Sound — Science STD 8 — Question

The process that you saw in Activity 7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?

Why have synthetic fibres become more popular than natural fibres?

Given below is a conversation between two farmers Heera and Beera.
Heera: Brother Beera, your maize crops look beautiful. They have grown pretty fast.
Beera: Yes, I have sprayed urea this time. What about you ?
Heera: Well, I am still relying on good old cow dung. I am saving money for buying a tractor.
Beera: That's good. Tractor saves a lot of time and labour.
Heera: Yes, it's been very labour intensive for me and now these weeds have come up.
Beera: Try weedicides, they are very effective.
Now, answer the following questions.
(i) List the practices which are not environment friendly and why ?
(ii) What is the advantage of modern agricultural Implements over traditional ones ?
(iii) Name one weedicide and the precaution, farmers must take during its application.

Explain why, sometimes when we take off the woolen sweater in a dark room, we can see tiny sparks of light and hear a cracking sound.

Beera, wants to practice crop rotation in his field. Suggest a Rabi crop and a Kharif crop which will replenish his field with nitrogen. Which crop replenishes nitrogen and why?

What is nylon? State the important properties of nylon.

Lila always eats only dal and rice in every meal. She often falls ill and has become prone to diseases. Can you suggest changes in her diet which can make her healthy and free from disease?

What is meant by an ‘embryo’? Can we identify the body features in an embryo?

State the harmful effects of taking drugs.