10 Science Solved Sample Paper 3 – Page 5
Question: 22. Why a leaf must be destarched before starting the experiment showing that light is necessary for photosynthesis? 
Answer: So that the previously formed starch in the leaves goes down into the stem and roots of the plant. Then, new starch is formed during the performing of the experiment on Photosynthesis, by keeping the plant in the sunlight. For destarching, the plant is first kept in the dark room for 72 hours.
(1). We use water bath for boiling leaf in alcohol.
(2). We cover a portion of leaf with black paper strip.
Question: 23. A student took an aluminium strip and dipped it in freshly prepared ferrous sulphate solution taken in a test tube. After some time, he observed some changes. Give the changes he observed and the balanced chemical reaction for that change. 
Answer: Green colour of Ferrous sulphate changed to colourless solution due the formation of Aluminium sulphate. Dark grey coloured Iron metal gets deposited on the Aluminium strip.
Al + FeSO4(aq) Green solution → Al2 —(SO4)3 (aq) Colourless solution + Fe (S) Iron metal
Question: 24. In the experiment to show that CO2 is given out during respiration, which chemical is taken in the small test tube to show the liberation of CO2 and how? 
Answer: KOH solution is taken in the small test-tube. CO2 gas liberated during respiration of germinating seeds is absorbed by KOH of this small test tube.
Question: 25. Which one of the following pairs represents homologous organs and why? 
(1). Wing of a bird and wing of a bat; (2). Wing of a bird and arm of a lizard
Answer: (2). Wing of a bird and arm of a lizard
Question: 26. While tracing the path of a ray of light through a glass prism it is advised that the distance between two pins you fix for drawing incident and emergent rays, should not be too large or too small. Why? Explain briefly. 
Answer: Reason: In order to locate the direction of incident ray and refracted ray with a greater accuracy.
Question: 27. You have been requested by an agency working for consumer awareness, to study the comparative cleansing capacity of different samples of soap. List two precautions that you will observe so as to get the reliable results. 
- The foaming capacity of soap depends on the type of water used.
- The foaming capacity of soap in distilled water is much more than in hard water.