Sorting Materials into Groups | CBSE Class 6 Science

What are Properties of Materials and Why Do We Sort Them?

In our daily lives, we group things to keep them organised. Your school library has books sorted by subject. A chemist's shop has medicines arranged in alphabetical order. This grouping is called sorting. In science, we sort objects and materials to understand them better. We don't sort them by name, but by their 'properties'. A property is a quality or characteristic of a material, like its colour, shape, or what it feels like. By grouping materials with similar properties, we can study them more easily and decide which material is best for a specific job. For example, we use glass for windows because it has the property of being transparent, but we use wood for a door because it is opaque and strong. So, sorting helps us in two main ways: for convenience and for understanding the nature of materials.

Key Properties Used for Sorting Materials

Appearance

This refers to how a material looks. A key aspect is 'lustre' or shininess. Materials like gold, steel, and copper are lustrous (shiny), while materials like wood, plastic, and chalk are non-lustrous (dull).

Hardness

This describes how easily a material can be compressed or scratched. Hard materials (like iron, stone) are difficult to compress. Soft materials (like cotton, sponge) are easy to compress.

Solubility

This is the ability of a substance to dissolve in a liquid (usually water). Substances that dissolve completely, like salt and sugar, are called soluble. Substances that do not dissolve, like sand and chalk powder, are called insoluble.

Transparency

This property relates to how much light can pass through a material. - Transparent: Objects can be seen clearly through them (e.g., glass, clean water). - Translucent: Objects can be seen, but not clearly (e.g., butter paper, frosted glass). - Opaque: We cannot see through them at all (e.g., wood, a brick wall).

Density (Floating/Sinking)

This property determines if an object will float or sink in water. Objects less dense than water (like wood, plastic ball, dry leaf) will float. Objects denser than water (like a stone, a metal key) will sink.

A Step-by-Step Guide to Testing Material Properties

1. Step 1: Test for Appearance (Lustre) — Take the material (e.g., a piece of iron and a piece of wood). Observe them in light. Does the surface shine? The iron piece shines, so it is lustrous. The wood does not, so it is non-lustrous. Sometimes, you may need to rub the surface with sandpaper to see the fresh, shiny layer.
2. Step 2: Test for Hardness — Take two materials, for example, a piece of chalk and a small rock. Try to scratch the surface of the chalk with the rock. The chalk gets scratched easily. Now try to scratch the rock with the chalk. It won't get scratched. This means the rock is harder than the chalk.
3. Step 3: Test for Solubility — Take a glass of water. Add one spoon of salt and stir well. After a minute, the salt disappears. This means salt is soluble in water. Now, take another glass of water and add one spoon of sand. Stir it. The sand settles at the bottom. This means sand is insoluble in water.
4. Step 4: Test for Transparency — Take a sheet of glass, a piece of butter paper, and a cardboard sheet. Try to look at a lighted bulb through each of them one by one. You can see the bulb clearly through the glass (transparent). You can see a blurry light through the butter paper (translucent). You cannot see the bulb at all through the cardboard (opaque).
5. Step 5: Test for Floating/Sinking — Fill a bucket with water. Gently place a dry leaf and a small stone in it. You will observe that the leaf stays on the surface of the water, while the stone goes to the bottom. This shows the leaf floats and the stone sinks.

Exam Tip: Grouping by Multiple Properties

In exams, you will often be asked to group objects based on more than one property. Don't get confused! Just check each property one by one. For example, a steel spoon is hard, lustrous, opaque, and it sinks in water. The key is to be systematic. Always remember that a single object can have many properties and can be part of many different groups depending on which property you are considering.

Practice Questions with Solutions

• Q: Why is it important to group materials? List two common properties used for grouping them. A: Step 1: Grouping materials makes it easier to study their properties and store them conveniently. It helps in organizing and understanding the vast variety of materials around us. Step 2: Two common properties used for grouping are appearance (e.g., shiny or dull) and hardness (e.g., hard or soft). Final answer: Grouping materials helps in easy study and storage. Properties: Appearance, Hardness.
• Q: Classify the following substances as soluble or insoluble in water: sugar, sand, salt, chalk powder. A: Step 1: Soluble substances dissolve completely in water, while insoluble substances do not. Step 2: Sugar and salt dissolve in water, so they are soluble. Sand and chalk powder do not dissolve in water, so they are insoluble. Final answer: Soluble: Sugar, Salt. Insoluble: Sand, Chalk powder.
• Q: Categorize the following materials based on their transparency: a glass window, a wooden door, butter paper, a brick wall. A: Step 1: Transparent materials allow light to pass through completely. Opaque materials do not allow any light to pass through. Translucent materials allow some light to pass through, but objects cannot be seen clearly. Step 2: Glass window is transparent. Wooden door and brick wall are opaque. Butter paper is translucent. Final answer: Transparent: Glass window. Opaque: Wooden door, Brick wall. Translucent: Butter paper.
• Q: Group the following objects based on their hardness: cotton, iron nail, sponge, stone. A: Step 1: Hard materials are difficult to compress or scratch. Soft materials are easy to compress or scratch. Step 2: Iron nail and stone are hard materials. Cotton and sponge are soft materials. Final answer: Hard: Iron nail, Stone. Soft: Cotton, Sponge.

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