The water in the container that you added salt to probably took longer to freeze than the plain water. Water freezes at a certain temperature - 32 degrees Fahrenheit. This temperature is called the freezing point of water, because pure water will always begin to freeze when it gets to 32 degrees. Salt is a mineral that lowers the freezing point of water. That means that when you added salt to the water, it lowered the freezing point of the water in the containers, so it did not start to freeze until it got really really cold from being in the freezer for a long time. The plain water started to freeze just as soon as it reached 32 degrees, but the salt water didn't start to freeze until it got much colder. The salt water was probably sort of slushy still by the time the plain water was frozen solid.
The ice from the cup without salt took much longer to thaw completely back into water than the cup with salt in it for the same reason. The freezing point of the salt water is still colder and now that it is out in the warmer air of the room, it is much farther from being as cold as it needs to be to continue to freeze into ice. It also thawed sooner than the plain water because there was less ice to melt since the salt water did not freeze all the way through.
The ocean can never freeze because there is so much salt in the water that the freezing point is very low. It just doesn't get cold enough. Any time a part of the ocean gets cold enough to start forming ice crystals, the salt around the ice will start to melt it by lowering the freezing point again so that it has to get colder to keep freezing. This is very important because many ocean animals would not survive if the water were to freeze!
For a fun and tasty experiment to see what happens when you mix ice and salt, try making ice cream in a plastic bag!
If you could be an ocean animal for a day, what would you want to be? Pick one you like or want to learn more about, then draw a picture of it. You can pretend to be that animal while you answer these questions:
What part of the ocean do you live in (near the surface, near the bottom, in a coral reef )?
What are your favorite things to eat (plants or animals)?
What animals are your predators (animals that might want to eat you)?
Are any animals your prey (what do you like to eat)?
What kind of shelter or protection do you like to have?
Do you use camouflage to protect yourself?
How do you move around?
What do you look like?
Do you have arms, fins, or tentacles? How many?
What kind of covering do you have on your body (scales, fur, skin)?
If you don't know the answers to some of the questions, ask a parent or older sibling to help you learn more about the animal from books or the internet. Here is one website to try (click on "Ocean").