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Chemistry - Elements, Compounds, and Mixtures

Grade 8IGCSE

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

🔑Concepts

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

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A Compound is a substance formed when two or more different elements are chemically bonded together in a fixed ratio. For example, Water (H2OH_2O) always contains two Hydrogen atoms for every one Oxygen atom.

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A Mixture consists of two or more substances (elements or compounds) that are physically blended but not chemically joined. The components of a mixture retain their individual properties and can be separated by physical methods like filtration or distillation.

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Chemical Symbols and Formulae: Elements are represented by symbols from the Periodic Table. Compounds are represented by formulae where subscripts indicate the number of atoms, such as glucose (C6H12O6C_6H_{12}O_6).

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Purity: A pure substance has a fixed melting and boiling point, whereas a mixture melts or boils over a range of temperatures.

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Particle Diagrams: In diagrams, elements are shown as single types of circles, compounds as joined different types of circles, and mixtures as a collection of unjoined different circles or molecules.

📐Formulae

General form of a compound: AxBy\text{General form of a compound: } A_xB_y

Percentage by mass=Total mass of element in compoundTotal formula mass of compound×100%\text{Percentage by mass} = \frac{\text{Total mass of element in compound}}{\text{Total formula mass of compound}} \times 100\%

Molecules of elements (Diatomic): H2,N2,O2,F2,Cl2,Br2,I2\text{Molecules of elements (Diatomic): } H_2, N_2, O_2, F_2, Cl_2, Br_2, I_2

💡Examples

Problem 1:

Identify whether the following are elements, compounds, or mixtures: (a) O2O_2 gas, (b) NaClNaCl dissolved in H2OH_2O, (c) CH4CH_4.

Solution:

(a) Element, (b) Mixture, (c) Compound.

Explanation:

O2O_2 consists of only one type of atom (Oxygen), so it is an element. NaClNaCl in H2OH_2O is a salt solution where components are physically mixed, making it a mixture. CH4CH_4 (Methane) contains Carbon and Hydrogen chemically bonded in a fixed 1:41:4 ratio, making it a compound.

Problem 2:

Calculate the total number of atoms present in one molecule of sulfuric acid, represented by the formula H2SO4H_2SO_4.

Solution:

2+1+4=72 + 1 + 4 = 7 atoms.

Explanation:

The formula H2SO4H_2SO_4 indicates there are 22 atoms of Hydrogen (HH), 11 atom of Sulfur (SS), and 44 atoms of Oxygen (OO). Summing these gives 2+1+4=72 + 1 + 4 = 7 atoms in total.

Problem 3:

Explain why air is considered a mixture rather than a compound.

Solution:

Air contains gases like N2N_2, O2O_2, and CO2CO_2 that are not chemically bonded and can vary in proportion.

Explanation:

In a compound, elements are joined in a fixed ratio (e.g., CO2CO_2). In air, the percentage of H2OH_2O vapor or CO2CO_2 can change depending on the environment, and the gases can be separated by physical means like fractional distillation of liquid air.