Ques. Out of two solar cookers, one was covered by a plane glass slab and the
other was left open. Which of the two solar cookers will be more efficient and why?
Ans. The solar cooker which is covered with plane glass slab will be more efficient because the plane glass slab traps the incoming solar radiations but does not allow the outgoing radiations and will help in increasing the temperature inside the solar cooker
Ques. Name two gases, other than carbon-di-oxide, that are given out during burning of fossil fuel and contribute towards acid rain formation
Ans. Sulphur-dioxide and nitrogen di-oxide
Ques. Alloys are used in electrical heating devices rather than pure metals. *Give one reason*.
Ans. This is because alloys have generally higher resistance than that of pure metals and therefore can produce higher temperatures.
Ques. During summer season, a milkman usually adds a very small amount of baking soda to fresh milk. *Give one reason*.
Ans. Milk is made slightly alkaline so that it may not get sour easily due to formation of lactic acid in it.
Ques. Baking soda is used in small amount in making bread and cake. It helps to make these soft and spongy. An aqueous solution of baking soda turns red litmus blue. It is also used in soda acid fire extinguisher. Use this information to answer the following questions:
(i) How does Baking Soda help to make cakes and bread soft and spongy?
(ii) How does it help in extinguishing fire?
(iii) is the pH value of baking soda solution lesser than or greater than 7?
Ans.
(i) Baking soda is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid like tartaric acid. When baking powder mixes with water then sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas, which gets trapped in wet dough and bubbles out slowly making the cake to rise and become soft and spongy.
(ii) Soda acid type fire extinguishers contain a solution of sodium hydrogencarbonate and sulphuric acid in seperate containers. When the knob of fire extinguisher is pressed, the sulphuric acid mixes with NaHCO3 to produce lot of carbon dioxide gas, which forms a blanket around the burning substance and cuts off its oxygen supply. Hence, fire gets extinguished.
(iii) Baking soda is alkaline in nature, therefore its pH is greater than 7 (approx. 8.5).
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