Solutions of NaOH should be kept capped or the NaOH will react with CO₂ in the atmosphere to ultimately produce Na₂CO₃. The end result is that the molarity of the NaOH will substantially change over time. There appears to be a two step reaction that leads to the production of Na₂CO₃

H₂O + CO₂ -> H₂CO₃ (note that CO₂ is an acid anhydride)

H₂CO₃ + 2 NaOH -> Na₂CO₃ + 2H₂O (neutralization reaction)

To learn the details of this demonstration see this site [local]. To see this phenomenon, fill a 2 L PET (polyethylene terephthlate)soft drink container with water and displace water by generating CO₂ (or use a lecture bottle of CO₂) into the 2L bottle by water displacement. Add 30 mL 6M NaOH into the bottle and recap it or stopper it tightly. Shake the bottle and watch it collapse. Using 6 M NaOH requires the use of goggles and careful handling.