I boiled apples in water until tender. I took apples out, then added lemon and sugar to water in pot and boiled it. When I poured it over the apples it made a nice thickened gel. By morning it's all watery. Why?
Some of the water content of the apples has migrated from the apples to the surrounding syrup. This is due to an effect called osmosis. The apple, like all living things is made out of cells that have (among other things) water inside them. The cell walls are semipermeable, meaning small molecules like water can pass through them, but larger molecules like sugars cannot. All dissolved substances (sugars, salt, etc.) attract water to themselves, so if you expose the apples to a highly sugary solution (like syrup), the water will be pulled from the apples into the syrup.
The syrup has very high sugar content and the apples much lower. The syrup pulls water out of the apples by a process known as osmosis, which tries to equalize the sugar concentration on each side of the permeable membranes of the apples' cells.
Sugar is hydrophilic, meaning it's attracted to water. Your water soaked/softened apples likely had a higher moisture content compared to your syrup. The water migrated out of the apples and equalized the sugar concentration. This process is referred to as osmosis.
Next time, try boiling the apples in that sugar syrup instead of just water.