Sorting Materials Into Groups

Soft substances are easy to scratch because they have a lower hardness compared to harder substances. When an object with a higher hardness comes into contact with a soft substance, it can easily leave marks or scratches on its surface. The softness of the material allows it to deform or be displaced more easily, making it susceptible to scratching.

Oxygen is a gas that dissolves in water and is important for marine life. Marine organisms rely on dissolved oxygen in water for respiration, just like terrestrial organisms rely on oxygen in the air. It is essential for the survival of aquatic organisms, as it is involved in various biochemical processes, including cellular respiration. Oxygen levels in water can fluctuate due to factors such as temperature, salinity, and photosynthesis. Therefore, maintaining adequate dissolved oxygen levels is crucial for the health and well-being of marine ecosystems.

Transparent materials are used for making the front glass (windscreen) of a car. This is because transparent materials allow light to pass through them without scattering, making it easier for the driver to see through the glass. Translucent materials allow some light to pass through but scatter it, while opaque materials do not allow any light to pass through. Since the front glass of a car needs to provide clear visibility, it is made from transparent materials.

Safety pins are commonly made using steel and plastic. Steel is used for the pin itself as it provides strength and durability. Plastic is used for the head of the safety pin, which is the part that is held and manipulated. Plastic is lightweight, inexpensive, and easy to mold into different shapes, making it suitable for the head of the safety pin.

The materials that are opaque do not allow light to pass through them. In this case, the mirror and the magnet are the opaque materials, as they do not allow light to pass through them. The paper stained with oil and the glass spectacles are not opaque, as they allow some light to pass through them. Therefore, the correct answer is (c) (i) and (iii).

Mirror is considered an opaque substance because it does not allow light to pass through it. Instead, it reflects light, creating a clear and distinct image. Opaque substances do not transmit light and therefore do not allow objects to be seen through them.

Soft drinks contain carbonic acid, which can react with the iron in the nail and cause it to lose its shine and appear dull. This reaction is known as corrosion. Mustard oil, coconut oil, and kerosene do not contain any acidic components that can react with the iron, so the nail would not lose its shine in those liquids.

The statement that materials are grouped for fun is not true. Materials are typically grouped for convenience, to study their properties, or according to their uses. Grouping materials for fun does not serve any practical purpose or contribute to scientific understanding.

While several physical properties can help distinguish between metals and non-metals, checking if the material is attracted to a magnet is often a reliable method. Many metals, such as iron, cobalt, and nickel, are magnetic, whereas non-metals are generally not. Other properties like shininess or ability to be cut might also indicate a metal, but they are not as definitive because some non-metals can also exhibit these characteristics. Floating in water is not a distinguishing feature between metals and non-metals.

Cotton thread and thermocol would float in a tumbler half filled with water because both substances have a lower density than water. Feather and thread would also float for the same reason. However, pin and oil drops would sink in water because they have a higher density than water. Similarly, a rubber band and a coin would sink in water because they are denser than water.