Answer 1) In the mixture of sand, NaCl and 
first separation was done by dissolving the mixture in water. In the first step the NaCl will get dissolved whereas, will be undissolved and sand will settle at the bottom. Here, the physical property of solubility of NaCl in water is taken into consideration. The collected water is then filtered off and evaporated to get the NaCl back from the mixture with some loss. No chemical change occurs in case of NaCl extraction from water.
Answer 2) When was to be removed from the undissolved part. The chemical property of was used. Where it gets dissolved in acidic medium. And this way we can extract and remove sand from it. We change the chemical composition of by adding HCl to the mixture and dissolve to form 
which gets dissolved into the solution of HCl. Here, first decantation occurs and then extraction is done. Second, extraction is done using potassium carbonate in it which separates from sand.
Answer 3) Here, Unknown mixture of salt and sand weighed =7.52 g (before washing);
Unknown mixture of salt and sand weighed =3.45 g (after washing);
To calculate the amount of salt in it, we can simply subtract the values of before and after washing change in weights.
The property of NaCl being soluble in water it will go away with washing leaving behind the sand only, after washing.
So, Weight of salt was = 7.52g - 3.45 g = 4.07g
To find the percentage of sand that was mixed with salt =
(3.45 g sand / 7.52g of mixture) X 100 = 45.9% sand was mixed with salt.
To verify whether the correct percentage of salt and sand was calculated we can recheck the value for salt as well.
(4.07 g of salt / 7.52g of mixture) X 100 = 54.1% salt
On adding we get, 45.9% + 54.1% = 100%.
Which confirms that the calculations are correct.
