Expert answer pls=>
How will you justify the following chemical changes?
1. Gasoline burning
2. Bread rising
3. Sun tanning
4. Milk turn into curd
1. This is a chemical reaction, because new substances are formed. The methane gas burns with oxygen in the air to form two new substances: carbon dioxide and water vapour. During this chemical reaction heat is also produced.
2. Here, due to fermentation , the yeast produces carbon dioxide and alcohol(ethanol) which is used to make the dough rise. There is a new substance formed.
3. So we know that melanin is the pigment causing tanning right? This melanin leaves the melanocytes and travels up through the epidermis where it REACTS with the UVA and UVB radiation through a chemical reaction ( new substance formed) darkening the pigmentation of the skin.
4. If you were making curd at home per se, you would add a little curd to milk and this curd contains Lactobacilli (pl. of Lactobacillus) The curd forms because of the chemical reaction between the lactic acid bacteria and casein. During fermentation, the bacteria use enzymes to produce energy (ATP) from lactose.
So since there is a new substance formed in all of this which is a sign of chemical change, it is considered a chemical change.