50 Science Chemistry Quiz Questions with Answers
Test your chemistry knowledge with 50 science chemistry quiz questions covering atoms, bonding, acids, reactions & more — each with a detailed explanation.
Chemistry is the branch of science that explains what everything around us is actually made of — from the air we breathe to the food we eat and the materials we build with. Understanding chemistry means understanding matter itself: how atoms combine, how substances react, and why the world behaves the way it does at a molecular level.
This 50 chemistry quiz questions covers a broad range of topics, including atomic structure, the periodic table, chemical bonding, acids and bases, reactions, and organic chemistry basics. Each question comes with a detailed explanation, so whether you get it right or wrong, you’ll walk away knowing a bit more chemistry than before.
1. What is the smallest unit of an element that retains its chemical properties?
A) Molecule
B) Atom
C) Proton
D) Compound
Answer: B) Atom An atom is the smallest unit of an element that still retains all the chemical properties of that element. Atoms are made up of protons, neutrons, and electrons — protons and neutrons cluster together in the nucleus, while electrons orbit around it in shells. When atoms combine chemically, they form molecules, but breaking an atom apart (as in nuclear reactions) changes its fundamental identity rather than just its chemical behavior.
2. Which subatomic particle has a negative charge?
A) Proton
B) Neutron
C) Electron
D) Nucleus
Answer: C) Electron Electrons carry a negative charge and orbit the nucleus of an atom in regions called electron shells or energy levels. Protons, found in the nucleus, carry a positive charge, while neutrons are neutral. The number of electrons in the outermost shell largely determines how an atom will react chemically, which is why chemists pay close attention to electron configuration when predicting bonding behavior.
3. What is the chemical symbol for sodium?
A) So
B) Sd
C) Na
D) S
Answer: C) Na Sodium’s chemical symbol, Na, comes from its Latin name “natrium.” Sodium is a highly reactive alkali metal that reacts violently with water, producing hydrogen gas and heat. In its pure form, it must be stored under oil to prevent it from reacting with moisture in the air. Combined with chlorine, it forms sodium chloride — ordinary table salt.
4. Which gas makes up approximately 78% of Earth’s atmosphere?
A) Oxygen
B) Carbon Dioxide
C) Nitrogen
D) Argon
Answer: C) Nitrogen Nitrogen makes up roughly 78% of Earth’s atmosphere by volume, making it by far the most abundant atmospheric gas — oxygen comes second at about 21%. Nitrogen is relatively inert under normal conditions due to the strong triple bond between its two atoms in the N₂ molecule, which is why it doesn’t easily react with other substances despite its abundance. It’s essential for life, forming a key part of amino acids and DNA.
5. What is the pH value of a neutral solution?
A) 0
B) 5
C) 7
D) 14
Answer: C) 7 A pH of 7 is considered neutral, meaning the solution is neither acidic nor basic — pure water at room temperature has a pH of 7. The pH scale ranges from 0 to 14, with values below 7 indicating acidity (more hydrogen ions) and values above 7 indicating alkalinity or basicity (more hydroxide ions). The scale is logarithmic, meaning each whole number change represents a tenfold difference in acidity or basicity.
6. Which element is essential for photosynthesis and found in chlorophyll?
A) Iron
B) Magnesium
C) Calcium
D) Potassium
Answer: B) Magnesium Magnesium sits at the center of the chlorophyll molecule, the pigment that gives plants their green color and enables them to absorb light energy for photosynthesis. Without magnesium, plants cannot produce chlorophyll effectively, leading to a yellowing condition called chlorosis. Magnesium is also important in human biology, playing a role in over 300 enzymatic reactions in the body.
7. What type of bond is formed when electrons are shared between atoms?
A) Ionic bond
B) Covalent bond
C) Metallic bond
D) Hydrogen bond
Answer: B) Covalent bond A covalent bond forms when two atoms share one or more pairs of electrons, typically between nonmetal elements. This is different from an ionic bond, where electrons are transferred from one atom to another, creating oppositely charged ions that attract each other. Water (H₂O) is a classic example of covalent bonding, where oxygen shares electrons with two hydrogen atoms.
8. What is the atomic number of an element determined by?
A) Number of neutrons
B) Number of protons
C) Number of electrons
D) Atomic mass
Answer: B) Number of protons An element’s atomic number is defined by the number of protons in its nucleus, and this number is what fundamentally distinguishes one element from another. For example, every atom with exactly 6 protons is carbon, regardless of how many neutrons it has. Elements are arranged on the periodic table in order of increasing atomic number, which is why hydrogen (1 proton) comes first.
