CIE iGCSE Co-ordinated Sciences-C3.1 Formulas- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences- C3.1 Formulas – Study Notes
CIE iGCSE Co-ordinated Sciences- C3.1 Formulas – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Formulas of Elements and Compounds
Chemical formulas show the composition of a substance. They tell us which elements are present and how many atoms of each are in one particle of the substance.
Formulas of Common Elements
Elements can exist in different forms. Some exist as single atoms (monatomic), others naturally bond in pairs to form molecules (diatomic), and some exist in other structures such as giant lattices or allotropes.
| Type of Element | Examples and Formulas |
|---|---|
| Monatomic gases | \( \text{He}, \text{Ne}, \text{Ar} \) |
| Diatomic molecules | \( \text{H}_2, \text{O}_2, \text{N}_2, \text{Cl}_2, \text{Br}_2, \text{I}_2, \text{F}_2 \) |
| Other elements | \( \text{Na}, \text{Mg}, \text{Al}, \text{C} \) (diamond, graphite, fullerene) |
Formulas of Simple Compounds
The formula of a compound shows the elements it contains and the ratio of their atoms.
Common Compounds
| Compound | Formula |
|---|---|
| Water | \( \text{H}_2\text{O} \) |
| Carbon dioxide | \( \text{CO}_2 \) |
| Ammonia | \( \text{NH}_3 \) |
| Methane | \( \text{CH}_4 \) |
| Sodium chloride | \( \text{NaCl} \) |
| Calcium carbonate | \( \text{CaCO}_3 \) |
| Magnesium oxide | \( \text{MgO} \) |
| Ethanol | \( \text{C}_2\text{H}_5\text{OH} \) |
| Ethanoic acid | \( \text{CH}_3\text{COOH} \) |
Example
A molecule contains 1 carbon atom and 4 hydrogen atoms. Deduce its formula and name.
▶️ Answer/Explanation
The formula is \( \text{CH}_4 \), which is methane.
Example
Deduce the formula of the compound formed between magnesium and chlorine.
▶️ Answer/Explanation
Magnesium forms \( \text{Mg}^{2+} \) ions and chlorine forms \( \text{Cl}^- \) ions.
To balance charges, 1 magnesium ion combines with 2 chloride ions.
Formula = \( \text{MgCl}_2 \).
Example
Write the balanced equation for the reaction of calcium carbonate with hydrochloric acid.
▶️ Answer/Explanation
The formulas are:
Calcium carbonate – \( \text{CaCO}_3 \)
Hydrochloric acid – \( \text{HCl} \)
Calcium chloride – \( \text{CaCl}_2 \)
Carbon dioxide – \( \text{CO}_2 \)
Water – \( \text{H}_2\text{O} \)
The balanced equation is:
\( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{CO}_2 + \text{H}_2\text{O} \)
Molecular Formula
The molecular formula of a compound shows the actual number and type of atoms present in a single molecule of that compound. It gives the exact composition of the molecule, not just the ratio of atoms.
Examples
Water – \( \text{H}_2\text{O} \) → 2 hydrogen atoms and 1 oxygen atom
Carbon dioxide – \( \text{CO}_2 \) → 1 carbon atom and 2 oxygen atoms
Glucose – \( \text{C}_6\text{H}_{12}\text{O}_6 \) → 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms
Example
State the molecular formula of ethanol and explain what it represents.
▶️ Answer/Explanation
The molecular formula of ethanol is \( \text{C}_2\text{H}_5\text{OH} \).
This shows that one molecule of ethanol contains:
2 carbon atoms
6 hydrogen atoms
1 oxygen atom
Deduction of Molecular Formulas
The molecular formula of a simple molecular compound can be deduced by counting the relative number of atoms of each element in a model or a diagrammatic representation. The subscripts in the formula show how many atoms of each element are present in one molecule.
Examples
A molecule with 1 carbon atom and 4 hydrogen atoms → \( \text{CH}_4 \) (methane)
A molecule with 2 hydrogen atoms and 1 oxygen atom → \( \text{H}_2\text{O} \) (water)
A molecule with 1 nitrogen atom and 3 hydrogen atoms → \( \text{NH}_3 \) (ammonia)
Example
A diagram shows 2 oxygen atoms bonded to 1 carbon atom. Deduce the molecular formula.
▶️ Answer/Explanation
The diagram shows 1 carbon atom and 2 oxygen atoms.
The molecular formula is \( \text{CO}_2 \) (carbon dioxide).
