Classify each of the following substances as an element or a compound: (a) hydrogen, (b) water, (c) gold, (d) sugar.

a) element

b) compound

c) element

d) compound

Explanation:

a) Hydrogen is the chemical element with the symbol H;  is the lightest element in the periodic table.

b) Water: It is a substance whose molecule is composed of two atoms of hydrogen and one of oxygen.

⇒ molecular formula: H2O

c) gold: is the chemical element with the symbol Au. It is a very dense, soft and intense yellow metal.

d) sugar: it is a compound whose chemical formula is C12H22O11

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.