Heat Transfer in Nature — Science STD 7 — Question

How would you test the presence of starch in leaves?

How is rusting of iron a chemical change? Explain with an example.

If an object travels different distances in equal time intervals, how do we calculate speed?

Observation : Table : 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 electricity
Iron nail	Bulb slows	Good conductor of electricity
Lump of sulphur (peasized)	Bulb does not glow	Poor conductor of electricity
CooDer wire	Bulb slows	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

A sewage treatment plant involves a few steps. Aeration tank, grit and sand removal tank, second sedimentation tank, bar screen, first sedimentation tank.
(i)Arrange all the above steps in the correct order in which they occur in the sewage treatment plant.
(ii)Which step gives most of the sludge?

Mary and Manoj were classmates and good friends. On turning 11, Mary developed a little bulge on the front of her neck. She visited the doctor who gave her medication and asked to take iodine-rich diet. Similarly, a bump developed on the front of Manoj’s neck when he turned 12. However, the doctor told him that it was a part of growing up. According to you, what could be the possible reason for advising Mary and Manoj differently?

The decaying matter is always moist and warm. Explain why?

Insects and leaves of plant have pores through which they exchange gases with the atmosphere. Can you write two points of difference between these pores with respect to their position, number and extension into the body?

Why do we cover plug pinholes which are within the reach of children with cellotape or a plastic cover when not in use?

The micro-organisms called decomposers play an important role in the forest. Discuss.