Vegetables become much more appetizing when after cooking the bright green of chlorophylls turn greyish-yellowish. This happens because the magnesium leaves the centre of chlorophylls molecules. Acidity favours this process, whereas alkalinity reduces it.
Therefore sometimes bicarbonate of soda is added to cooking water to keep bright the colours of broccoli, green beans and peas. But it alters the taste and destroys vitamins. To preserve chlorophylls without altering taste and nutritive principles, you have to boil vegetables in little water, leaving the saucepan without lid.
In a little water magnesium won’t leave chlorophylls so easily. It is even better to cut vegetables in small pieces and cook them directly in a pan: they are more crispy, preserve bright colours and keep all their vitamins and minerals almost intact.