9. Which of these is a noble gas?
A) Chlorine
B) Neon
C) Sodium
D) Sulfur
Answer: B) Neon Neon is a noble gas, part of Group 18 on the periodic table, known for having a full outer electron shell that makes these elements extremely unreactive. Noble gases rarely form compounds under normal conditions because their electron configuration is already stable. Neon is famously used in neon signage, where an electric current passed through the gas causes it to glow a distinctive reddish-orange color.
10. What is the process called when a solid changes directly into a gas without becoming a liquid?
A) Evaporation
B) Condensation
C) Sublimation
D) Deposition
Answer: C) Sublimation Sublimation occurs when a substance transitions directly from a solid to a gas, skipping the liquid phase entirely. Dry ice — solid carbon dioxide — is a well-known example, turning into carbon dioxide gas at room temperature without ever becoming liquid. This happens because the substance’s vapor pressure exceeds atmospheric pressure at that temperature, allowing molecules to escape directly from the solid surface.
11. Which acid is commonly found in the human stomach?
A) Sulfuric acid
B) Hydrochloric acid
C) Nitric acid
D) Acetic acid
Answer: B) Hydrochloric acid Hydrochloric acid, produced by cells in the stomach lining, helps break down food and kill harmful bacteria that might be ingested. It creates a highly acidic environment, typically with a pH between 1.5 and 3.5, which activates digestive enzymes like pepsin. The stomach protects itself from this powerful acid through a thick layer of mucus lining its walls.
12. What is the chemical formula for water?
A) CO2
B) H2O
C) O2
D) H2O2
Answer: B) H2O Water’s chemical formula, H₂O, indicates that each molecule consists of two hydrogen atoms covalently bonded to one oxygen atom. This simple molecule has a bent shape due to the two lone electron pairs on the oxygen atom, which gives water its polarity — a property responsible for many of its unique behaviors, such as its ability to dissolve a wide range of substances and its relatively high boiling point compared to similarly sized molecules.
13. Which metal is liquid at room temperature?
A) Iron
B) Mercury
C) Aluminum
D) Zinc
Answer: B) Mercury Mercury is the only metal that exists in liquid form at standard room temperature, with a melting point of about -38.83°C. This unusual property comes from its unique electron configuration, which results in relatively weak metallic bonding between its atoms. Mercury was historically used in thermometers and barometers, though its use has declined significantly due to its toxicity.
14. What do you call a substance that speeds up a chemical reaction without being consumed?
A) Reactant
B) Catalyst
C) Product
D) Solvent
Answer: B) Catalyst A catalyst speeds up the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without being consumed or permanently altered in the process. Enzymes are biological catalysts that make life-sustaining reactions in our bodies happen fast enough to be useful. Industrial processes also rely heavily on catalysts — the catalytic converter in a car, for instance, uses platinum and palladium to speed up reactions that reduce harmful emissions.
15. Which element has the chemical symbol “Fe”?
A) Fluorine
B) Francium
C) Iron
D) Ferrite
Answer: C) Iron Iron’s symbol, Fe, comes from its Latin name “ferrum.” Iron is one of the most widely used metals in the world, essential for building infrastructure and manufacturing steel, and it’s also biologically important as a core component of hemoglobin, the protein in red blood cells that carries oxygen throughout the body. Earth’s core is believed to be composed largely of iron and nickel.
16. What is the term for a substance that cannot be broken down into simpler substances by chemical means?
A) Compound
B) Mixture
C) Element
D) Solution
Answer: C) Element An element is a pure substance made up of only one type of atom and cannot be broken down into simpler substances through ordinary chemical reactions. There are 118 known elements, organized on the periodic table, ranging from naturally occurring elements like hydrogen and oxygen to synthetically created ones like oganesson. Compounds, by contrast, are made of two or more elements chemically bonded together and can be broken down into their constituent elements.
17. Which gas is produced when metals react with acids?
A) Oxygen
B) Carbon Dioxide
C) Hydrogen
D) Nitrogen
Answer: C) Hydrogen When reactive metals such as zinc, magnesium, or iron come into contact with acids like hydrochloric or sulfuric acid, they typically produce hydrogen gas along with a salt. This reaction happens because the metal displaces the hydrogen ions in the acid. The classic laboratory test for hydrogen gas is the “squeaky pop” test, where a lit splint is held near the gas and produces a distinctive popping sound as the hydrogen ignites.
