8th grade Science: Orals

What is an atom?

An atom is the smallest unit of matter. An atom is the smallest particle of an element that retains it's chemical identity in all physical and chemical changes

What are Isotopes?

Some elements in nature have atoms with same atomic number but different mass number. Such atoms of same element with different mass number are called isotopes

What is Atomic number?

The number of protons in the nucleus of an atom is the atomic number of that element. It is denoted by ‘Z’.

What is Atomic mass number?

The total number of protons and neutrons in an atom is the atomic mass number of that element. It is denoted by ‘A'.

What is a Moderator in Nuclear Reactor?

To control the chain reaction in the nuclear reactor, it is necessary to decrease the speed and number of neutrons. Graphite and heavy water are used as moderator for reducing the speed of neutrons.

Examples of two liquid elements

Mercury, Bromine

Two examples of Gaseous elements

Hydrogen, Oxygen

Two examples of Solid element

Iron, Copper

Two examples of Homogeneous mixture

Air and seawater

Two examples of Colloid

Milk, blood

Two examples of inorganic compounds

Iron oxide (rust), sodium bicarbonate (soda)

Two examples of organic compounds

Glucose, Urea

Two Elements with valency 1

Hydrogen and chlorine

Two Elements with valency 2

Oxygen, Sulphur

Two examples of Metalloids

Arsenic, silicon

Two examples of complex compound

Chlorophyll, hemoglobin

Why stainless steel vessels in kitchen have copper coating on the bottom?

Copper is a better conductor of heat than stainless steel. Stainless steel vessels used in kitchen have copper coating to get heated up faster

Why are copper and brass vessels are cleaned with lemon?

Copper reacts with moisture CO2 in air to form copper carbonate, due to which copper loses it's shine. Citric acid present in lemon neutralizes copper carbonate. There copper and brass vessels are cleaned with lemon

Why is Sodium metal is kept in kerosene?

Sodium metal is highly reactive element. It reacts with water vapour present in air to form sodium hydroxide and hydrogen. The hydrogen released catches fire in presence with oxygen. Therefore sodium metal is kept in kerosene

What is element?

Matter that is made up of only one kind of substance is element, eg oxygen, nitrogen

What is Compound?

When two or more elements are combined together in a fixed proportion, it is called a compound. Eg, sugar, water

What is a mixture?

When two or more elements are mixed in any proportion and do not react chemically form a mixture. Eg, air, milk

Why is all the mass of atom concentrated in the nucleus?

Almost entire mass of an atom is concentrated in the atomic nucleus because the nucleus contains protons and neutrons. Electron is in the extranuclear part and it's mass is negligible

Why is Atom electrically neutral?

The number of electrons in the extranuclear part is equal to the number of protons in the nucleus. Therefore electrical charges are balanced and and the atom is electrically neutral

Why is Atomic mass number a whole number?

The total number of protons and neutron in an atom is the atomic mass number of that number. Therefore, atomic mass number is a whole number.

Explain protons

Protons are positively charged. Mass of a proton is 1.66×10 raised to minus 27kg. They're present inside the nucleus

Explain Neutrons

Neutrons are neutral, they don't have charge. Mass of neutron is 1u. They're present inside the nucleus.

Explain Electrons

They're negatively charged. Mass is negligible. They're present outside the nucleus in different shells.

What is a suspension?

A heterogeneous mixture of a liquid and a solid is called a suspension. Eg. Sand + water.

What is a Colloid?

A heterogeneous mixture in which the particles cannot be seen with naked eyes and have a diameter of 10 raised to minus 5m are called colloid

What is a solute?

The components of a solution which is present in less proportion is a solute

What is a solvent?

The component in a solution which is present in a large proportion is solvent

What is a solution

A homogeneous mixture of two of more substances is called a solution