Chemical Reactions

CloseA balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides. This follows the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. For example, in 2H₂ + O₂ → 2H₂O, there are 4 hydrogen atoms and 2 oxygen atoms on both sides.

CloseThe law of conservation of mass, given by Antoine Lavoisier, states that mass can neither be created nor destroyed in a chemical reaction. Balancing a chemical equation ensures that the total mass of reactants equals the total mass of products, which is a direct application of this law.

CloseMelting ice is a physical change because it only changes the state of water from solid to liquid without forming any new substance. The chemical composition H₂O remains the same. In contrast, cooking food, rusting iron, and burning a candle all produce new substances with different chemical properties.

CloseCoefficients are the numbers written in front of chemical formulas in an equation. They indicate the number of molecules or moles of that substance involved in the reaction. For example, in 2H₂ + O₂ → 2H₂O, the coefficient 2 before H₂ and H₂O shows that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.

CloseIn a chemical equation, substances written on the left side of the arrow are reactants (starting materials), and substances written on the right side are products (new substances formed). Here, magnesium (Mg) and oxygen (O₂) react to form magnesium oxide (MgO), so MgO is the product.

CloseBurning coal is a chemical reaction because coal (carbon) reacts with oxygen to form new substances like carbon dioxide and heat. Cutting wood and tearing paper are physical changes that only change the size and shape. Freezing water is also a physical change where liquid water becomes solid ice without changing its chemical composition.

CloseA balanced equation has an equal number of atoms of each element on both sides. In option B, the left side has 4 hydrogen atoms (2 x H₂ = 4H) and 2 oxygen atoms (O₂), while the right side has 4 hydrogen atoms (2 x H₂O = 4H) and 2 oxygen atoms (2 x H₂O = 2O). The other options have unequal numbers of atoms on the two sides.

CloseIn this reaction, zinc (Zn) and hydrochloric acid (HCl) are the reactants written on the left side. The arrow points to the products on the right side: zinc chloride (ZnCl₂) and hydrogen gas (H₂). Therefore, hydrogen is formed as a product, not used as a reactant.

CloseThe law of conservation of mass, proposed by Antoine Lavoisier in 1789, states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. Mass is neither created nor destroyed; it is simply rearranged as atoms break old bonds and form new ones. This principle is the foundation for balancing chemical equations.

CloseIn a chemical reaction, the starting materials are called reactants, and the new substances formed as a result of the reaction are called products. For example, when magnesium burns in oxygen, magnesium oxide is the product. Products are always written on the right side of the arrow in a chemical equation.

CloseBy convention, in a chemical equation, the reactants (substances that undergo change) are always written on the left-hand side before the arrow. The products (new substances formed) are written on the right-hand side after the arrow. For example, in CaO + H₂O → Ca(OH)₂, CaO and H₂O are the reactants on the left side.

CloseThe chemical formula of water is H₂O, which shows that one molecule of water contains two hydrogen atoms and one oxygen atom. H₂ is hydrogen gas, H₂O₂ is hydrogen peroxide (a different compound used as a disinfectant), and HO₂ is the hydroperoxyl radical, which is unstable and not water.

CloseIn chemical equations, state symbols are written in brackets after each formula to indicate the physical state of the substance. (s) stands for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution (dissolved in water). For example, in NaCl(s), sodium chloride is in solid form.

CloseThe Greek letter delta (Δ) is commonly written above or below the arrow in a chemical equation to indicate that heat is supplied to carry out the reaction. For example, in CaCO₃(s) → CaO(s) + CO₂(g) with Δ written above the arrow, it shows that calcium carbonate decomposes only when heated strongly.

CloseFor a chemical equation to be balanced, the number of atoms of each element must be exactly the same on both the reactant and product sides. This is achieved by placing appropriate coefficients before the formulas. You should never change the subscripts inside a formula, as that would change the identity of the compound.

CloseReactants are the starting substances that undergo chemical change during a reaction. They are consumed as the reaction proceeds and are written on the left side of the arrow. For example, in the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, methane (CH₄) and oxygen (O₂) are the reactants.

