Explain why water has a high specific heat capacity

Water has a high specific heat capacity because a large amount of thermal energy needs to be absorbed in order to break the hydrogen bonds among water molecules before the individual water molecules can begin to move about more freely and therefore, causing an increase in temperature.  

Explain why ice floats

At O˚C, water molecules become locked into a crystalline lattice structure; each water molecule bonded to four other molecules through hydrogen bonding. This is the solid state of water called ice. The hydrogen bonds keep the water molecules far apart enough to give ice a density of about 10% less than that of liquid water. The crystalline structure provides a larger volume for the same amount of liquid water molecules. Therefore, ice is less dense compared to liquid water and ice floats on liquid water.

Describe the variation in melting points and electrical conductivities of the Period 3 elements from sodium to chlorine in terms of their structure and bonding.

Sodium, magnesium and aluminium have giant metallic structures with strong metallic bonds. They have relatively high melting point. Silicon has a giant covalent structure with strong covalent bonds. This accounts for its exceptionally high melting point. The rest are simple molecules with weak intermolecular van der Waals forces. The strength of the van der Waals force and the melting point increases in the order: Ar ˂ Cl₂ ˂ P₄ ˂ S₈. Sodium, magnesium and aluminium have mobile electrons. They are conductors. Silicon is a metalloid. The energy gap between the conduction band and the valence bond is small. It is a semi-conductor. Phosphorous, sulphur, chlorine and argon do not have mobile electrons. They are non-conductors.

State the general properties of ionic compounds and covalent compounds.

General properties of ionic compounds:

• Solid at room conditions.
• High melting point and boiling point.
• Soluble in water but insoluble in non-polar solvents.
• Do not conduct electricity in the solid state.
• Conduct electricity in molten state or in aqueous solutions.

General properties of covalent compounds:

• Low boiling point and melting point.
• Insoluble in water but soluble in non-polar solvents.
• Do not conduct electricity, either in the solid or molten state.

Weak acids and weak bases

Weak acids

• Weak acids dissociate only slightly in an aqueous solution to produce a low concentration of H₃O⁺.
• % dissociation less than 100% or α ˂˂ 1.

Weak bases

• Weak bases dissociate only slightly in an aqueous solution to produce a low concentration of OH-.
• % dissociation less than 100% or α ˂˂ 1. 

Strong acids and strong bases

Strong acid

• Strong acids dissociate completely in an aqueous solution to produce high concentration of H₃O⁺.
• 100% ionization or 100% dissociation or α = 1

 

Strong base

• Strong bases dissociate completely in an aqueous solution to produce high concentration of OH^-.
• 100% ionization or 100% dissociated or α=1

Structures and properties of metallic bonds

• Metals can be bent (ductile) and can be stretched (malleable) because the layers of atoms in metals slide over each other when force is applied but will not break due to attractive force between electrons and metal ions.
• Metals conduct electricity as it has free electrons which carry current.
• Metals conduct heat as it has free electrons which gains energy when heated and moves faster to collide with metal atoms, releasing heat in collisions.
• Metals have high melting and boiling points because the bonds between metals are very strong. Hence very high heat energy needed to break the bonds.