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Is Matter Around Us Pure - Types of Pure Substances

Grade 9CBSE

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

🔑Concepts

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On the basis of their chemical composition, pure substances can be classified as either elements or compounds.

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An Element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Robert Boyle was the first scientist to use the term element in 1661.

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Elements are divided into three categories: Metals (e.g., AuAu, AgAg, CuCu), Non-metals (e.g., HH, OO, II, CC), and Metalloids (e.g., BB, SiSi, GeGe) which have intermediate properties.

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A Compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

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The properties of a compound are totally different from the properties of its constituent elements. For example, H2H_2 is combustible and O2O_2 supports combustion, but their compound H2OH_2O is used as a fire extinguisher.

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In a compound, the constituents can be separated only by chemical or electrochemical reactions.

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A pure substance consists of a single type of particles (atoms or molecules) and has a fixed melting and boiling point.

📐Formulae

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

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

Pure Substance→Elements+Compounds\text{Pure Substance} \rightarrow \text{Elements} + \text{Compounds}

2H2(g)+O2(g)→Chemical Change2H2O(l)2H_2(g) + O_2(g) \xrightarrow{\text{Chemical Change}} 2H_2O(l)

💡Examples

Problem 1:

Classify the following as elements or compounds: (a) Iron, (b) Iron Sulphide, (c) Mercury, (d) Methane (CH4CH_4).

Solution:

(a) Iron (FeFe): Element; (b) Iron Sulphide (FeSFeS): Compound; (c) Mercury (HgHg): Element; (d) Methane (CH4CH_4): Compound.

Explanation:

Iron and Mercury consist of only one type of atom and cannot be broken down further. Iron Sulphide and Methane consist of different atoms chemically bonded in fixed proportions.

Problem 2:

Explain why the formation of water from H2H_2 and O2O_2 is considered a chemical change, while mixing iron filings and sulphur powder is a physical change.

Solution:

The formation of H2OH_2O results in a new substance with entirely different properties than H2H_2 and O2O_2, involving a chemical bond. Mixing FeFe and SS allows them to retain their individual properties and they can be separated by a magnet.

Explanation:

In a compound like H2OH_2O, the composition is fixed and chemical properties change. In a mixture of FeFe and SS, no chemical reaction occurs unless heated to form the compound FeSFeS.