Deduction of Ionic Formulas
The formula of an ionic compound can be deduced from:
1. The relative numbers of positive and negative ions in a model or diagram
2. The charges on the ions, ensuring that the overall charge in the compound is zero
Examples
Sodium ion \( \text{Na}^+ \) and chloride ion \( \text{Cl}^- \) combine in a 1:1 ratio → \( \text{NaCl} \)
Magnesium ion \( \text{Mg}^{2+} \) and oxide ion \( \text{O}^{2-} \) combine in a 1:1 ratio → \( \text{MgO} \)
Calcium ion \( \text{Ca}^{2+} \) and chloride ion \( \text{Cl}^- \) combine in a 1:2 ratio → \( \text{CaCl}_2 \)
Example
Deduce the formula of the ionic compound formed between aluminium ions and oxide ions.
▶️ Answer/Explanation
Aluminium ion = \( \text{Al}^{3+} \)
Oxide ion = \( \text{O}^{2-} \)
To balance charges, 2 aluminium ions (+6) combine with 3 oxide ions (−6).
Formula = \( \text{Al}_2\text{O}_3 \)
Word Equations
Word equations are a simple way of showing how reactants form products in a chemical reaction. They use the full names of the substances instead of chemical symbols or formulas.
The general format is:
Reactants → Products
Word equations clearly show the substances that take part in the reaction and the substances that are formed, but they do not give information about the exact number of atoms or molecules.
Examples
Magnesium + Oxygen → Magnesium oxide
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water
Methane + Oxygen → Carbon dioxide + Water
Example
Write the word equation for the reaction between zinc and hydrochloric acid.
▶️ Answer/Explanation
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
This shows that zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.
Example
Write the word equation for the combustion of ethanol in oxygen.
▶️ Answer/Explanation
Ethanol + Oxygen → Carbon dioxide + Water
This shows that ethanol burns in oxygen to produce carbon dioxide and water.
Balancing and Interpreting Symbol Equations
A chemical (symbol) equation shows the reactants and products in a chemical reaction using their chemical formulas. For the equation to be correct, it must be balanced — meaning the number of atoms of each element is the same on both sides.
Steps for balancing equations:
1. Write down the correct formulas of reactants and products.
2. Count the number of atoms of each element on both sides.
3. Use coefficients (numbers in front of formulas) to balance the atoms.
4. Do not change the chemical formulas, only adjust the coefficients.
Example of balancing
Word equation: Hydrogen + Oxygen → Water
Unbalanced symbol equation: \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \)
Balanced: \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \)
Example
Balance the equation: \( \text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3 \)
▶️ Answer/Explanation
Count atoms: LHS has 1 Fe, RHS has 2 Fe → place 4 Fe on LHS and 2 Fe₂O₃ on RHS.
Count oxygen: LHS has 3 O₂ = 6 O atoms, RHS has 2×3 = 6 O atoms.
Balanced equation: \( 4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 \)
State Symbols
State symbols are used in equations to show the physical state of each substance:
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)
Example with state symbols
Calcium carbonate (s) + Hydrochloric acid (aq) → Calcium chloride (aq) + Carbon dioxide (g) + Water (l)
Symbol equation: \( \text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{CO}_2(g) + \text{H}_2\text{O}(l) \)
Example
Write the balanced symbol equation with state symbols for the combustion of methane.
▶️ Answer/Explanation
\( \text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l) \)
Methane burns in oxygen to form carbon dioxide gas and liquid water.
Ionic Equations
Ionic equations show only the particles that actually take part in the reaction. Spectator ions (ions that do not change) are left out.
Example of ionic equation
Full equation: \( \text{NaOH}(aq) + \text{HCl}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l) \)
Ionic form: \( \text{Na}^+(aq) + \text{OH}^-(aq) + \text{H}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq) + \text{H}_2\text{O}(l) \)
Cancel spectator ions (\( \text{Na}^+, \text{Cl}^- \)) → Net ionic equation: \( \text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) \)
Example
Deduce the ionic equation for the reaction between silver nitrate solution and sodium chloride solution.
▶️ Answer/Explanation
Full equation: \( \text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq) \)
Ionic form: \( \text{Ag}^+(aq) + \text{NO}_3^-(aq) + \text{Na}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) + \text{Na}^+(aq) + \text{NO}_3^-(aq) \)
Cancel spectator ions (\( \text{Na}^+, \text{NO}_3^- \)) → Net ionic equation: \( \text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) \)