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A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the Fig. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it? (a) Explain why the milk sample was illuminated. Name the phenomenon involved. (b) Same results were not observed with a salt solution. Explain. (c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Answer: (a) Milk is a colloidal substance. Particulate matter present in the milk makes the light scatter which results in Tyndall effect.... View Article

A child wanted to separate the mixture of dyes constituting a sample of ink. He marked a line by the ink on the filter paper and placed the filter paper in a glass containing water as shown in Fig. The filter paper was removed when the water moved near the top of the filter paper. (i) What would you expect to see, if the ink contains three different coloured components? (ii) Name the technique used by the child. (iii) Suggest one more application of this technique.

Answer: (i) If the ink has three different coloured components, the filter paper will have spots of different colours at different heights, as... View Article

Iron filings and sulphur were mixed together and divided into two parts, ‘A’ and ‘B’. Part ‘A’ was heated strongly while Part ‘B’ was not heated. Dilute hydrochloric acid was added to both the Parts and evolution of gas was seen in both the cases. How will you identify the gases evolved?

Answer: Part A: When iron filings and sulphur powder are mixed and heated, they undergo a chemical reaction and form ferrous sulphide (FeS). It... View Article

Non-metals are usually poor conductors of heat and electricity. They are non-lustrous, non-sonorous, non-malleable and are coloured. (a) Name a lustrous non-metal. (b) Name a non-metal which exists as a liquid at room temperature. (c) The allotropic form of a non-metal is a good conductor of electricity. Name the allotrope. (d) Name a non-metal which is known to form the largest number of compounds. (e) Name a non-metal other than carbon which shows allotropy. (f) Name a non-metal which is required for combustion.

Answer: (a) Iodine is a lustrous non-metal. (b) Bromine is liquid at room temperature (c) Graphite is an allotropic form of Carbon (non-metal)... View Article