Chapter 21, Problem 22PS

Interpretation Introduction

Interpretation: To write the reaction of potassium and hydrogen and predict the properties of the compound formed.

Concept introduction: The reaction of elements with hydrogen produces either ionic metal hydrides or molecular compounds.  The reaction of hydrogen with metals produces ionic metal hydrides. The electronegativity of hydrogen is more than metals and hence, metals bear positive charge and hydrogen has a negative charge. Thus, the oxidation number of hydrogen in ionic metal hydrides is $-1$.

Alkali metals are very reactive and form compounds with chlorides, oxides, hydrides and water. There is a transfer of electrons from metals to non-metals and thus ions are formed where metals have a positive charge and non-metals are negatively charged. Thus, alkali metals generally form ionic compounds and react vigorously to produce compounds with non-metals.

Physical properties of a substance are used to describe the solid by observing certain parameters related to it. Physical properties of any substance include colour, appearance, state, odour, melting and boiling point, texture, density and solubility.

The colour of a substance depends on various factors which are related to the number of electrons present in the substance. Matter can be classified as solid, liquid or gas. Matter in the solid state has fixed shape and volume, Matter in the liquid state has fixed volume but takes shape of the container in which it is filled. Matter in the gaseous state does not have fixed shape and volume. The appearance, texture and colour can be determined by looking at the substance. The melting and boiling point is determined by heating the substance.

The chemical property of a substance is predicted by its reaction with other compounds, chemical stability and heat of formation and combustion of that substance.