Question

Name two soft metals that can be cut with a knife.

Answer 1

Hint: We can say that a soft metal is metal which is easy to dent and can be cut without shattering. It is not hard and is able to give little or no resistance to pressure and weight. It is smooth and fluffy on touch.

Complete answer:
Sodium and potassium are the metals which can be cut with a knife. The reason is that sodium has only one electron present in its outer orbital called the one valence electron. As it is the first element of period 3 so it has a large size. So this means that the force of attraction between particles which binds the metallic ions with electrons is weak. This means that the metallic bonding of sodium is weaker than other metals metallic bonding. Freshly cut sodium is bright but becomes dull gray when it reacts with oxygen in air. Hence it can be cut with a knife. Potassium can be cut with a knife because it has an empty space in the unit cell. So it has weaker metallic bonding due to which it can be cut with a knife. When the potassium is cut it has a bright shiny surface which changes to dull colour rapidly. This process of changing in colour from shiny to dull is called tarnishing. These soft metals can be reacted with cold water and form hydroxides. When sodium reacts with cold water the reaction is
$$2Na+2H_{2}O\to 2NaOH+H_2$$.
When potassium reacts with cold water the reaction is
$$2K+2H_{2}O\to 2KOH+H_2$$.

Note: Potassium is a soft metal with a silver luster and has a low melting point. It is a good conductor of electricity and heat. It imparts lavender colour flame and green colour water vapor. Sodium is silver white color metal with malleable and ductile property. It is the metal that can easily be cut with a knife. It is a good conductor of heat and electricity.