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52 21. Classic chemistry experiments Testing the pH of oxides Topic Metal and non-metal oxides. Timing 20–30 min. Description Students test the pH of various metal and non-metal oxides. Apparatus and equipment (per group) ▼ Test-tubes. Chemicals (per group) Provide solutions of : ▼ Nitric acid (labelled Nitrogen oxide and water) ▼ Sodium hydroxide (labelled Sodium oxide and water) –3 0.2 mol dm –3 0.2 mol dm –3 ▼ Potassium hydroxide (labelled Potassium oxide and water) 0.2 mol dm (Irritant) (Irritant) (Irritant) –3 ▼ Phosphoric acid (labelled Phosphorus (V) oxide and water) 0.2 mol dm ▼ Calcium hydroxide (labelled Calcium oxide and water) –3 0.2 mol dm ▼ Universal Indicator (in bottle with dropping pipette). In the past, chemistry teachers have labelled these as oxide solutions. It is understood that these are not solutions, the oxides do not dissolve – they have reactions with water. While it is important to maintain accuracy, it is suggested that an ‘…and water’ approach is taken. This is accurate and it allows the teacher to focus on the teaching point without the need to cover another set of reactions. However, teachers may wish to cover these reactions with water, this may depend on the age and ability of the class. Teaching tips Students are often confused with the difference between bases and alkalis. Background theory In general metal oxides are basic, non-metal oxides are acidic. Some metal oxides react with water to form alkaline solutions. Some metal oxides do not react with water. They test neutral in water because they are insoluble but are bases and react with acids. Non-metal oxides react with water to form acids. Safety Wear eye protection. If other concentrations of solution are used, ensure they are less –3 than 0.5 mol dm . Answers 1. Sodium, potassium and calcium oxides are metallic. 2. All the metal oxides are basic. 3. Some metal oxides do not react to form hydroxides in water therefore they are neutral. Classic chemistry experiments 53 Testing the pH of oxides Introduction In this experiment the pH of various oxides is tested. Solutions of various oxides Universal Indicator What to record Name of oxide Colour of Universal Indicator pH value Acid, alkali or neutral Nitrogen oxide Sodium oxide Potassium oxide Phosphorus(V) oxide Calcium oxide What to do 3 1. Using separate test-tubes, collect a sample (about 2 cm ) of each oxide in water. 2. Add three drops of Universal Indicator solution to each sample. 3. Record the results in a table showing the oxide, the colour of the Universal Indicator, the pH and whether the oxide is acidic, alkaline or neutral in water. Safety Wear eye protection Questions 1. Which compounds in the table are metal oxides? 2. Comment on your results for the metal oxides? 3. Some metal oxides do not react with water. Predict the pH of these compounds.