Transparent nanoporous silica aerogel was synthesized at ambient pressure using sodium silicate (water glass) precursor in a sol-gel process. Effect of solvents used for exchange on physical properties was investigated. Trimethylchlorolsilane (TMCS) was used as a hydrophobic reagent in the surface modification process. The volume % of TMCS was varied by changing its volume from 2 to 12 ml added in 50 ml of hexane and toluene solvent. Also the effects pH and temperature on the gelation of hydrosol and the physical properties of the nanoporous silica aerogels (particle size, pore volume, surface area, porosity, and pore diameter) were investigated. The thermal stability of the nanoporous silica aerogel was studied in the temperature range 25 to 800°C. The optical transmittance of the aerogel was found to decrease from 20 to 80% with an increasing volume % of TMCS in solvents. The surface chemical modification was confirmed by the FTIR spectroscopy. The modified aerogels were thermally stable up to 350°C.