The World of Metals and Non-metals — Question 15

For making tools, a ironsmith first heats a piece of iron until it becomes red-hot. This is because heating makes the iron soft, more malleable and easier to bend, hammer and mould into the desired shape for tools.
Class 7 The World of Metals and Non metals Question Answer (Activities)
Activity 4.1: Let Us Explore (Pages 42-43)
Observation:
Table 4.1: Appearance, hardness, and effect of hammering on different objects or materials
Object/Material
Appearance (lustrous/non-lustrous)
Hard/Soft
Effect of hammering (fattens/ breaks into pieces)
Piece of copper
Lustrous
Hard
Flattens
Piece of aluminium
Lustrous
Hard
Flattens
Iron nail
Lustrous
Hard
Flattens
Piece of coal
Non-lustrous
Soft
breaks into pieces
Lump of sulphur (pea-sized)
Non-lustrous
Soft
breaks into pieces
Block of wood
Non-lustrous
Hard but not hard as metal
Neither flattens nor breaks into pieces
Conclusion:
• Materials like copper, aluminium and iron are lustrous, hard and can be flattened into thin sheets.
• Materials like coal, sulphur are non-lustrous, soft and breaks into pieces. Wood is neither malleable nor brittle.
Activity 4.2: Let Us Investigate (Pages 45 – 46)
Observation: Metal spoon and metal coin produce a ringing sound. On the other hand, coal and wood produce dull sounds.
Conclusion: Metals are sonorous, they produce a ringing sound.
Activity 4.3: Let us Investigate (Pages 46-47)
Observation: Metal spoon is hotter to touch than wooden spoon.
Conclusion: Heat transfers through the metal spoon easily, making it hotter. While heat transfers through the wooden spoon poorly. Hence, metals are good conductors of heat, but wood is a poor conductor of heat.
Activity 4.4: Let Us Design And Create (Pages 47-48)
Observation: Table 4.2: Conduction of electricity by different objects or materials
Object/ Material
Observation (bulb glows/ does not glow)
Good conductor of electricity or poor conductor of electricity
Piece of aluminium foil
Bulb slows
Good conductor of electricitv
Iron nail
Bulb glows
Good conductor of electricitv
Lump of sulphur (peasized)
Bulb does not glow
Poor conductor of electricity
CooDer wire
Bulb glows
Good conductor of electricity
Piece of coal
Bulb does not glow
Poor conductor of electricity
Piece of dry wood
Bulb does not glow
Poor conductor of electricity
Stone
Bulb does not glow
Poor conductor of electricity
Conclusion:
• Materials like aluminium, iron and copper allows electricity to pass through them. So, they are good conductors of electricity.
• Materials like sulphur, coal, dry wood, stone, eraser and nylon do not allow electricity to pass through them. So, they are poor conductors of electricity.
Activity 4.5: Let Us Experiment (Pages 49-50)
Observation: Table 4.3: Formation of brown deposit on iron nails
Glass Bottle
A
Conditions
Observations
Presence of water (Yes/No)
Presence of air (Yes/No)
A
No
Yes
No brown denosit on iron nail
B
Yes
No
No brown denosit on iron nail
C
Yes
Yes
Brown deposits on iron nail
Conclusion:  The presence of both air and water is essential for the brown deposit to be formed on iron (rusting of iron).
Activity 4.6: Let Us Investigate (demonstration activity) (Page 51)
Observation:
• Magnesium ribbon burns with a dazzling white flame and changes into a white powder (ash).
• The solution changes the colour of red litmus paper to blue.
Conclusion:
• Magnesium burns with oxygen in the air and produces magnesium oxide.
• The oxides of metals are generally basic in nature.
Activity 4.7: Let Us Experiment (demonstration activity) (Pages 52-53)
Observation: The solution obtained after the addition of water to the glass jar turns blue litmus paper to red.
Conclusion:
• Sulphur burns with oxygen in the air and produces sulphur dioxide gas.
• On dissolving sulphur dioxide gas in water, sulphurous acid is formed, which turns blue litmus paper to red. Thus, oxides of non-metals are acidic in nature.
Activity 4.8: Let Us Explore (Page 53)
Observation: When sulphur is placed in water, no reaction occurs.
Conclusion: Sulphur does not react with water.