Teacher’s Guide Focus-on Science Grade 7 29 © Praxis Publishing Singapore Pte. Ltd. move up and down in the ocean. Understanding density and floating/sinking allows students to gain a better understanding of how fluids behave and the properties of matter. Induction 1 Begin the lesson by asking students whether they understand what density is and if they have ever noticed how certain objects float while others sink in water. To assess their understanding, elicit responses with the following questions. (a) What is density? (b) How is it measured? (c) Can the densities of objects with the same mass differ? How? (d) Can you name a few examples of common objects that float or sink and explain how the density of these objects affects their behaviour? (e) How do the density of an object and the density of the liquid it is placed in determine if it will float or sink? 2 Allow students to respond to the questions, then look over their responses and ask follow-up questions. Provide more information as needed to assist identify any gaps in understanding. Teaching and Learning Activities Textbook pp. 58–59 1 Introduce the concept of density and explain that it refers to how much mass is contained within a certain volume. Write the formula for density (density = mass/volume) and mention that it is expressed in terms of gram per cubic centimetre or kilogram per cubic metre. 2 Go through the ‘Science Facts’ about the concept that Archimedes used to prove that King Hiero II’s crown was not made of pure gold with the students. Let them know that they will study Archimedes’ Principle in more detail in Grade 9. 3 Assign Activity 4 to students to work in pairs and review their work upon completion. Encourage them to discuss their answer and provide feedback to each other as a form of peer assessment to promote collaborative learning and improve their understanding of the topic. 4 Divide students into groups. Give each group a set of objects and water containers. Tell them to measure the mass and volume of each object and calculate their densities. After that, ask them to predict whether every object will float or sink before putting the objects in the water to see if their predictions were accurate. Let them record their observations and have a class discussion based on their findings. Textbook p. 60 5 Assign students to complete Activity 5 independently as a home activity. Then, relate the results of the activity with the question in ‘Think About It’. 6 Explain to the students on how the concept of density and floating/sinking relate to real-world scenarios, such as why ships float. Then, go through the ‘Science Facts’ that is related to the text with them. Textbook pp. 61–62 7 Ask the students to conduct Activity 6 which is similar to the previous activity in step 4. However, in Activity 6, students will measure the mass of different objects with the same volume and calculate their density to explore their relationship. ©Praxis Publishing_Focus On Science
Teacher’s Guide Focus-on Science Grade 7 30 © Praxis Publishing Singapore Pte. Ltd. 8 Help them to recall what they learned on p. 22 in the Textbook and carry out Activity 7 to determine the density of solids using water displacement method. Assessment and Evaluation 1 Assess the students’ understanding of the concept through their independent work, pair work and group activities. 2 Assess students’ scientific and critical thinking skills, attitudes and values through observation of their participation during Activities 6 and 7. 3 Have students complete Exercises 1–5 Workbook pp. 26–28 . 4 Provide feedback to students on their performance, highlighting areas where they have achieved and areas where they need to improve. Closure • To summarise the important ideas and concepts taught in the lesson, consider using the key points provided through strategies such as a class discussion, a summary statement, a quiz, a visual organiser or a reflective writing assignment. You may also ask students to summarise them in their own words. Allow students to ask any additional questions for clarification or clear up any doubts. Density 1 Density of a substance is the mass per unit volume of the substance. 2 The formula for density is density = mass/volume. 3 Density is measured in units such as gram per cubic centimetre or kilogram per cubic metre. Relationship between Density and Floating or Sinking 4 When placed in a fluid, objects that have a lower density than the fluid will float, while those with a higher density would sink. 5 The upward force, also known as buoyant force or upthrust of a fluid on an object is equal to the weight of the fluid displaced by the object. 6 Designing ships and submarines are two examples of how the relationship between density and floating/sinking is used in real-world situations. • Ask students to reflect on what they have learned in this chapter by completing the sections ‘Recall’ Textbook p. 63 and ‘Concept Map’ Workbook p. 31 . • Let students complete the ‘Objective Questions’ Workbook pp. 29–30 and ‘Online Quick Quiz’ Workbook p. 32 to check their mastery of this chapter. • To reinforce students’ critical thinking skills, have them answer the questions in ‘Put on Your Thinking Cap’ Textbook p. 64 and ‘HOTS Daily Application’ Workbook p. 32 that require them to analyse, evaluate or create based on the knowledge they have acquired. • Give students a preview of what will be covered in the upcoming chapter to help them get ready. Textbook Answers Activity 4 p. 59 2–4 Air 0.00129 g cm–3 (gas), petrol 0.8 g cm–3 (liquid), ice 0.92 g cm–3 (solid), water 1.0 g cm–3 (liquid), seawater 1.03 g cm–3 (liquid), rubber 1.52 g cm–3 (solid), glass 2.5 g cm–3 (solid), aluminium 2.7 g cm–3 (solid), diamond 3.5 g cm–3 (solid), iron 7.9 g cm–3 (solid), gold 19.3 g cm–3 (solid), platinum 21.4 g cm–3 (solid) 5 Gas 6 Solid ©Praxis Publishing_Focus On Science
