Atoms & Molecules - Practice Test, Class 9 Science - Class 9 MCQ

Indivisibility of an atom was proposed by Dalton.
Explanation:
- John Dalton was an English chemist who proposed the atomic theory in the early 19th century.
- The atomic theory suggested that all matter is made up of tiny, indivisible particles called atoms.
- Dalton's theory stated that atoms are the smallest unit of matter and cannot be divided further.
- He believed that atoms of the same element are identical in size, mass, and other properties, while atoms of different elements have different properties.
- Dalton's atomic theory also proposed that atoms combine in simple, whole number ratios to form compounds.
- This theory laid the foundation for our modern understanding of atoms and their behavior.
- Although Dalton's atomic theory has undergone modifications with the discovery of subatomic particles, the concept of atoms being indivisible still holds true in the sense that they are the fundamental building blocks of matter.

Law of Constant Proportion
The Law of Constant Proportion, also known as the Law of Definite Proportions, states that in a chemical compound, the elements are always present in definite proportions by mass. This means that the ratio of the masses of the elements in a compound is always the same, regardless of the amount or source of the compound.
Explanation:
- The given statement states that all samples of carbon dioxide (CO2) contain carbon and oxygen in the mass ratio of 3:8.
- This means that for every 3 parts of carbon, there are 8 parts of oxygen by mass in carbon dioxide.
- According to the Law of Constant Proportion, the ratio of carbon to oxygen in carbon dioxide will always be 3:8, regardless of the source or quantity of the compound.
- This law applies to all chemical compounds, and it helps to establish the fixed composition of substances.
- The Law of Constant Proportion is one of the fundamental principles of chemistry and is essential for stoichiometry calculations and understanding the behavior of substances.
- It provides a basis for determining the empirical formula of a compound and allows scientists to predict and analyze chemical reactions and reactions stoichiometry.
- In summary, the given mass ratio of carbon and oxygen in carbon dioxide is in agreement with the Law of Constant Proportion, which states that the elements in a compound are always present in fixed proportions by mass.

Explanation:
The Latin name for an element is often derived from its chemical symbol. In the case of potassium, the chemical symbol is "K". The Latin name for potassium is "Kalium". Here is a detailed explanation:
Potassium:
- Potassium is a chemical element with the symbol K and atomic number 19.
- It is a soft, silvery-white alkali metal that is highly reactive.
- Potassium is an essential mineral that plays a vital role in various bodily functions.
- It is important for muscle contractions, nerve function, and maintaining a healthy balance of fluids in the body.
- Potassium is found in many foods, including fruits, vegetables, and whole grains.
Kalium:
- Kalium is the Latin name for potassium.
- The Latin name is derived from the chemical symbol "K" for potassium.
- The use of Latin names for elements is common in scientific and chemical contexts.
- It helps to avoid confusion and provides a standardized way of referring to elements.
Conclusion:
- In conclusion, the Latin name "Kalium" refers to the chemical element potassium, which has the symbol "K".
- This Latin name is commonly used in scientific and chemical literature.

Answer:
The smallest particle of a substance that is capable of independent existence is a molecule.
Explanation:
- A molecule is made up of two or more atoms bonded together.
- Atoms are the basic units of matter, but they cannot exist independently as they are constantly interacting and bonding with other atoms.
- When atoms combine chemically, they form molecules.
- Molecules can be made up of atoms of the same element (e.g., oxygen gas - O2) or different elements (e.g., water - H2O).
- Molecules have their own unique properties and can exist independently.
- Molecules can further combine to form more complex structures like cells, tissues, and organisms.
Therefore, the correct answer is B: Molecule.

The number 16 in 16O8 represents the atomic mass of the isotope Oxygen-16 (O-16).
Explanation:
- The atomic mass of an element is the weighted average mass of all the isotopes of that element, taking into account their relative abundance.
- Oxygen has several isotopes, including Oxygen-16, Oxygen-17, and Oxygen-18, with atomic masses of 16, 17, and 18 respectively.
- The number 16 in 16O8 specifically refers to Oxygen-16, which is the most abundant and stable isotope of oxygen.
- The atomic mass is expressed in atomic mass units (u) or Dalton (Da), where 1 atomic mass unit is defined as 1/12th the mass of a carbon-12 atom.
- The atomic mass is determined by the number of protons and neutrons in the nucleus of an atom.
- In the case of Oxygen-16, it has 8 protons and 8 neutrons, giving it an atomic mass of 16.
- The number 8 in 16O8 represents the number of electrons in an oxygen atom, as the number of electrons is equal to the number of protons in a neutral atom.
Therefore, in the compound 16O8, the number 16 stands for the atomic mass of Oxygen-16.

