Hard water is water that does not give lather easily with soap because it contains dissolved calcium and magnesium salts. Temporary hardness is due to bicarbonates of Ca and Mg (Ca(HCO3)2, Mg(HCO3)2) and can be removed by boiling or by Clark's method (adding calculated slaked lime). Permanent hardness is due to chlorides and sulphates of Ca and Mg and is removed by adding washing soda (Na2CO3) or by the ion-exchange (permutit/resin) method.

Solubility of hydroxides increases down the group (Mg(OH)2 is sparingly soluble while Ba(OH)2 is fairly soluble) because lattice energy decreases faster than hydration energy. Solubility of sulphates decreases down the group (MgSO4 is soluble but BaSO4 is almost insoluble) because hydration energy decreases faster for the larger ions. Flame colours: calcium gives brick-red, strontium gives crimson-red and barium gives apple-green.

Diborane (B2H6) has two boron atoms and six hydrogen atoms. Four hydrogen atoms (two on each boron) are terminal and are bonded by normal two-centre two-electron (2c-2e) B-H bonds. The remaining two hydrogen atoms are bridging: each forms a three-centre two-electron (3c-2e) B-H-B bond, often called a banana bond. Each boron is sp3 hybridised. The molecule is electron-deficient because it does not have enough valence electrons for normal two-electron bonds throughout.

B2H6 (2 bridging + 4 terminal H)