Complete the table and answer the following questions concerning chemical properties and allotropy.

18. Complete the table below.

• (i) Colour when hot: Yellow
• (ii) Colour when cold: White
• (iii) Colour when hot: Reddish-brown
• (iv) Colour when cold: Yellow

a) Calcium oxide is a basic oxide, and hydrogen chloride is an acidic gas. They would react chemically (acid-base reaction), so calcium oxide cannot be used to dry hydrogen chloride gas.

b) One drying agent for hydrogen chloride gas is concentrated sulfuric acid.

19.

a) Allotropy is the property of some chemical elements to exist in two or more different forms in the same physical state. These forms, called allotropes, have different physical properties but similar chemical properties.

b)

i) The allotrope is graphite.

ii) One property of graphite is that it is soft and slippery. This is because graphite has a layered structure where carbon atoms within each layer are strongly bonded, but the layers themselves are held together by weak intermolecular forces (van der Waals forces). These weak forces allow the layers to slide past each other easily.

20.

a) The substance responsible for the light blue precipitate is copper(II) hydroxide ($\text{Cu(OH)}_2$).