Science : Ch 01 – Chemical Reactions and Equations

Both these processes produce a cooling efect rather than heating. Dry ice absorbs heat from the environment to convert into gas. This feature makes it a good refrigerant. Processes of dilution of sulphuric acid and condensation of water vapours produce a large amount of heat. There fore these are exothermic processes.

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.

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

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.

In Combination and Exothermic reaction, Calcium oxide (CaO), reacts vigorously with water to produce calcium hydroxide (slaked lime) and produces a large amount of heat. Since calcium oxide and water combine to form a single product, calcium hydroxide. So this is a combination reaction.

A double displacement reaction involves a displacement of two ions of two different compounds that results in the formation of new compounds. Here, only in reaction (ii), ions of both reactants are exchanged and two entirely different compounds are formed