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Nature of Matter: Elements, Compounds, and Mixtures - Pure Substances vs. Mixtures

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Matter is classified into two main types based on chemical composition: Pure Substances and Mixtures.

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A Pure Substance consists of only one type of particles (atomsatoms or moleculesmolecules). These are further divided into Elements and Compounds.

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Elements are the simplest form of matter that cannot be broken down into simpler substances by chemical reactions. Examples include Iron (FeFe), Oxygen (O2O_2), and Gold (AuAu).

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Compounds are substances formed when two or more elements combine chemically in a fixed ratio by mass. For example, in Water (H2OH_2O), the ratio of Hydrogen to Oxygen by mass is always 1:81:8.

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Mixtures contain two or more substances (elements or compounds) mixed together physically in any proportion, without any chemical bond formation.

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Homogeneous Mixtures have a uniform composition throughout (e.g., Sugar solution, Air).

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Heterogeneous Mixtures have a non-uniform composition where the components can be seen separately (e.g., Mixture of SandSand and SaltSalt, OilOil and WaterWater).

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The properties of a compound are entirely different from its constituent elements, whereas a mixture retains the properties of its constituents.

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Components of a mixture can be separated by physical methods (filtration, evaporation), but components of a compound can only be separated by chemical or electrochemical reactions.

📐Formulae

Mass Ratio in H2O=2×Mass of H:1×Mass of O=2:16=1:8\text{Mass Ratio in } H_2O = 2 \times \text{Mass of } H : 1 \times \text{Mass of } O = 2:16 = 1:8

Mass Ratio in CO2=1×Mass of C:2×Mass of O=12:32=3:8\text{Mass Ratio in } CO_2 = 1 \times \text{Mass of } C : 2 \times \text{Mass of } O = 12:32 = 3:8

Percentage of an element in a compound=Mass of element in 1 moleMolar mass of compound×100\text{Percentage of an element in a compound} = \frac{\text{Mass of element in 1 mole}}{\text{Molar mass of compound}} \times 100

💡Examples

Problem 1:

Explain why a mixture of Iron (FeFe) filings and Sulphur (SS) powder is considered a mixture, while Iron Sulphide (FeSFeS) is considered a compound.

Solution:

  1. In the mixture of FeFe and SS, the iron particles can be separated using a magnet. 2. In FeSFeS, the iron and sulphur are chemically bonded and cannot be separated by a magnet. 3. The mixture shows the individual properties of FeFe and SS, but FeSFeS has entirely different properties from its constituent elements.

Explanation:

A mixture retains the physical properties of its constituents and can be separated by physical means. A compound is a new substance formed by chemical change with a fixed composition.

Problem 2:

Classify the following as homogeneous or heterogeneous mixtures: (a) Soda water, (b) Wood, (c) Air, (d) Soil.

Solution:

(a) Soda water: Homogeneous, (b) Wood: Heterogeneous, (c) Air: Homogeneous (if filtered/dust-free), (d) Soil: Heterogeneous.

Explanation:

Homogeneous mixtures have a single phase and uniform distribution of particles, while heterogeneous mixtures have visible boundaries of separation between constituents.

Problem 3:

Calculate the ratio by number of atoms for the elements in Ammonia (NH3NH_3).

Solution:

In Ammonia (NH3NH_3), there is 11 atom of Nitrogen (NN) and 33 atoms of Hydrogen (HH). Therefore, the ratio of atoms N:HN:H is 1:31:3.

Explanation:

The chemical formula of a compound represents the fixed ratio of the number of atoms of the constituent elements.