18. What is the atomic mass unit primarily based on?
A) Hydrogen-1
B) Oxygen-16
C) Carbon-12
D) Helium-4
Answer: C) Carbon-12 The atomic mass unit (amu) is defined as exactly one-twelfth the mass of a carbon-12 atom, chosen as the standard reference because carbon-12 is stable and easy to measure precisely. This standardization allows scientists to compare the relative masses of different atoms consistently. Before carbon-12 was adopted as the standard in 1961, oxygen-16 had been used as the reference point.
19. Which type of reaction releases energy in the form of heat?
A) Endothermic
B) Exothermic
C) Isothermic
D) Photothermic
Answer: B) Exothermic An exothermic reaction releases energy, usually as heat, into its surroundings, causing the temperature around the reaction to rise. Combustion reactions, like burning wood or gasoline, are common examples of exothermic processes. This is the opposite of an endothermic reaction, which absorbs energy from its surroundings, often causing a noticeable drop in temperature, as seen when certain salts dissolve in water.
20. What is the name of the horizontal rows on the periodic table?
A) Groups
B) Periods
C) Families
D) Series
Answer: B) Periods The horizontal rows on the periodic table are called periods, and elements within the same period have the same number of electron shells, though their chemical properties vary significantly across the row. The vertical columns, in contrast, are called groups or families, and elements within the same group tend to share similar chemical properties because they have the same number of electrons in their outermost shell.
21. Which of the following is considered a base?
A) Vinegar
B) Lemon juice
C) Sodium hydroxide
D) Battery acid
Answer: C) Sodium hydroxide Sodium hydroxide, commonly known as lye or caustic soda, is a strong base with a pH well above 7. Bases release hydroxide ions (OH⁻) when dissolved in water and typically feel slippery to the touch, taste bitter, and can neutralize acids in what’s called a neutralization reaction. Sodium hydroxide is widely used in soap making, drain cleaners, and various industrial chemical processes.
22. What is the name for two or more atoms chemically bonded together?
A) Ion
B) Molecule
C) Isotope
D) Electron
Answer: B) Molecule A molecule forms when two or more atoms bond together chemically, whether they’re the same element (like O₂, oxygen gas) or different elements (like H₂O, water). Molecules are held together by covalent bonds, where atoms share electrons. The properties of a molecule can be vastly different from the properties of its individual constituent atoms — hydrogen and oxygen are both gases, but together as water, they form a liquid at room temperature.
23. Which element is the most abundant in the Earth’s crust?
A) Iron
B) Silicon
C) Oxygen
D) Aluminum
Answer: C) Oxygen Oxygen makes up roughly 46% of the Earth’s crust by mass, making it the single most abundant element there, largely because it combines readily with silicon and other elements to form the silicate minerals that make up most rocks. Silicon comes in second at about 28%. This is somewhat counterintuitive, since oxygen is most commonly associated with the atmosphere, but the vast majority of Earth’s oxygen is actually locked up in solid mineral form within the crust.
24. What is the process of a liquid turning into a gas called?
A) Condensation
B) Evaporation
C) Freezing
D) Sublimation
Answer: B) Evaporation Evaporation is the process by which a liquid gradually transforms into a gas, typically occurring at the surface of the liquid at temperatures below its boiling point. It happens because some molecules at the surface have enough kinetic energy to escape into the air, even though the bulk liquid hasn’t reached boiling temperature. This is different from boiling, which occurs throughout the liquid at a specific temperature, forming bubbles of vapor.
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25. Which of these elements is a halogen?
A) Helium
B) Chlorine
C) Sodium
D) Calcium
Answer: B) Chlorine Chlorine belongs to the halogen group (Group 17) on the periodic table, along with fluorine, bromine, iodine, and astatine. Halogens are highly reactive nonmetals that readily form salts when combined with metals — sodium chloride, ordinary table salt, is a well-known example. Chlorine gas is toxic in concentrated form but is widely used in diluted amounts to disinfect drinking water and swimming pools.
