Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Distinguish between physical and chemical changes.
• Identify evidence of a chemical change.
• Write balanced chemical equations with correct symbols.
• Apply the Law of Conservation of Mass.
• Define reactants, products, coefficients, and subscripts.

Lesson summary

Chemical changes are fundamental to understanding how the world around us works. From the burning of fuel in our cars (or paraffin stoves) to the digestion of food in our bodies, chemical reactions are constantly occurring. In South Africa, understanding these processes is particularly important. For example, knowing how to safely use and store chemicals in households and industries, understanding environmental issues like acid rain from coal burning, and appreciating the chemical processes involved in mining and agriculture are all crucial. This week, we will delve into the differences between physical and chemical changes and learn how to represent these changes using chemical equations.

Lesson notes

2.1 Physical vs. Chemical Changes The core distinction lies in whether the identity of the substance changes.

Physical Change: Alters the form or appearance of a substance, but not its chemical composition. The substance is still the same material, just in a different form.

Examples include: Changes of state: Melting (solid to liquid), boiling (liquid to gas), freezing (liquid to solid), sublimation (solid to gas), condensation (gas to liquid).

Think of ice melting into water: it's still H₂O. Dissolving salt in water is also a physical change as we can evaporate the water to obtain the salt again.

Changes in size or shape: Crushing a can, cutting paper, dissolving sugar in water.

Mixtures: Combining sand and water (they don't react). Important

Note: While some physical changes may appear to involve a change in identity (like dissolving), the original substance can be recovered by physical means.

Chemical Change: Results in the formation of new substances with different chemical compositions and properties. This involves the breaking and forming of chemical bonds. Chemical changes are often irreversible (or require significant energy to reverse).

Examples include: Burning: Wood burning to ash and gases. The wood is no longer present; it's been transformed into entirely new substances.

Rusting: Iron reacting with oxygen and water to form iron oxide (rust).

Cooking: Baking a cake involves chemical reactions that change the ingredients into something new.

Digestion: Food being broken down into simpler substances in your body.

Electrolysis: Using electricity to decompose water into hydrogen and oxygen.

Signs of Chemical Change: Change in colour Formation of a precipitate (a solid forming in a solution) Gas evolution (bubbles) Change in temperature (heat is either released - exothermic reaction, or absorbed - endothermic reaction) New odour 2.2 Representing Chemical Changes: Chemical Equations Chemical equations are symbolic representations of chemical reactions. They use chemical formulas to show the reactants (the substances that react) and the products (the substances formed). Reactants are written on the left side of the equation. Products are written on the right side of the equation. An arrow (→) separates the reactants and products and indicates the direction of the reaction. The arrow can also have text above or below it, indicating conditions necessary for the reaction, such as heat (Δ) or a catalyst.

State Symbols: These indicate the physical state of the substance: (s) - solid (l) - liquid (g) - gas (aq) - aqueous (dissolved in water)

Coefficients: These are numbers placed in front of the chemical formulas to balance the equation. They indicate the number of moles of each substance involved in the reaction.

Subscripts: These are small numbers written below and to the right of an element symbol in a chemical formula. They indicate the number of atoms of that element in a molecule or formula unit.

Example: 2H₂(g) + O₂(g) → 2H₂O(l)

Reactants: H₂ (hydrogen gas), O₂ (oxygen gas)

Products: H₂O (liquid water)

Coefficients: 2 in front of H₂ and H₂O Subscripts: 2 in H₂ and O₂ State Symbols: (g) for hydrogen and oxygen, (l) for water 2.3 Balancing Chemical Equations: Law of Conservation of Mass The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. This means that the number of atoms of each element must be the same on both sides of the chemical equation. Balancing an equation ensures that this law is obeyed.

Steps for Balancing Equations: Write the unbalanced equation: Make sure you have the correct chemical formulas for all reactants and products. Count the number of atoms of each element on both sides of the equation. Choose an element to balance. Start with elements that appear in only one reactant and one product. Adjust the coefficients in front of the chemical formulas to make the number of atoms of that element equal on both sides. Never change the subscripts within a chemical formula. Repeat steps 3 and 4 for all other elements in the equation.

Check your work: Make sure the number of atoms of each element is the same on both sides of the balanced equation.