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Nature of Matter: Elements, Compounds, and Mixtures - What Are Pure Substances?

Grade 8CBSE

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

🔑Concepts

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A pure substance is a form of matter that has a constant chemical composition and characteristic properties. It consists of only one type of atoms or molecules.

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Pure substances are classified into two categories: Elements and Compounds.

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Elements are the most basic form of matter that cannot be broken down into simpler substances by chemical reactions. Examples include metals like Iron (FeFe), non-metals like Oxygen (O2O_2), and metalloids like Silicon (SiSi).

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Compounds are substances composed of two or more elements chemically combined with one another in a fixed proportion by mass. For example, Water (H2OH_2O) and Sodium Chloride (NaClNaCl).

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Unlike mixtures, pure substances have fixed melting and boiling points. For instance, pure water always boils at 100∘C100^\circ C at 1 atm1\text{ atm} pressure.

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

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In a compound, the properties of the constituent elements are entirely different from the properties of the compound itself. For example, Hydrogen (H2H_2) is combustible and Oxygen (O2O_2) supports combustion, but their compound Water (H2OH_2O) is used to extinguish fires.

📐Formulae

Mass Ratio of H2O=1:8\text{Mass Ratio of } H_2O = 1 : 8

Mass Ratio of NH3=14:3\text{Mass Ratio of } NH_3 = 14 : 3

Mass Ratio of CO2=3:8\text{Mass Ratio of } CO_2 = 3 : 8

Atomicity=Number of atoms present in one molecule of a substance\text{Atomicity} = \text{Number of atoms present in one molecule of a substance}

💡Examples

Problem 1:

Determine the ratio by mass of the combining elements in Ammonia (NH3NH_3). (Atomic mass: N=14uN = 14u, H=1uH = 1u)

Solution:

Mass of Nitrogen (N)=14×1=14uMass of Hydrogen (H)=1×3=3uRatio (N:H)=14:3\begin{array}{r} \text{Mass of Nitrogen (N)} = 14 \times 1 = 14u \\ \text{Mass of Hydrogen (H)} = 1 \times 3 = 3u \\ \hline \text{Ratio (N:H)} = 14:3 \end{array}

Explanation:

According to the Law of Constant Proportions, in a chemical substance like Ammonia, the elements are always present in definite proportions by mass. Here, for every 1414 units of Nitrogen, there are 33 units of Hydrogen.

Problem 2:

Identify which of the following are pure substances: Ice, Milk, Iron, Hydrochloric Acid, Calcium Oxide, Mercury, Brick, Air.

Solution:

The pure substances are: Ice (Compound: H2OH_2O), Iron (Element: FeFe), Hydrochloric Acid (Compound: HClHCl), Calcium Oxide (Compound: CaOCaO), and Mercury (Element: HgHg).

Explanation:

Pure substances must consist of only one type of constituent particle. Milk is a mixture of water, fats, and proteins. Brick and Air are also mixtures of various substances with variable compositions.