26. What is the chemical name for rust?
A) Iron sulfide
B) Iron oxide
C) Iron chloride
D) Iron carbonate
Answer: B) Iron oxide Rust forms when iron reacts with oxygen and moisture in the air over time, producing hydrated iron oxide, commonly represented as Fe₂O₃·nH₂O. This process, called oxidation, gradually weakens iron and steel structures, which is why protective coatings like paint, galvanization, or oiling are used to prevent metal from being exposed to the combination of oxygen and water that drives rusting.
27. Which particle has no electrical charge?
A) Proton
B) Electron
C) Neutron
D) Ion
Answer: C) Neutron Neutrons carry no electrical charge, making them electrically neutral, unlike protons (positive) and electrons (negative). Neutrons reside in the nucleus alongside protons and contribute to an atom’s mass without affecting its charge. The number of neutrons can vary among atoms of the same element, resulting in different isotopes — for example, carbon-12 and carbon-14 both have 6 protons but differ in their neutron count.
28. What is the name given to a solution with a pH less than 7?
A) Basic
B) Neutral
C) Acidic
D) Alkaline
Answer: C) Acidic A solution with a pH below 7 is classified as acidic, meaning it has a higher concentration of hydrogen ions (H⁺) than pure water. Common acidic substances include lemon juice (around pH 2), vinegar (around pH 2.5), and black coffee (around pH 5). The lower the pH number, the stronger the acidity, since the scale is logarithmic — a solution with pH 2 is ten times more acidic than one with pH 3.
29. Which gas do humans exhale in higher concentration than they inhale?
A) Oxygen
B) Nitrogen
C) Carbon Dioxide
D) Hydrogen
Answer: C) Carbon Dioxide Humans inhale air that’s roughly 0.04% carbon dioxide, but exhale air containing about 4% carbon dioxide — a hundredfold increase. This happens because cellular respiration in the body’s cells consumes oxygen and produces carbon dioxide as a waste product, which is then transported by the blood to the lungs and expelled. This exchange is a fundamental part of how the body generates energy from food.
30. What is the term for the number of atoms of each element in a compound?
A) Molar mass
B) Chemical formula
C) Valence
D) Atomic weight
Answer: B) Chemical formula A chemical formula specifies the exact number and type of atoms present in a molecule of a compound, such as CO₂ for carbon dioxide, indicating one carbon atom bonded to two oxygen atoms. Chemical formulas are essential shorthand in chemistry, allowing scientists to communicate the composition of substances precisely and to balance chemical equations, which must have equal numbers of each type of atom on both sides.
31. Which of the following best describes an isotope?
A) Atoms with different numbers of protons
B) Atoms with different numbers of neutrons
C) Atoms with different numbers of electrons
D) Atoms of different elements
Answer: B) Atoms with different numbers of neutrons Isotopes are variants of the same element that have the same number of protons (and therefore the same atomic number) but differ in their number of neutrons, which changes their atomic mass. Carbon-12, carbon-13, and carbon-14 are all isotopes of carbon. Some isotopes are stable, while others, like carbon-14, are radioactive and decay over time — a property used in techniques like carbon dating to determine the age of ancient organic materials.
32. What is the primary component of natural gas?
A) Propane
B) Methane
C) Butane
D) Ethane
Answer: B) Methane Methane (CH₄) is the primary component of natural gas, typically making up 70-90% of its composition, with smaller amounts of ethane, propane, and other hydrocarbons mixed in. It’s a colorless, odorless gas — the distinct smell associated with natural gas leaks is actually added artificially using a chemical called mercaptan, specifically so that leaks can be detected by smell for safety reasons.
33. Which of these substances is amphoteric, meaning it can react as both an acid and a base?
A) Sodium chloride
B) Water
C) Sulfuric acid
D) Sodium hydroxide
Answer: B) Water Water is amphoteric, meaning it can act as either an acid or a base depending on what it’s reacting with. In the presence of a stronger acid, water can accept a proton and behave as a base, while in the presence of a stronger base, it can donate a proton and behave as an acid. This dual behavior is central to water’s role as a near-universal solvent and its involvement in countless chemical and biological reactions.
34. What is the name of the law stating that matter cannot be created or destroyed in a chemical reaction?
A) Law of Definite Proportions
B) Law of Conservation of Mass
C) Avogadro’s Law
D) Law of Multiple Proportions
Answer: B) Law of Conservation of Mass The Law of Conservation of Mass, established by Antoine Lavoisier in the late 18th century, states that mass is neither created nor destroyed during a chemical reaction — the total mass of the reactants equals the total mass of the products. This principle underlies the process of balancing chemical equations, ensuring that the number of atoms of each element remains the same before and after a reaction occurs.
