1-chemical changes, such as the effect of heat on calcium carbonate are irreversible, whereas physical changes such as hard boiling an egg, are reversible.

2-Hydrogen chloride has the structure H-Cl, and sodium chloride has the structure Na-Cl.

3-in the periodic table, carbon comes in the same group as silicon, so CO2 has very similar properties to SiO2.

4-when you burn sodium in chlorine, you start off with seperate atoms of sodium and the sodium gives an electron to the chlorine because the sodium and chlorine atoms want to have 8 electrons in their outer shells. the ions of sodium and chlorine then stick together to make sodium chloride.

5-Imagine you had a flask which contained just one single molecule of water. At 0 degrees celcius the water molecule would turn to an ice molecule, and at 100 degrees celcius it would turn to steam molecule.