Science : Ch 01 – Chemical Reactions and Equations

A physical change involves only change in physical state whereas a chemical change results in the formation of new substances. Boiling of water, melting of ice and dissolution of salt are physical changes as no new products are formed.

The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is 2:1 by volume. In a decomposition reaction, a compound is broken down into its simpler forms. During electrolysis, water is broken down into oxygen gas and hydrogen gas due to the passage of electric current through it.

Inert gases like helium or nitrogen both can be used for the storage of fresh sample of oil for a long time, as they do not react with most elements, including oxygen

Decomposition reaction can be defined as the reaction which involves decomposition of one compound to more than one substance. Endothermic reactions require heat energy to form products.

When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid. So formed remains in the solution.

In standard conditions, hydrogen and oxygen exist in a gaseous state, whereas water exists in a liquid state.

It is not atoms but the bonds between these atoms break and form during chemical reactions. Atoms of elements can rearrange but cannot change into other element.

The chemical reaction between dilute hydrochloric acid and granulated zinc is represented as follows:_x000D__x000D_Zn + 2HCl → ZnCl₂ + H₂↑

Melting of candle wax on heating is an specimen of physical change no chemical reaction is taking place only the change of state from solid to liquid state.

Combination reactions form a new product with the combination of more than one reactant molecules.