35. Which element is used in the core of a nuclear reactor as fuel?
A) Carbon
B) Uranium
C) Iron
D) Helium
Answer: B) Uranium Uranium, particularly the isotope uranium-235, is commonly used as fuel in nuclear reactors because its nucleus can be split apart in a process called nuclear fission, releasing a tremendous amount of energy. This energy heats water to produce steam, which drives turbines to generate electricity. Uranium fuel is typically “enriched” to increase the concentration of the fissile uranium-235 isotope before use in reactors.
36. What do you call the negatively charged ion?
A) Cation
B) Anion
C) Isotope
D) Radical
Answer: B) Anion An anion is an ion that carries a negative charge, formed when an atom or molecule gains one or more extra electrons. This contrasts with a cation, which carries a positive charge due to having lost electrons. In table salt (sodium chloride), the chloride ion (Cl⁻) is an anion, while the sodium ion (Na⁺) is a cation — the electrostatic attraction between these oppositely charged ions is what forms the ionic bond holding the compound together.
37. Which process describes a substance dissolving in water?
A) Sublimation
B) Solvation
C) Precipitation
D) Distillation
Answer: B) Solvation Solvation refers to the process by which solvent molecules, such as water, surround and interact with the particles of a solute, effectively dissolving it. When the solvent is specifically water, this process is often called hydration. Water is an especially effective solvent because of its polarity, allowing it to surround and stabilize both positive and negative ions, which is why so many salts and ionic compounds dissolve readily in it.
38. What is the boiling point of water at sea level in Celsius?
A) 90°C
B) 100°C
C) 110°C
D) 120°C
Answer: B) 100°C Water boils at 100°C (212°F) at standard atmospheric pressure, which is defined at sea level. This boiling point isn’t fixed everywhere, though — at higher altitudes, where atmospheric pressure is lower, water boils at a lower temperature, which is why cooking instructions sometimes need adjusting in mountainous regions. Conversely, under higher pressure, such as inside a pressure cooker, water can be heated above 100°C without boiling, allowing food to cook faster.
39. Which of the following is a synthetic polymer?
A) Cellulose
B) Silk
C) Polyethylene
D) Wool
Answer: C) Polyethylene Polyethylene is a synthetic polymer made by chemically linking together many repeating units of ethylene monomers into long chains. It’s the most widely produced plastic in the world, used in everything from plastic bags and bottles to piping. Unlike natural polymers such as cellulose, silk, and wool, which occur in living organisms, polyethylene and other synthetic polymers are manufactured through industrial chemical processes.
40. What is the term for the minimum energy needed to start a chemical reaction?
A) Bond energy
B) Activation energy
C) Potential energy
D) Kinetic energy
Answer: B) Activation energy Activation energy is the minimum amount of energy required to get a chemical reaction started, essentially the energy barrier that reactants must overcome before they can transform into products. Even reactions that release energy overall (exothermic reactions) usually require some initial input of energy to begin — striking a match, for example, provides the activation energy needed to start the combustion reaction, even though burning releases far more energy than it took to ignite.
41. Which metal is used to galvanize iron to prevent rusting?
A) Copper
B) Zinc
C) Tin
D) Lead
Answer: B) Zinc Galvanization is the process of coating iron or steel with a layer of zinc to protect it from corrosion. Zinc protects the underlying metal in two ways: it acts as a physical barrier against moisture and oxygen, and even if the coating is scratched, zinc is more reactive than iron and will corrode preferentially, sacrificially protecting the iron underneath in a process called cathodic protection.
42. What is the chemical formula for table salt?
A) NaCl
B) KCl
C) CaCl2
D) MgCl2
Answer: A) NaCl Table salt’s chemical formula, NaCl, represents sodium chloride, formed through an ionic bond between a positively charged sodium ion and a negatively charged chloride ion. This compound forms a crystal lattice structure, which is why salt appears as small cubic crystals. Salt is essential to human physiology, playing a critical role in nerve function, muscle contraction, and maintaining proper fluid balance in the body.
