CBSE Class 7 Science Advanced Chemistry Classification of Matter Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. In a ______ properties of the constituents are retained.
Answer:
mixture
In simple words: The substances in a mixture are only mixed, so each keeps its own properties.

Teacher's Note:
1. Constituents keep their properties in a mixture, not in a compound.
2. That is why they can be separated by physical methods.

Question 2. A mixture of alcohol and water is an example of ______ mixture.
Answer:
homogeneous
In simple words: Alcohol and water mix completely and look uniform.

Teacher's Note:
1. Alcohol and water are miscible liquids.
2. Miscible liquids always make homogeneous mixtures.

Question 3. A substance which is formed by the chemical combination of two or more elements is called a ______.
Answer:
compound
In simple words: Chemical combination of elements gives a compound.

Teacher's Note:
1. Remember: chemical combination means compound, physical mixing means mixture.
2. Water and carbon dioxide are examples.

Question 4. Boiling point is the temperature at which ______ is converted to ______ at one atmosphere pressure.
Answer:
liquid, gas
In simple words: At the boiling point a liquid changes into gas (vapour).

Teacher's Note:
1. The boiling point of water is 100°C at normal atmospheric pressure.
2. Pressure matters, so the phrase 'one atmosphere' is included.

Question 5. Nonmetals usually exist in ______ state.
Answer:
gaseous
In simple words: Most nonmetals are gases at room temperature, though some, like carbon and sulphur, are solids.

Teacher's Note:
1. Learn the exceptions: carbon, sulphur, phosphorus and iodine are solids, bromine is a liquid.
2. Metals, on the other hand, are mostly solids.

Question 6. ______ show the properties of metals and nonmetals.
Answer:
Metalloids
In simple words: Metalloids lie in between metals and nonmetals in their properties.

Teacher's Note:
1. Germanium, arsenic, antimony, selenium and tellurium are metalloids.
2. The name tells you they are 'metal-like'.

Question 7. With an increase in the surface area of a liquid, the rate of evaporation ______.
Answer:
increases
In simple words: A bigger surface lets more molecules escape at the same time.

Teacher's Note:
1. The rate of evaporation is directly proportional to surface area.
2. A liquid in a wide plate evaporates faster than in a narrow bottle.

Question 8. The conversion of a liquid to its solid state on cooling is called ______.
Answer:
solidification (freezing)
In simple words: Cooling a liquid until it becomes a solid is called solidification or freezing.

Teacher's Note:
1. Water changes to ice at 0°C.
2. It is the opposite of melting.

Question 9. During freezing, heat is ______.
Answer:
released
In simple words: A liquid has to give out heat to become a solid.

Teacher's Note:
1. Freezing needs the liquid to lose heat to its surroundings.
2. Melting is the opposite, as it absorbs heat.

Question 10. Atomicity of ozone is ______.
Answer:
three
In simple words: A molecule of ozone, \(\ce{O3}\), has three oxygen atoms.

Teacher's Note:
1. Atomicity is the number of atoms in a molecule of an element.
2. Oxygen gas \(\ce{O2}\) has atomicity two.

Teacher's Note:
1. Sublimation fits two entries, a and e; the question warns that this can happen.
2. Link each method with the property it uses.

Teacher's Note:
1. A solid matches two entries here.
2. Use Table 1.2 of the notes to check properties.

Teacher's Note:
1. Draw the triangle of the three states and mark each change.
2. Sublimation (solid to gas) is the extra option here.

Question 14. Define cohesive and adhesive forces.
Answer:
Cohesive force: the force of attraction between molecules of the same substance.
Adhesive force: the force of attraction between molecules of two different substances. For example, water sticks to glass because of the adhesive force between water and glass.
In simple words: Same substance, cohesive; different substances, adhesive.

Teacher's Note:
1. Co- sounds like 'together', so cohesion is among like molecules.
2. Mention one example to complete the answer.

Question 15. Define melting and boiling points.
Answer:
Melting point: the temperature at which a solid changes into a liquid on heating at normal atmospheric pressure (0°C for ice).
Boiling point: the temperature at which a liquid changes into gas from the bulk of the liquid at normal atmospheric pressure (100°C for water).
In simple words: Melting point is where solid becomes liquid; boiling point is where liquid becomes gas throughout.

Teacher's Note:
1. Always quote the pressure condition 'normal atmospheric pressure'.
2. Give the values for water as examples.

Question 16. Name the factors which affect the rate of evaporation.
Answer:
1. Surface area of the liquid
2. Temperature
3. Humidity of the air
In simple words: Evaporation is faster with more surface area, higher temperature and drier air.

Teacher's Note:
1. Area and temperature increase the rate; humidity decreases it.
2. List all three factors for full marks.

Question 17. Define atomicity.
Answer:
The number of atoms present in one molecule of an element is called its atomicity. For example, a hydrogen molecule \(\ce{H2}\) has two atoms, so its atomicity is two.
In simple words: Atomicity counts the atoms in a molecule of an element.

Teacher's Note:
1. Give an example such as \(\ce{O2}\) or \(\ce{P4}\).
2. Atomicity is used only for elements here.

Teacher's Note:
1. Mention the kind of atoms in each.
2. Add one example of each.

Question 19. Give two examples of metals which can exist in liquid state below 35°C.
Answer:
Mercury and gallium.
In simple words: These two metals are liquid at or near room temperature.

Teacher's Note:
1. Mercury is liquid at room temperature; gallium melts at about 30°C.
2. Most other metals are solid at room temperature.