CBSE Class 9 Science Chapter 2: Is Matter Around Us Pure NCERT Solutions

Solution: A pure substance is defined as a material that is composed of only one type of particle. This means that all the constituent particles within the substance have the same chemical identity. Pure substances can be further categorized into two main types: elements, which consist of only one kind of atom, and compounds, which are formed when two or more different elements combine chemically in a fixed ratio.

Solution:

The key differences between homogeneous and heterogeneous mixtures are:

• Composition: Homogeneous mixtures have a uniform composition throughout, meaning the components are evenly distributed. Heterogeneous mixtures have a non-uniform composition, with components not evenly distributed.
• Visibility of Components: In homogeneous mixtures, the components are not visually distinguishable. In heterogeneous mixtures, the components may be visually distinguishable, or at least the composition varies from one part to another.
• Phases: Homogeneous mixtures typically exist in a single phase. Heterogeneous mixtures can consist of two or more phases.
• Examples: Common examples of homogeneous mixtures include salt dissolved in water, sugar in water, copper sulphate in water, alloys (like brass), and air. Examples of heterogeneous mixtures include mixtures of salt and iron filings, salt and sulphur, oil and water, and chalk powder in water.

Solution:

The primary distinction between homogeneous and heterogeneous mixtures lies in their composition and appearance:

Homogeneous Mixtures: These mixtures exhibit a uniform composition and appearance throughout. The components are evenly distributed at a molecular level, making them indistinguishable even under a microscope. Examples include:

• Mixtures of salt in water
• Mixtures of sugar in water
• Mixtures of copper sulphate in water
• Iodine dissolved in alcohol
• Alloys (like brass, steel)
• Air (a mixture of gases)

Heterogeneous Mixtures: These mixtures have a non-uniform composition. The components are not evenly distributed, and their composition varies from one part of the mixture to another. Often, different components can be visually identified. Examples include:

• Mixtures of sodium chloride (salt) and iron filings
• Mixtures of salt and sulphur
• Mixtures of oil and water
• Chalk powder mixed with water
• Wheat flour mixed with water
• Milk and water (though milk appears uniform, it's a colloid, a type of heterogeneous mixture)

Solution:

Solutions, sols, and suspensions are all types of mixtures, but they differ significantly in the size of their constituent particles and their properties:

Solution: A solution is a homogeneous mixture where the solute particles are extremely small (typically less than 1 nanometer in diameter). These particles dissolve completely and are evenly distributed throughout the solvent. They do not settle down even when left undisturbed, and they do not scatter light (i.e., the Tyndall effect is not observed). Examples include salt in water, sugar in water, and alloys.

Sol (Colloid): A sol is a type of heterogeneous mixture where the solute particles are larger than those in a true solution (typically between 1 nanometer and 1 micrometer in diameter) but are still too small to be seen with the naked eye. These particles appear to be uniformly spread throughout the mixture, but they are not truly dissolved. Sols exhibit the Tyndall effect, meaning they scatter a beam of light. The particles in a sol do not settle down easily. Examples include milk of magnesia, mud, and milk.

Suspension: A suspension is a heterogeneous mixture in which the solute particles are relatively large (typically greater than 1 micrometer in diameter). These particles are visible to the naked eye and remain suspended throughout the bulk of the medium. They tend to settle down over time if left undisturbed. Suspensions also exhibit the Tyndall effect. Examples include chalk powder mixed with water and wheat flour mixed with water.