Teacher’s Guide Focus-on Science Grade 7 31 © Praxis Publishing Singapore Pte. Ltd. Activity 5 p. 60 3 The density of egg is higher than the density of water, so the egg sinks. 6 By adding salt, the density of water is increased and its density is higher than that of the egg. Thus, the egg floats. Area is the total surface covered by an object. Think About It p. 60 This is because of the extremely high concentration of dissolved mineral salts in the water, making it denser than plain fresh water. As our body is less dense than the density of the water in the Dead Sea, we can float more easily. Activity 6 p. 61 Discussion: 1 Density of an object is defined as the mass per unit volume of the object. 2 The density increases when the mass increases. 3 Cork → Glass → Iron → Copper → Lead 4 Density = Mass Volume 2.3 = Mass 8 Mass = 2.3 × 8 = 18.4 g 5 sink, float Conclusion: increases, volume Activity 7 p. 62 Discussion: 1 Density = Mass Volume 2 irregular Conclusion: The density of regular or irregular-shaped solids can be obtained using the water displacement method. Recall p. 63 1 mass, discrete 2 liquid 3 regular, vibrate, cannot, shape 4 randomly, freely, cannot, can 5 far apart, directions, easily, easily 6 released 7 sublimation, evaporation (either order) 8 physical, chemical 9 density, kilogram 10 float, sink ©Praxis Publishing_Focus On Science
Teacher’s Guide Focus-on Science Grade 7 32 © Praxis Publishing Singapore Pte. Ltd. Put on Your Thinking Cap p. 33 1 Iron has the highest melting point. Its high melting point shows that its particles need more energy to overcome the forces of attraction between them to break away from their fixed positions. Thus, the greater the forces of attraction between particles, the higher the melting point of the substance. 2 The density of pure water is 1 g cm–3 and the volume of water given is 73.6 cm3 . Mass of water = Density of water × Volume of water = 1 × 73.6 = 73.6 g When water turns into ice, its mass remains unchanged. Therefore, 73.6 cm3 of water will freeze to form 73.6 g of ice. As given, the density of ice = 0.92 g cm–3. Volume of ice = Mass of ice Density of rice = 73.6 g 0.92 g cm–3 = 80 cm3 3 Fruit ripening is a chemical change that occurs as a result of a number of changes. The colour, texture and taste of a fruit change as it ripens. These changes are not reversible. Once the fruit has ripened, it cannot return to its raw state. Project: Build a Submarine Model Textbook p. 65 Ask students to apply what they have learned about density and floating versus sinking to a real-world situation, such as how submarines can control their buoyancy. Encourage students to practice logical thinking and critical thinking skills and make connections between the theoretical aspects of science and their practical applications. 1 Introduction (a) Refresh the concept of density and water displacement method. (b) Explain to students how different materials and designs can affect the buoyancy of an object. 2 Material selection (a) Instruct the students to research objects that can float or sink, as well as designs for a submarine model, from the Internet and books. (b) Identify various objects that can either float or sink in water. (c) Have the students test the abilities of objects to float or sink in water by using a water tank, basin or other containers filled with water. (d) Ask the students to confirm the materials they will use and calculate the total cost. 3 Planning and execution (a) Have students design a submarine model using the objects they have identified and tested, and plan how to make the model float and sink. (b) Encourage students to follow their plan carefully. (c) Provide guidance and support as needed throughout the process, such as advising on the suitability of materials and objects (e.g. cardboard, plastic bottles, pencils) used to build the submarine model or providing guidance on how to make the model float and sink. (d) Motivate them to redesign if the model does not work. Listen to students’ proposals to improve the model and make necessary adjustment. (e) Remind students to take note of the points of improvement to share with the class. ©Praxis Publishing_Focus On Science
Teacher’s Guide Focus-on Science Grade 7 33 © Praxis Publishing Singapore Pte. Ltd. 4 Reflection and presentation (a) Have each group demonstrate to the class how their submarine model can float and sink. (b) Discuss with the class after listening to their experience of designing and building the model. (c) Ask the students what they have learned about density and water displacement. 5 Assessment (a) Assess the students’ research skills before starting the planning stage, as well as their planning and execution skills. (b) Evaluate the submarine model based on its ability to float and sink. (c) Evaluate the student’s report, which should include steps 2 to 7 of the procedure, to assess their understanding of density and water displacement. ©Praxis Publishing_Focus On Science