CloseAt the atomic level, a chemical reaction involves breaking the chemical bonds between atoms in the reactant molecules and forming new bonds to create product molecules. This rearrangement of atoms is what produces new substances with different properties. Freezing, melting, and boiling are physical changes that do not involve breaking or making chemical bonds.

CloseA balanced chemical equation ensures that the total mass of reactants equals the total mass of products. This is because the number of atoms of each element is the same on both sides, and atoms have fixed masses. For example, in 2Mg + O₂ → 2MgO, the total mass of 2 magnesium atoms and one oxygen molecule equals the mass of two magnesium oxide formula units.

CloseIron (Fe) and sulfur (S) are two different elements. When they react chemically, they form iron sulfide (FeS), which is a compound because it contains two or more elements chemically combined in a fixed ratio. A compound has properties completely different from its constituent elements, unlike a mixture where components retain their individual properties.

CloseThe arrow in a chemical equation is read as “gives”, “yields”, “forms”, or “produces”. It shows the direction of the reaction, pointing from the reactants (which are consumed) to the products (which are formed). For example, the equation C + O₂ → CO₂ is read as “carbon plus oxygen gives carbon dioxide”.

CloseThe defining characteristic of a chemical reaction is the formation of one or more new substances with chemical properties different from the original reactants. This is accompanied by changes such as color change, gas evolution, temperature change, or precipitate formation. Physical changes, like change in shape, do not produce new substances.

CloseRusting of iron is a chemical change because iron (Fe) reacts with oxygen (O₂) and moisture to form a new compound called hydrated iron oxide (rust), which has completely different properties from iron. Melting wax, breaking chalk, and dissolving salt are physical changes because no new substance is formed; the original material can be recovered.

CloseIn chemical equations, (l) is the state symbol for liquid. It indicates that the substance is in the liquid state at room temperature or under the reaction conditions. For example, H₂O(l) represents liquid water, while H₂O(g) would be water vapor or steam. The other common state symbols are (s) for solid, (g) for gas, and (aq) for aqueous solution.

CloseCoefficients are numbers written immediately before the chemical formulas in an equation. They indicate the number of molecules or moles of that substance participating in the reaction. For example, in 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g), the coefficient 3 before Fe and 4 before H₂O and H₂ are placed before the formulas, not after or below them.

CloseIn a chemical reaction, products are the new substances that are formed as a result of the reaction. They are written on the right side of the arrow. For example, in the reaction 2H₂ + O₂ → 2H₂O, water (H₂O) is the product because it is the substance formed. Reactants and starting materials refer to the original substances, while a catalyst speeds up the reaction without being consumed.

Close(aq) stands for aqueous, which means the substance is dissolved in water to form a solution. For example, NaCl(aq) represents a solution of sodium chloride (table salt) dissolved in water. This is different from (l) which is pure liquid, and (s) which is solid. Many ionic compounds in reactions are written as (aq) because they dissociate into ions in water.

CloseIn a chemical equation, the products are always written on the right-hand side of the arrow. The arrow points from the reactants (left side) to the products (right side). For example, in the equation NaOH + HCl → NaCl + H₂O, sodium chloride (NaCl) and water (H₂O) are the products and appear on the right side.

CloseAccording to the law of conservation of mass, the total mass of reactants in a balanced chemical equation is always equal to the total mass of products. This is because the number of atoms of each element is the same on both sides, and atoms have fixed masses. The number of molecules may change (e.g., 2H₂ + O₂ → 2H₂O has 3 molecules on the left and 2 on the right), but mass remains constant.

CloseBurning magnesium is a chemical reaction where magnesium metal reacts with oxygen to form a new substance called magnesium oxide (MgO). This is evident from the bright white flame and the white powdery ash formed. Cutting paper, boiling water, and freezing milk are physical changes where no new substance is created; the original material can be recovered.

CloseIn chemical equations, (g) is the state symbol for gas. It indicates that the substance is in the gaseous state under the reaction conditions. For example, O₂(g) represents oxygen gas, and CO₂(g) represents carbon dioxide gas. Other state symbols include (s) for solid, (l) for liquid, and (aq) for aqueous solution. Including these symbols makes the chemical equation more informative.