The molecular mass of ammonia is 17 grams.
Explanation:
Ammonia is a compound that consists of one nitrogen atom and three hydrogen atoms. To calculate the molecular mass of ammonia, we need to find the sum of the atomic masses of each atom in the molecule.
1. Nitrogen atomic mass:
- Nitrogen (N) has an atomic mass of 14 grams per mole.
2. Hydrogen atomic mass:
- Hydrogen (H) has an atomic mass of 1 gram per mole.
3. Molecular mass calculation:
- Ammonia (NH₃) contains one nitrogen atom and three hydrogen atoms.
- Multiply the atomic mass of nitrogen (14 grams/mole) by its number of atoms (1).
- Multiply the atomic mass of hydrogen (1 gram/mole) by its number of atoms (3).
- Add the results together: (1 * 14) + (3 * 1) = 14 + 3 = 17 grams/mole.
Therefore, the molecular mass of ammonia is 17 grams.

Explanation:
Definition:
- An atomic mass unit (amu) is a unit of mass used to express atomic and molecular weights.
- It is defined as 1/12th of the mass of a carbon-12 atom.
Given:
- One atomic mass unit is equal to the weight of:
A. One atom of hydrogen
B. 1/16th of an oxygen atom
C. 1/12th of a natural carbon atom
D. 1/12th of the C-12 isotope of carbon

- To determine which option is correct, we need to understand the definitions and relationships between the given options.
Option A: One atom of hydrogen

- The atomic mass of hydrogen is approximately 1.008 amu, which is not equal to 1 amu.
Option B: 1/16th of an oxygen atom
- The atomic mass of oxygen is approximately 16 amu.
- 1/16th of the atomic mass of oxygen would be approximately 1 amu.
- This means that one atomic mass unit could be equal to 1/16th of an oxygen atom, but it is not the standard definition.
Option C: 1/12th of a natural carbon atom
- The atomic mass of carbon is approximately 12 amu.
- 1/12th of the atomic mass of carbon would be approximately 1 amu.
- This means that one atomic mass unit could be equal to 1/12th of a natural carbon atom, but it is not the standard definition.
Option D: 1/12th of the C-12 isotope of carbon
- The atomic mass of the C-12 isotope of carbon is exactly 12 amu.
- 1/12th of the atomic mass of the C-12 isotope of carbon is exactly 1 amu.
- This matches the standard definition of an atomic mass unit.
Conclusion:
- Based on the definitions and relationships between the given options, the correct answer is D. One atomic mass unit is equal to 1/12th of the C-12 isotope of carbon.

Explanation:
When a chemical reaction takes place, there are certain changes that occur. However, one of the options listed does not change during a chemical reaction. Let's break down each option and explain why it does or does not change:
A: Volume
- Volume can change during a chemical reaction. For example, if a gas is produced, the volume of the system will increase.
- Therefore, volume can change during a chemical reaction.
B: Mass
- According to the law of conservation of mass, mass is conserved during a chemical reaction.
- This means that the total mass of the reactants is equal to the total mass of the products.
- Therefore, mass does not change during a chemical reaction.
C: Physical properties
- Physical properties refer to characteristics such as color, texture, density, and boiling point.
- In a chemical reaction, new substances are formed with different physical properties.
- Therefore, physical properties can change during a chemical reaction.
D: Chemical properties
- Chemical properties refer to the ability of a substance to undergo a chemical change.
- During a chemical reaction, substances react to form new substances with different chemical properties.
- Therefore, chemical properties can change during a chemical reaction.
Answer: B
- Mass does not change during a chemical reaction according to the law of conservation of mass.
- The total mass of the reactants is equal to the total mass of the products.
- Therefore, mass is the option that does not change when a chemical reaction takes place.

Dalton was the first scientist to use the symbols for elements in a very specific sense. Symbols for some elements as proposed were proposed by Dalton are as follows