The SI unit is kilogram, symbol kg; gram, g, is also used. Everyday speech often calls a balance reading “weight”, but mass and weight are distinct physical quantities. Their detailed relationship belongs to later study.

Volume is the space occupied by matter. A bag takes up space on a seat; water takes up space inside a tumbler. Identical tumblers have the same capacity, but one may contain less water than the other. Capacity is how much the container can hold; the present water volume need not equal it.

A 500 mL label describes volume, not mass. Milk and water labelled 500 mL have equal stated volumes, but that label alone does not establish equal masses. Do not exchange g and mL as though they measure the same property.

Matter has mass and occupies space. Air is matter even though clean air is invisible. Sand, pebbles, water and the cup itself are also matter. Materials are types of matter selected to make objects.

Use a space between number and unit: 7 kg, 500 mL and 2 m³. Unit symbols do not take a plural s. The chapter uses L for litre and mL for millilitre; the SI unit of volume is cubic metre, m³.

A traditional classification example

The p.117 box describes ten pairs used in Ayurvedic classification: heavy/light, slow/fast, cold/hot, oily/dry, smooth/rough, solid/liquid, soft/hard, stable/moving, subtle/gross and non-slimy/slimy. This is historical context reported by NCERT; the list is not presented as a modern measurement standard or a treatment recommendation.

Use the chapter’s word-matching and grouping exercises as retrieval prompts. Explain which observation supports a property, and avoid treating a single clue as a unique identification.

1. Grouping and definitions · questions 1–4, 7 and 9

   Propose a useful kitchen sorting rule and explain transparent storage containers. Distinguish matter, soluble, transparent and lustrous. Correct claims that wood is translucent, that all glass is opaque, or that matter occupies no space. An invisible example can be air; a shiny example need not be uniquely identified as one metal.

2. Design for use · questions 5–6

   Compare chair materials against support, portability, comfort and ease of cleaning; no single material automatically wins every requirement. Food-waste containers need leak resistance and cleaning; broken-glass containers need rigid, puncture-resistant walls; dry paper needs a suitable dry container. Consider the whole design, not just the material name.

3. Combine evidence · questions 8–10

   A sample that resists pressing but dissolves can be hard and soluble; one that compresses and remains separate can be soft and insoluble. These clues narrow possibilities but need not identify a unique substance. Test pairs such as glucose–water and vinegar–water against oil–water and flour–water, recording the solvent and what remains after standing.

4. Recycling and reuse · further learning

   Investigate what a local recycler accepts and which material properties matter. A broken object may still contain useful material, but not every material or mixture is accepted everywhere. Classify household samples and propose a useful object from discarded materials. Evaluate plastic by its specific use, durability and disposal route rather than a blanket “good” or “bad” label.

Select an answer and check the explanation. Distinguish the object, material, observed property and test conditions before choosing. You can revise your choice and try again.