A scientist use a filter weighing 4.020 grams to collect particles. After pumping 750mL of air through the filter, it weighed 4.031 grams. What was the concentration of particle mass in the processed air, in milligrams per liter (mg/L)?

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Ido not understand. 

Hbr dissociates to form h+ and br-ca(oh)2 dissociates to form ca2+ and 2oh-the h+ and oh- react to form waterassuming the reactants are dissolved in water, the net reaction is h+ + oh- —> h2oif you’re asked to show all components of the reaction, the complete reaction is 2hbr + ca(oh)2 —> cabr2 + 2h2o