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Nature of Matter: Elements, Compounds, and Mixtures - Classification: Elements and Compounds

Grade 8CBSE

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

🔑Concepts

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Matter can be classified as pure substances or mixtures. Pure substances consist of only one type of particles and include elements and compounds.

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An Element is a substance that consists of only one type of atom and cannot be broken down into simpler substances by chemical means. For example, Iron (FeFe) and Oxygen (O2O_2).

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Elements are further classified into Metals (e.g., NaNa, MgMg), Non-metals (e.g., CC, SS), and Metalloids (e.g., SiSi, GeGe).

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A Compound is a substance formed when two or more elements chemically combine in a fixed proportion by mass. For example, Water (H2OH_2O) and Carbon Dioxide (CO2CO_2).

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The properties of a compound are entirely different from its constituent elements. For example, H2H_2 is a gas and O2O_2 is a gas, but they combine to form H2OH_2O, which is a liquid at room temperature.

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The components of a compound can only be separated by chemical or electrochemical methods, not by physical methods like filtration or evaporation.

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Law of Constant Proportions: In a chemical substance, the elements are always present in definite proportions by mass.

📐Formulae

Mass Ratio of H2O=1:8 (Hydrogen : Oxygen)\text{Mass Ratio of } H_2O = 1:8 \text{ (Hydrogen : Oxygen)}

Mass Ratio of NH3=14:3 (Nitrogen : Hydrogen)\text{Mass Ratio of } NH_3 = 14:3 \text{ (Nitrogen : Hydrogen)}

Mass Ratio of CO2=3:8 (Carbon : Oxygen)\text{Mass Ratio of } CO_2 = 3:8 \text{ (Carbon : Oxygen)}

2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O

💡Examples

Problem 1:

Explain why H2OH_2O is considered a compound while a mixture of H2H_2 and O2O_2 gas is not.

Solution:

H2OH_2O is a compound because Hydrogen and Oxygen are chemically bonded in a fixed ratio of 1:81:8 by mass, and its properties are different from the constituent gases. A mixture of H2H_2 and O2O_2 can have any ratio, and the gases retain their individual properties.

Explanation:

Compounds have fixed compositions and unique properties, whereas mixtures retain the properties of their constituents and have variable compositions.

Problem 2:

Calculate the mass of Oxygen required to react completely with 3 g3\text{ g} of Hydrogen to form water, given the mass ratio in H2OH_2O is 1:81:8.

Solution:

Given ratio Mass of HMass of O=18\frac{\text{Mass of } H}{\text{Mass of } O} = \frac{1}{8}. If mass of H=3 gH = 3\text{ g}, then 3x=18⇒x=3×8=24 g\frac{3}{x} = \frac{1}{8} \Rightarrow x = 3 \times 8 = 24\text{ g}.

Explanation:

According to the Law of Constant Proportions, elements always combine in a fixed ratio by mass. Therefore, 24 g24\text{ g} of Oxygen is needed.

Problem 3:

Classify the following as elements or compounds: O2O_2, NaClNaCl, FeFe, CaCO3CaCO_3.

Solution:

Elements: O2O_2 (Oxygen), FeFe (Iron). Compounds: NaClNaCl (Sodium Chloride), CaCO3CaCO_3 (Calcium Carbonate).

Explanation:

O2O_2 and FeFe consist of only one type of atom, whereas NaClNaCl and CaCO3CaCO_3 consist of multiple elements chemically bonded together.