Chapter 6 — Materials Around Us

Chapter 6 introduces the concept of materials — substances used to make objects. Students learn to classify materials based on properties like lustre, hardness, transparency (transparent/translucent/opaque), solubility in water, mass, and volume. The chapter concludes with the definition of matter as anything that occupies space and has mass.

Transparent materials allow things to be seen clearly through them (e.g., glass, water, air, cellophane paper). Translucent materials allow objects to be seen but not clearly (e.g., frosted glass, butter paper). Opaque materials block vision completely — you cannot see through them at all (e.g., wood, cardboard, metals).

Lustrous materials have a shiny surface, usually metals like iron, copper, zinc, aluminium, and gold. Non-lustrous materials do not have a shiny surface — examples include paper, wood, rubber, and jute. However, not all shiny materials are metals: surfaces can be made to look shiny by polishing or coating with plastic or wax, so they may not be metals.

Soluble materials completely dissolve and disappear when mixed in water (e.g., sugar, salt). Insoluble materials do not mix with water and do not disappear even after stirring for a long time (e.g., sand, sawdust, chalk powder).

Anything that occupies space and has mass is called matter. Mass gives the quantity of matter (measured in grams or kilograms) and volume is the space occupied by matter (measured in litres or millilitres). All materials around us — water, sand, pebbles, wood, air — are examples of matter.

The SI unit of mass is kilogram (kg). Volume is measured in litres (L) and millilitres (mL); the SI unit for volume is cubic metre (m³), where 1 m³ = 1000 L. When writing units: use 'kg' not 'kgs', always leave a space between the number and the unit (e.g., 7 kg, 500 mL).

ORS stands for Oral Rehydration Solution, used to treat dehydration due to diarrhoea or illness. Home-made ORS is prepared by dissolving 6 teaspoons of sugar and half a teaspoon of common salt in one litre of boiled and cooled water. Sugar and salt are soluble in water, which is why they dissolve to form ORS.

Terracotta is baked clay. The earliest pottery in the Indian subcontinent dates back 7,000–8,000 years in the Ganga plains (Lahuradewa) and Baluchistan (Mehrgarh). The Sindhu-Sarasvatī (Harappan) Civilisation (2600–1900 BCE) developed sophisticated pottery with a bright red surface painted with black geometric designs.

The Ashtanga Hridaya (Sutra sthana 1.18) describes 20 properties — ten pairs of opposite qualities (guṇa) used to describe all physical matter in Ayurveda. Examples include: guru (heavy)–laghu (light), hima (cold)–uṣhṇa (hot), mṛidu (soft)–kaṭhina (hard), sāndra (solid)–drava (liquid). These properties apply to matter, living systems, environment, and food.

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Different sports require balls with specific properties — such as the height of bounce, texture, size, and hardness. For example, a tennis ball bounces higher than a cricket ball. Since these properties depend on the material, different materials are chosen to suit the requirements of each sport.

In common language, weight is sometimes used to mean mass because it is determined by weighing. However, mass is the quantity of matter in an object (measured in kg or g), while weight is a related but different concept that students will learn about in higher classes.