Friday, February 22, 2013

8th Science Hots Questions[CBSE Holy faith a b c of Science]


8th Science: Chapter:  Reproduction and Reaching of adolescence

1. How are human twins produced?
Ans: There are two way twins are formed. (a) Twins occur when a single egg is fertilized to form one zygote which then divides into two separate embryos.(b) Twins occur when two separate eggs  are fertilized by two separate sperm.
2. Whose sex chromosomes is responsible for sex of unborn baby, either father’s or mother’s sex hormones.
Ans: The sex chromosomes of the father determine the sex of an unborn baby.
3. Why ace and pimple appear in adolescent age of human being?
Ans: This is because during puberty the secretion of sweat glands and sebaceous glands (oil glands) increases in the skin.
4. What is Adam’s apple?
At puberty, the voice box or the larynx begins to grow. The growing voice box in boys can be seen as bump in the throat called Adam’s apple.
5. Why does reproductive age of women starts from menarche and end at menopause?
Women start producing eggs when first menstrual flow begins called menarche. Stoppage of menstruation is termed menopause. This is why reproductive age of women starts from menarche and end at menopause.
8th Science: Chapter: The Cell
1. What is Eutely?
Ans: The phenomenon of fix number of cell in the organs of body is called Eutely.
2. Why chromosomes are called hereditary vehicle?
Ans: chromosomes transmit genetic information for one generation to next so they are called hereditary vehicle.
3. What is autolysis? Name the organelles of cell which perform the process of autolysis?
Ans: Autolysis is the process of self-digestion of cell or tissue by their own enzyme .  Lysosomes perform the process of autolysis.
4. Why are Chloroplast found only in plants?
Ans: Chloroplasts (Green plastids) are responsible for the process of photosynthesis.  Since only plants carry out photosynthesis, Chloroplast found only in plants.
8th Science: Chapter:  Combustion and Flame
1. Why should water not be used to extinguish fire caused due to faulty wiring?
Ans: This is because water is good conductor of electricity and current can flow in body.
2. Why red phosphorous stored inside water?
Ans: This is because red phosphorous has low ignition temperature and catch fire at room temperature.
3. Why some material burns with flame while other without flame?
Ans: This is because combustible substance that vaporizes on combustion produce flame.
8th Science: Chapter: Air and water pollution
1. Why in some village of Gujarat and Rajasthan hunchbacks are common sights?
Ans: This is because in some village of Gujarat and Rajasthan fluoride level has gone up to 11.0mg/l .
2. What is an algal bloom in pond?
Ans: Excessive quantities of chemicals which get washed from the fields act as nutrients for algae to flourish. This is called bloom.
3. What are the harmful effects of ozone whole? Name some appliances that use CFC?
Ans: Ozone holes have been found over Antarctic and arctic region. It is feared that excessive amount of UV radiation will reach the earth and will cause skin cancer in humans. Chlorofluorocarbons (CFCs) which are used in refrigerators, air conditioners and aerosol sprays.  
4. How does green house effect comes into play in our natural environments?
Ans: A part of the Sun’s radiation that falls on the earth is absorbed by harm full gases like CO2 does not allow it to escape into space. The trapped radiations further warm the green house earth. This is called the greenhouse effect.
5. What is eutrofication?
Ans:  Eutrofication is the gradual increase in concentration of phosphorous, nitrogen and other plant nutrient in an aquatic echo system.

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