Question

Which of these is not an element?

1. On heating it gives off a gas and leaves residue.

2. It burns in air to form carbon dioxide and water.

3. It changes into solid at 273 K and to gas at 373 K.

I don't know about elements so can you please explain the answer?

Answer 1

1,2,3 all is correct .

A chemical element is a species of atomshaving the same number of protons in their atomic nuclei (that is, the same atomic number, or Z). 118 elements are identified, of which the first 94 occur naturally on Earthwith the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radionuclides, which decay over time into other elements. Iron is the most abundant element (by mass) making up Earth, while oxygen is the most common element in the Earth's crust.