Lesson Notes By Weeks and Term v4 - SHS 1
SCIENCE AND MATERIALS IN NATURE
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SCIENCE AND MATERIALS IN NATURE
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Subject: General Science
Class: SHS 1
Term: 2nd Term
Week: 5
Grade code: 3.1.1.LI.3
Strand code: 1
Sub-strand code: 1
Content standard code: 3.1.1.CS.1
Indicator code: 3.1.1.LI.3
Theme: EXPLORING MATERIALS
Subtheme: SCIENCE AND MATERIALS IN NATURE
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Oxygen is one of the most important substances on Earth. It is the gas we breathe to stay alive, the gas that helps fire burn, and a key element used in hospitals and by artisans in our communities. In this lesson, we will move from just knowing *about* oxygen to learning how we can actually produce it in a school laboratory. We will investigate its properties and connect its uses to real-life situations we see every day in Ghana, from the welder's shop in Madina to the emergency ward at the Komfo Anokye Teaching Hospital. Understanding how to prepare and identify oxygen is a fundamental skill in science that helps us appreciate the materials in our natural world.
A. What is Oxygen? Oxygen is a chemical element with the symbol O. In nature, it usually exists as a molecule made of two oxygen atoms bonded together, so we write its formula as O₂. It is a colourless, odourless, and tasteless gas that makes up about 21% of the air we breathe. B. Laboratory Preparation of Oxygen The most common and safest way to prepare oxygen in a school laboratory is by the decomposition of hydrogen peroxide (H₂O₂). Decomposition: This is a chemical reaction where one compound breaks down into two or more simpler substances. Hydrogen Peroxide (H₂O₂): This is a liquid that looks like water (H₂O) but has an extra oxygen atom. It is unstable and can break down on its own, but very, very slowly. Catalyst: To speed up the decomposition, we use a catalyst. A catalyst is a substance that increases the rate of a chemical reaction without being used up in the reaction itself. For this experiment, the catalyst is Manganese (IV) oxide (MnO₂), a black powder. Apparatus Needed: Flat-bottom flask Thistle funnel Delivery tube Water trough Beehive shelf (or gas jar stand) Gas jars (at least 3) Spatula Hydrogen peroxide solution (approx. 20 volume) Manganese (IV) oxide powder The Procedure (Step-by-Step) Set-up: Place a small amount (about two spatulas full) of manganese (IV) oxide powder into the flat-bottom flask. Set up the apparatus as shown in the diagram (a two-holed stopper is fitted to the flask; the thistle funnel goes through one hole and the delivery tube through the other). Crucial Safety Point: Ensure the tip of the thistle funnel is below the level where the hydrogen peroxide will be. This prevents the oxygen gas from escaping through the funnel instead of the delivery tube. Fill the water trough with water. Fill the gas jars with water, cover them with a lid, and invert them in the trough, resting on the beehive shelf. Place the end of the delivery tube under the beehive shelf. Reaction: Carefully pour hydrogen peroxide solution through the thistle funnel into the flask. You will immediately see bubbles of gas (effervescence) as the hydrogen peroxide decomposes. This gas is oxygen. Allow the first few bubbles to escape, as this is just the air that was in the flask. Collection of Oxygen Gas: Place an inverted, water-filled gas jar over the end of the delivery tube on the beehive shelf. The oxygen gas will bubble up and displace the water in the gas jar. This method is called downward displacement of water or collection over water. When the gas jar is full of gas (i.e., all the water is pushed out), slide a lid over its mouth while it is still under water, and then remove it and place it upright on the bench. Collect 2-3 jars of the gas for testing. The Chemical Equation The reaction can be represented by a chemical equation. Word Equation: Hydrogen Peroxide → Water + Oxygen *(in the presence of Manganese (IV) oxide)* Symbol Equation (unbalanced): H₂O₂ → H₂O + O₂ Balancing the Equation: We need the same number of each type of atom on both sides. Left side: 2 Hydrogen (H), 2 Oxygen (O) Right side: 2 Hydrogen (H), 3 Oxygen (O) (1 in H₂O, 2 in O₂) To balance the oxygen, we place a '2' in front of H₂O₂ and a '2' in front of H₂O. Balanced Chemical Equation: 2H₂O₂(aq) --[MnO₂]--> 2H₂O(l) + O₂(g) (aq) = aqueous (dissolved in water) (l) = liquid (g) = gas [MnO₂] written over the arrow indicates it is the catalyst. C. Properties and Tests for Oxygen
Physical Properties: It is a colourless, odourless, and tasteless gas. It is slightly soluble in water (this is why we can collect it over water, and also why fish can breathe in water). It is slightly denser than air.
Chemical Properties / Tests: It supports combustion (burning): Oxygen itself does not burn, but things burn in it. Test for Oxygen: Take a glowing splint (a piece of wood that has been lit and then blown out so it is just glowing red) and insert it into a gas jar of oxygen. The glowing splint will relight. This is the definitive test for oxygen gas. It is a reactive non-metal: It reacts with many elements to form compounds called oxides. For example, when iron rusts, it is reacting with oxygen to form iron oxide.
Guided Practice (With Solutions)