Which of the following are elements? (Select all that apply.)
KCl
Na
C
H2O

Na(Sodium) and C(Carbon) only

Na, C

Explanation:

1. KCl, or potassium chloride, is a combination of two elements- so it's a chemical compound.

2. Na, or sodium, can be found on the periodic table of elements. Therefore, it is an element.

3. C, or carbon, can also be found on the periodic table of elements. Therefore, it is an element.

4. H20, or water, is a combination of 2 hydrogen atoms that bonded into an oxygen atom. Combination of 2 or more elements is a chemical compound.

Therefore, the answers are Na and C.

the answer is 1 i believe

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.