Give Scientific Reason.

Question 13 Marks

Methane is used as a domestic fuel.

Answer

Methane is highly inflammable.
It burns by reacting with oxygen to give a bluish flame.
It burns completely and producers 213 Kcal/ mol of heat.
Being the smallest hydrocarbon, the proportion of CO2 released in the combustion of methane is small.
Therefore methane is used as a domestic fuel.

Question 23 Marks

Graphite is used as a lubricant.

Answer

Lubricants are used to reduce friction and wear and tear of mechanical parts.
Graphite is smooth and slippery and hence is useful in reducing friction. Hence, graphite is used as a lubricant.

Question 33 Marks

Graphite is a good conductor of electricity whereas diamond is a non-conductor of electricity.

Answer

In diamond, each carbon atom uses all its four electronic to get bonded to another carbon atom, whereas in graphite each carbon uses there out of four electrons during bonding.
Hence in diamond no free electrons are left whereas in graphite free electrons are available.
Due to the presence of electron which are free to flow graphite is a good conductor of electricity, whereas due to the absence of free electrons diamond is a non-conductor of electricity.

Question 43 Marks

Covalent compounds have low melting and boiling points.

Answer

When substances melt or boil, bonds between the molecules are broken due to heat supplied.
On covalent compounds, the intermolecular forces of attraction are weak.
Hence, intermolecular forces in covalent compounds are broken easily due to which they have low melting and boiling points.

Question 53 Marks

Biogas is an eco-friendly fuel.

Answer

Biogas is formed by the decomposition of animal dung, dry leaves, wet garbage in a biogas plant.
This produces methane gas also called biogas.
Biogas is a very cheap fuel option which meets the demand for cooking gas.
Biogas is eco-friendly as it contains about 55% to 60% of methane and rest is carbon dioxide, hence, on combustion it does not produce harmful gases which cause pollution.
Biogas is a fuel which is convenient to use and in addition to this it produces a very good manure as a side product of the process.
Hence, biogas is an eco-friendly fuel.

Question 63 Marks

Limewater turns milky when $CO_2$ is passed through it.

Answer

Limewater traditionally means a weak solution of the alkali calcium hydroxide $Ca(OH)_2.$
When $CO_2$ is passed through limewater, it reacts with calcium hydroxide to form insoluble particulates (precipitate) of Calcium carbonate $(CaCO_3).$
Calcium carbonate is weak basic salt and this gives a milky white precipitate.
Hence, lime water turns milky when $CO_2$ gas is passed through it.

Question 73 Marks

Graphite is not used in ornaments.

Answer

Question 83 Marks

Graphite is a conductor of electricity.

Answer

In graphite, each carbon is bonded to three other carbon atoms in such a way that a hexagonal layered structure is formed.
Due to this structure, graphite has free electrons available.
These free electrons move continuously within the entire layer.
Hence, graphite is a good conductor of electricity.

Science Give Scientific Reason.