Question

 

Why did we add bicarbonate solution in the tube we put the Elodea plant when experimenting with light intensity and photosynthesis?

		it provides minerals for the plant
		It provides CO2 for the plant
		it provides oxygen for the plant
		it is a hypotonic solution

You have an unknown substance and you get a (+) result with the Benedict’s and a negative result with the Barfoed’s test: which of the following could be your unknown?

		glucose
		fructose
		sucrose
		lactose
		starch

You have an unknown mixture of 2 substances and you get a (+) result on both the Benedict’s and Barfoed’s tests: which of the following could be in your mixture?

		glucose and sucrose
		glucose and lactose
		lactose and maltose
		a or b
		a, b, or c

Why is acetone used as the solvent for the paper chromatography separation of pigments?

		acetone is hydrophilic
		pigments are hydrophobic
		acetone is volatile
		acetone is clear

Answer 1

The function of sodium bicarbonate in the above experiment is to provide carbon dioxide for the photosynthesis to occur. When the concentration of sodium bicarbonate is increased, the rate of photosynthesis will also increase. Water, carbon dioxide and light energy is required for photosynthesis to occur. So when sodium bicarbonate is increased, the carbon dioxide is more supplied hence, the photosynthesis increases. Therefore, it provides CO​​​2​ for the plant, is correct.

Benedict's test is used to detect the presence of reducing sugar therefore, it gives positive test for all monosaccharides and some disaccharides like lactose, maltose, because they all reducing and negative for non-reducing sugars. Barfoed's test is also used to detect reducing sugar but is also used to distinguish monosaccharides from reducing disaccharides. Because monosaccharides give the reaction in 1-2 mins while reducing disaccharides give the reaction in 7-12 mins. Therefore, glucose, fructose and lactose give positive Benedict's test while out of those 3 only lactose give negative barfoed's test. Hence, lactose is correct option.

From the above explanation it is clear that any mixture having glucose will give both the test positive. Hence, option 4 is correct.

Acetone is less polar than water therefore it gives better resolution of pigments on paper. Hence, acetone make it clear to see the result of paper chromatography which make it better than water to be used in experiment. Hence, option 4 is correct.