Q1.
Add 3–4 drops of iodine solution into each test tube with the help of a dropper. Mix the content of each test tube separately and observe. Record your observations in Table 9.1.
Answer
Iodine is the test for starch: it turns blue-black where starch is present, and stays its own brownish-orange colour where starch is absent.
| Table 9.1: Action of saliva on starch | |||
|---|---|---|---|
| Test tube | Initial colour before adding iodine | Final colour after adding iodine | Possible reason for the change in colour, if any |
| A: Boiled rice | Milky white (rice + water) | Blue-black | The rice was not chewed, so no saliva acted on it. Starch is still present, and iodine gives a blue-black colour with starch. |
| B: Chewed boiled rice | Milky white (rice + water) | No change — stays brownish-orange; or only a very light blue-black | While chewing, the digestive juice in saliva broke the starch into simple sugars. Little or no starch is left, so iodine cannot give its blue-black colour. |
Why it happens: the two test tubes differ in only one thing — whether saliva was mixed with the rice. So whatever difference appears in the colour must be caused by saliva. That is what makes this a fair test.
Check it yourself: if tube B still turns fairly dark, you did not chew long enough. Repeat with 60–90 seconds of chewing and the colour will fade further.