Samacheer Class 8 Science - Matter

A chemical symbol is a shorthand method of representing an element using one or two letters, where the first letter is always a capital letter and the second letter, if present, is a lowercase letter. A chemical symbol represents two important pieces of information about an element. First, it represents the name of the element itself. For example, the symbol O stands for the element oxygen, the symbol H stands for hydrogen, and the symbol C stands for carbon. Second, a chemical symbol represents one atom of that element. So when we write O, it means one atom of oxygen, and when we write H, it means one atom of hydrogen. This dual representation makes chemical symbols extremely useful in writing chemical formulas and equations, as they allow us to quickly identify both the element and the number of atoms present in a compound or reaction.

Metalloids possess several distinctive properties that set them apart from both metals and non-metals. First, metalloids are all solid at room temperature, maintaining a solid state under normal conditions. Second, metalloids can form alloys with other metals, combining with metallic elements to create useful materials with enhanced properties. Third, some metalloids such as silicon and germanium can act as electrical conductors under specific conditions, and for this reason they are called semiconductors. These semiconductors have the ability to conduct electricity better than non-metals but not as well as metals, and their conductivity can be controlled and modified, making them extremely valuable in electronic devices. Additionally, metalloids display mixed characteristics in their physical properties. For example, silicon appears lustrous like a metal, but it is not malleable or ductile like metals; instead, it is brittle, which is a characteristic more commonly associated with non-metals. Furthermore, metalloids are much poorer conductors of heat and electricity compared to true metals, reflecting their intermediate nature between metallic and non-metallic elements.

No, we cannot store pickle in an aluminium utensil because of a chemical reaction that occurs between the metal and the acidic content of the pickle. Aluminium is a reactive metal, and pickles, particularly lemon pickle, contain acids such as citric acid and acetic acid. When acidic foods come into contact with aluminium, a chemical reaction takes place in which the acid reacts with the aluminium metal. This reaction produces hydrogen gas and aluminium salts, which contaminate the food. The reaction not only spoils the taste and quality of the pickle but also makes it unfit for consumption as the aluminium compounds formed can be harmful if ingested. Additionally, the reaction may cause the aluminium utensil to corrode and develop holes or pits. Therefore, it is advisable to store acidic foods like pickles in glass, ceramic, or stainless steel containers instead of aluminium utensils.

Cooking utensils are made from aluminium and brass because both materials are excellent conductors of heat, allowing them to distribute heat evenly during cooking and ensuring efficient food preparation. Aluminium is particularly favored because it forms a protective layer of aluminium oxide on its surface, which prevents further oxidation and corrosion, thereby increasing the durability and lifespan of the utensils. Additionally, aluminium is relatively inexpensive and lightweight compared to many other metals, making it economically viable for mass production of kitchenware. Brass, an alloy of copper and zinc, is also chosen for certain utensils because of its good heat conductivity and aesthetic appeal. Both materials are non-toxic and safe for food contact, which is an essential requirement for cooking utensils. These properties combined make aluminium and brass ideal choices for manufacturing durable, efficient, and affordable cooking utensils.

1. Water (hydrogen oxide): Used for drinking and as a solvent.
2. Table salt (sodium chloride): An essential part of the diet and a preservative for meat and fish.
3. Sugar (sucrose): Used to prepare sweets, toffees and fruit juices.
4. Baking soda (sodium bicarbonate): Used in fire extinguishers, baking powder, cakes and bread.

Baking soda, also known as sodium bicarbonate, has multiple important uses including serving as a fire extinguisher by releasing carbon dioxide when heated, being a key ingredient in the preparation of baking powder, and being used in the preparation of cakes and bread to help them rise and achieve a light texture. Bleaching powder is widely used as a bleaching agent in the textile and paper industries, functions as a disinfectant for surfaces and equipment, and is employed in the sterilisation and purification of drinking water to eliminate harmful microorganisms and ensure water safety. Quick lime, or calcium oxide, is an essential compound used in the manufacture of cement, which is a fundamental building material in construction, and is also used in the production of glass for windows, containers, and various optical applications. Each of these compounds plays a crucial role in different industrial and domestic applications.

Wires cannot be drawn from materials such as stone or wood because these materials lack the necessary physical and electrical properties required for wire drawing. Stone and wood are brittle materials that fracture and break when subjected to the tensile stress involved in the wire-drawing process, whereas metals are malleable and ductile, allowing them to be stretched into thin wires without breaking. Additionally, stone and wood are non-conductors of electricity, meaning they do not allow the flow of electric current, whereas wires are typically made from metals like copper and aluminium that are excellent conductors of electricity. The combination of brittleness, lack of malleability, and poor electrical conductivity makes stone and wood unsuitable for wire production.