Case Based Question Answer: Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

Section (I): Passage-Based Questions

Passage: Everything around us, like air, water, and food, is made of matter. Matter can be mixtures, which have more than one substance mixed together, or pure substances, which have only one type of particle. Mixtures, like sprout salad or air, can be uniform, where components are evenly mixed, or non-uniform, where components are visible. Pure substances include elements, like iron or oxygen, which cannot be broken down, and compounds, like water or sugar, which are made of elements combined chemically. Mixtures can be separated by physical methods, but compounds need chemical methods to break apart.

Q1. Why is air considered a uniform mixture?
Air is a uniform mixture because its gases, like nitrogen and oxygen, are evenly mixed and cannot be seen separately, even with a microscope. Each gas keeps its own properties, such as oxygen supporting burning. This makes air appear the same throughout.

Q2. How can iron be separated from a mixture with sand?
Iron can be separated from sand using a magnet because iron is magnetic and sand is not. The magnet attracts iron particles, leaving sand behind. This physical method works due to their different properties.

Q3. Why does water behave differently from its elements, hydrogen and oxygen?
Water is a compound where hydrogen and oxygen are chemically combined in a fixed ratio, giving it unique properties, like extinguishing fire. Hydrogen is flammable, and oxygen supports burning, but water has a new, different nature.

Q4. What happens when sugar is heated strongly?
When sugar is heated strongly, it turns brown, then black, forming carbon and releasing water droplets. This shows sugar is a compound made of carbon, hydrogen, and oxygen. The process breaks it into simpler substances.

Section (II): Table-Based Questions

Table: Properties of Substances

Q1. Which substance in the table can be separated from a mixture using a magnet?
Iron can be separated using a magnet because it is magnetic, as shown in the table. This property allows it to be attracted while other substances, like sulfur or sugar, remain unaffected. This makes separation easy.

Q2. Why does iron sulfide have different properties from iron and sulfur?
Iron sulfide is a compound formed by chemically combining iron and sulfur, giving it a unique black appearance and different reactions, like producing a smelly gas with acid. Iron is magnetic and sulfur is yellow, but iron sulfide has new properties. This shows a chemical change occurred.

Q3. Based on the table, why is sugar considered a compound?
Sugar is a compound because it has a uniform composition and cannot be separated physically, as shown by its non-magnetic and non-reactive properties in the table. When heated, it breaks into carbon and water, proving it is made of multiple elements. This distinguishes it from a mixture.

Q4. Which substance in the table is an element, and how can you tell?
Iron and sulfur are elements because they cannot be broken down into simpler substances and have distinct properties, like iron’s magnetism or sulfur’s yellow color. Iron sulfide and sugar are compounds, as they are made of multiple elements combined chemically. The table shows their unique behaviors.