43. Which of the following describes an exothermic reaction most accurately?
A) Absorbs heat from surroundings
B) Releases heat to surroundings
C) Requires a catalyst
D) Occurs only in liquids
Answer: B) Releases heat to surroundings An exothermic reaction releases energy, usually in the form of heat, into its surrounding environment as the reaction proceeds, often causing a noticeable rise in temperature. Combustion, neutralization reactions between acids and bases, and many oxidation reactions are typically exothermic. This is because the products of the reaction have lower total energy than the reactants, with the difference released as heat.
44. What is the name for a chemical reaction where a compound breaks down into simpler substances?
A) Synthesis reaction
B) Decomposition reaction
C) Displacement reaction
D) Combustion reaction
Answer: B) Decomposition reaction A decomposition reaction occurs when a single compound breaks down into two or more simpler substances, often requiring an input of energy such as heat, light, or electricity to proceed. A classic example is the decomposition of water into hydrogen and oxygen gas through electrolysis. This is essentially the reverse of a synthesis reaction, in which simpler substances combine to form a more complex compound.
45. Which element has the highest electronegativity?
A) Oxygen
B) Fluorine
C) Chlorine
D) Nitrogen
Answer: B) Fluorine Fluorine has the highest electronegativity of any element on the periodic table, meaning it has the strongest tendency to attract electrons toward itself within a chemical bond. This extreme reactivity makes fluorine the most reactive nonmetal, capable of forming compounds with nearly every other element, including some noble gases. Its high electronegativity is why fluorine compounds, like those used in non-stick cookware coatings, tend to be exceptionally stable once formed.
46. What is the primary gas responsible for the greenhouse effect that humans have significantly increased?
A) Oxygen
B) Carbon Dioxide
C) Nitrogen
D) Hydrogen
Answer: B) Carbon Dioxide Carbon dioxide is the primary greenhouse gas that human activity, particularly the burning of fossil fuels like coal, oil, and natural gas, has significantly increased in the atmosphere since the Industrial Revolution. Greenhouse gases trap heat by absorbing and re-emitting infrared radiation, preventing it from escaping directly into space. While water vapor and methane are also greenhouse gases, carbon dioxide’s long atmospheric lifetime and the sheer scale of human emissions make it the central focus of climate science and policy.
47. Which of the following is a physical change rather than a chemical change?
A) Burning paper
B) Rusting iron
C) Melting ice
D) Baking a cake
Answer: C) Melting ice Melting ice is a physical change because it alters the state of the water (from solid to liquid) without changing its chemical composition — it’s still H₂O before and after. Physical changes are generally reversible and don’t produce new substances. Burning paper, rusting iron, and baking a cake, by contrast, are all chemical changes, since they involve chemical reactions that transform the substances into entirely new compounds that can’t easily be reversed.
48. What is the name for the outermost electron shell of an atom?
A) Core shell
B) Valence shell
C) Nucleus shell
D) Ionic shell
Answer: B) Valence shell The valence shell is the outermost electron shell of an atom, and the electrons within it, called valence electrons, are primarily responsible for determining how the atom will chemically bond with other atoms. Atoms tend to react in ways that allow them to achieve a full or stable valence shell, often by gaining, losing, or sharing electrons — this drive toward stability is the basis for most chemical bonding behavior described in chemistry.
49. Which acid is found in vinegar?
A) Citric acid
B) Acetic acid
C) Lactic acid
D) Formic acid
Answer: B) Acetic acid Vinegar’s sour taste and characteristic smell come from acetic acid, which typically makes up about 4-8% of its composition, with the rest being mostly water. Acetic acid is a weak acid, meaning it doesn’t fully dissociate into ions in water, unlike strong acids such as hydrochloric or sulfuric acid. Vinegar has been used for thousands of years, both as a food preservative, since the acidic environment inhibits the growth of many harmful bacteria, and as a cleaning agent.
50. What is the term for the amount of a substance that contains 6.022 × 10²³ particles?
A) Gram
B) Mole
C) Molecule
D) Atomic mass unit
Answer: B) Mole A mole is a unit used in chemistry to count particles — atoms, molecules, or ions — and is defined as exactly 6.022 × 10²³ particles, a number known as Avogadro’s number. This unit allows chemists to work conveniently with the enormous quantities of atoms and molecules involved in even small chemical samples, bridging the gap between the microscopic scale of individual atoms and the macroscopic scale of grams and liters that can actually be measured in a lab.
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