Elements, Compounds and Equations

Elements, Compounds and Equations

Overview

This topic forms the foundation of chemistry at the upper-primary level and is essential for MAHA TET Paper II candidates. Understanding how matter is classified into elements, compounds and mixtures—and how substances interact through chemical equations—enables teachers to build scientific thinking in students aged 11–14.

For the exam, expect questions that test your ability to distinguish between pure substances and mixtures, identify elements and compounds from given examples, and balance simple chemical equations. This topic connects directly to other science areas like atoms, molecules, acids-bases and metals, making it a high-value area for revision. Teachers must also know how to explain these abstract concepts using everyday examples and simple experiments.

Mastering this topic requires clarity on definitions, memorisation of common examples, and the mechanical skill of balancing equations—all of which are tested in MCQ format.

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Key Concepts

• Element: A pure substance made of only one type of atom. It cannot be broken down into simpler substances by ordinary chemical means. Examples: iron (Fe), oxygen (O), gold (Au), carbon (C).
• Compound: A pure substance made of two or more elements chemically combined in a fixed ratio. It has properties different from its constituent elements. Examples: water (H₂O), carbon dioxide (CO₂), common salt (NaCl).
• Mixture: A combination of two or more substances (elements or compounds) that are not chemically combined. Components retain their individual properties and can be separated by physical methods. Examples: air, seawater, brass.
• Homogeneous vs Heterogeneous Mixtures: Homogeneous mixtures have uniform composition throughout (solutions like sugar in water). Heterogeneous mixtures have non-uniform composition with visible boundaries (sand and iron filings).
• Physical vs Chemical Change: Physical change alters form but not chemical identity (ice melting). Chemical change produces new substances with different properties (iron rusting).
• Chemical Equation: A symbolic representation of a chemical reaction showing reactants (left side) and products (right side) with an arrow indicating the direction of reaction.
• Law of Conservation of Mass: In a chemical reaction, total mass of reactants equals total mass of products. This is why equations must be balanced.
• Balanced Equation: An equation where the number of atoms of each element is equal on both sides.

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Formulas / Key Facts

Common Compounds to Remember:

• Water: H₂O
• Carbon dioxide: CO₂
• Ammonia: NH₃
• Sulphuric acid: H₂SO₄
• Sodium chloride: NaCl
• Calcium carbonate: CaCO₃
• Glucose: C₆H₁₂O₆

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Worked Examples

Example 1: Classification Question

Classify the following as element, compound or mixture: (a) Brass (b) Oxygen gas (c) Carbon dioxide (d) Air

Solution:

• (a) Brass — Mixture (alloy of copper and zinc; components not in fixed ratio)
• (b) Oxygen gas — Element (contains only oxygen atoms, O₂)
• (c) Carbon dioxide — Compound (CO₂; carbon and oxygen chemically combined in fixed 1:2 ratio)
• (d) Air — Mixture (contains N₂, O₂, CO₂, water vapour etc. in variable proportions)

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Example 2: Balancing a Chemical Equation

Balance the equation: H₂ + O₂ → H₂O

Step-by-step:

1. Count atoms on each side:
   • Left: H = 2, O = 2
   • Right: H = 2, O = 1
2. Oxygen is unbalanced. Put coefficient 2 before H₂O:
   • H₂ + O₂ → 2H₂O
   • Now: Left O = 2, Right O = 2 ✓
   • But Right H = 4, Left H = 2 ✗
3. Put coefficient 2 before H₂:
   • 2H₂ + O₂ → 2H₂O
   • Left: H = 4, O = 2
   • Right: H = 4, O = 2 ✓

Balanced equation: 2H₂ + O₂ → 2H₂O

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Example 3: Writing Chemical Equation from Word Equation

Write the balanced chemical equation: Magnesium burns in oxygen to form magnesium oxide.

Solution:

1. Word equation: Magnesium + Oxygen → Magnesium oxide
2. Write symbols: Mg + O₂ → MgO
3. Count atoms:
   • Left: Mg = 1, O = 2
   • Right: Mg = 1, O = 1
4. Balance oxygen by putting 2 before MgO: Mg + O₂ → 2MgO
5. Now Mg is unbalanced. Put 2 before Mg: 2Mg + O₂ → 2MgO

Balanced equation: 2Mg + O₂ → 2MgO

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Common Mistakes

• Confusing compound with mixture → A compound has fixed ratio and uniform properties everywhere; a mixture has variable composition. Salt dissolved in water is a mixture (solution), but NaCl itself is a compound.
• Changing subscripts while balancing → Students often change H₂O to H₂O₂ to balance oxygen. This is wrong—H₂O₂ is hydrogen peroxide, a completely different substance. Only add coefficients before formulas.
• Forgetting diatomic elements → Oxygen, nitrogen, hydrogen, and halogens exist as O₂, N₂, H₂, Cl₂ etc. in free state. Writing just "O" instead of "O₂" leads to incorrect balancing.
• Assuming all gases are elements → Carbon dioxide and ammonia are gases but are compounds, not elements. State of matter does not determine classification.
• Ignoring state symbols in equations → For complete representation, equations should include (s), (l), (g), (aq). Example: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

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Quick Reference

• Element = one type of atom; Compound = two or more elements chemically joined; Mixture = physically combined, separable.
• Compounds have fixed ratio and new properties; mixtures have variable ratio and original properties.
• Law of Conservation of Mass: atoms are neither created nor destroyed—balance both sides.
• Balancing trick: start with the most complex molecule, balance metals first, then non-metals, leave H and O for last.
• Separation methods for mixtures: filtration, evaporation, distillation, magnetic separation, chromatography.
• Common diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ (remember: HOFBrINCl).