Chemical reactions?
Which of these are chemical reactions?
1) Heating of iodine crystals to get purple vapour.
2) Melting of candle wax
4) Film photography
8) Heating of sugar to get a black substance
16) Dissolving potassium chloride in water
32) Burning methane to get water and carbon dioxide
Insert your answer as the sum of the option numbers.
E.g. If you think 1, 8 and 32 are the chemical reactions, you should input 41 as the answer (1+8+32=41).
The answer is 44.
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1 solution
1) Heating of iodine(s) sublimes it to iodine(g) involving only a phase transition which is a physical change.
2) Melting of candle wax (considering only melting part and not the combustion) is also a phase transition and can be reversed if cooled, so it is a physical change.
4) The simplest example is the black and white film (negative) in which silver halide (preferably AgCl) turns to metallic silver which blocks some part of incoming light and forms the black part of the photograph while chlorine is released as a gas. This is thus a chemical reaction.
8) When sugar is heated, the heat causes the sugar's atoms to combine with the oxygen in the air, forming new groups of atoms. Energy is released in this reaction in the form of smoke and black soot. It is also a chemical reaction.
16) When potassium chloride is dissolved in water and stirred then you get a clear solution of aqueous potassium chloride which is not formed by the reaction of potassium chloride with water but the association of molecules with the molecules of water by attractive forces.
32) Burning methane in air gives carbon dioxide and water with the release of energy in form of heat, light and sound. The process does not preserve the properties of methane and we get new compounds with different properties thus it is a chemical change.
Required answer is thus 32+8+4 = 44.