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Introduction to Chemistry - Element Molecule Compound and Mixture

Grade 7CBSE

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

🔑Concepts

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Matter is anything that occupies space and has mass. It is classified into pure substances (Elements and Compounds) and impure substances (Mixtures).

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An Element is a pure substance made up of only one kind of atom. It cannot be broken down into simpler substances by chemical means. Symbols like FeFe (Iron) and AuAu (Gold) are used to represent them.

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A Molecule is the smallest particle of an element or a compound that can exist independently. For example, a molecule of Oxygen is O2O_{2} and a molecule of Water is H2OH_{2}O.

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Atomicity refers to the number of atoms present in one molecule of an element. For example, Helium (HeHe) is monoatomic, while Phosphorus (P4P_{4}) is polyatomic.

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A Compound is a substance formed when two or more elements combine chemically in a fixed ratio by mass. The properties of a compound are entirely different from its constituent elements.

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A Mixture is formed when two or more substances (elements or compounds) are physically mixed in any proportion without a chemical reaction. They can be Homogeneous (uniform composition, e.g., sugar in water) or Heterogeneous (non-uniform composition, e.g., sand in water).

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The Valency of an element is its combining capacity. For example, in HClHCl, the valency of Chlorine (ClCl) is 11.

📐Formulae

Water: H2O\text{Water: } H_{2}O

Carbon Dioxide: CO2\text{Carbon Dioxide: } CO_{2}

Sodium Chloride (Salt): NaCl\text{Sodium Chloride (Salt): } NaCl

Sulfuric Acid: H2SO4\text{Sulfuric Acid: } H_{2}SO_{4}

Methane: CH4\text{Methane: } CH_{4}

Glucose: C6H12O6\text{Glucose: } C_{6}H_{12}O_{6}

Ammonia: NH3\text{Ammonia: } NH_{3}

💡Examples

Problem 1:

Calculate the ratio by mass of Carbon and Oxygen in a molecule of Carbon Dioxide (CO2CO_{2}). Given atomic mass of C=12C = 12 and O=16O = 16.

Solution:

12:(2×16)=12:32=3:812 : (2 \times 16) = 12 : 32 = 3 : 8

Explanation:

One molecule of CO2CO_{2} contains 11 atom of Carbon and 22 atoms of Oxygen. The ratio of their masses is 1232\frac{12}{32}, which simplifies to 3:83:8.

Problem 2:

What is the difference between 2H2H and H2H_{2}?

Solution:

2H2H represents two separate atoms of Hydrogen, while H2H_{2} represents one molecule of Hydrogen consisting of two atoms bonded together.

Explanation:

The coefficient 22 in 2H2H indicates the quantity of atoms, whereas the subscript 22 in H2H_{2} indicates that the atoms are chemically bonded to form a diatomic molecule.

Problem 3:

Classify the following as Element, Compound, or Mixture: Iron filings, Air, Common Salt, and Oxygen gas.

Solution:

Iron filings: Element; Air: Mixture; Common Salt (NaClNaCl): Compound; Oxygen gas (O2O_{2}): Element.

Explanation:

Iron and Oxygen consist of only one type of atom. Salt consists of NaNa and ClCl in a fixed 1:11:1 ratio. Air contains various gases like N2,O2, and CO2N_{2}, O_{2}, \text{ and } CO_{2} mixed physically in varying proportions.