Remember it as it is ! - Form 4 syllabus

1) During heating of a solid substance, why the temperature remains constant?
A : The heat energy absorbed by the particles/molecules is used to overcome the forces of attraction between particles/molecules.

2) During cooling of a liquid substance, why the temperature remains constant?
A : The heat energy released is exactly balanced by the heat loss to the surroundings as the particles/molecules attract one another to form a solid.

3) Proton number = Number of protons in the nucleus of an atom.

4) Nucleon number = Total number of protons and neutrons in the nucleus of an atom.

5) Isotopes = Atoms of the same element with the same number of protons but different number of neutrons.

6) Empirical formula = A chemical formula that gives the simplest whole number ratio of atoms of each element in a compound.

7) Molecular formula = A chemical formula that gives the actual number of atoms of each element in a compound.

8) Why group 18 elements are inert?
A : The atoms have achieved stable electron arrangements.

9) Why group 18 elements exist as monoatomic gases?
A : Atoms of group 18 elements have achieve stable electron arrangements. Their atoms do not donate, accept or share electron with atoms of other elements.

10) Why are group 1 elements have the same chemical properties?
A : The atoms have the same number of valence electron.

11) Why the melting and boiling points of an ionic compound is higher than a covalent compound?
A : An ionic compound consists of oppositely charged ions that are attracted to one another by strong electrostatic force. More heat energy is required to overcome this strong force. Thus, melting and boiling points of an ionic compound is high.
Whereas, a covalent compound is made up of neutral molecules that are held together by a weak intermolecular force of attraction. Therefore, only small amount of heat energy is required to overcome this weak force. The melting and boiling points of a covalent compound is low.

12) Why an ionic compound in aqueous or liquid state can conduct electricity but a covalent compound cannot do so?
A : An aqueous or molten ionic compound has free moving ions, thus it can conduct electricity. But a covalent compound only contains neutral molecules that make it cannot conduct electricity.

13)Why are group 17 elements have the same chemical properties?
A : The atoms have the same number of valence electrons.

14) Strong / weak acid : An acid that ionises completely / partially in water to give high / low concentration of hydrogen ions.

15) Strong / weak alkali : An alkali that ionises completely / partially in water to give high / low concentration of hydroxide ions.

Empirical formulae of metal oxides

Experimentally, there are two methods to determine the empirical formulae of metal oxides. The methods depend on the reactivity of the metal.


Method 1 : To determine the empirical formula of oxide of a more reactive metal than hydrogen (in the Reactivity series = metals that are positioned higher than hydrogen in the series).

The principle is that to react the reactive metal with oxygen to produce metal oxide. This is because the metal can react readily with oxygen. In the laboratory, the apparatus set up that is appropriate for the purpose is like the diagram shown below :



In determining empirical formulae of metal oxides, metals that are said to be more reactive than hydrogen include calcium, magnesium, aluminium and zinc.

Taking magnesium as an example, the chemical equation of the reaction is
2Mg + O2 --> 2MgO.



Method 2 : To determine the empirical formula of a less reactive metal than hydrogen (in the Reactivity series = metals that are positioned lower than hydrogen in the series).

For this method, the principle is that to react (reduced) the less reactive metal with hydrogen gas to give its pure metal and water. This method is used because it is more convenient rather than waiting for a slow reaction between the metal and oxygen to react completely.

Below is the apparatus set up used in the laboratory to determine the empirical formulae of oxides of less reactive metals:



These kind of metals include tin, lead and copper. For an example, the reaction between copper(II) oxide and hydrogen is described as follows :
CuO + H2O --> Cu + H2.