Test: Chemical Changes and Reactions - Class 9 MCQ

Heating mercuric chloride causes it to decompose into mercury and chlorine gas. This reaction exemplifies a decomposition reaction, where a single compound breaks down into simpler substances upon the application of heat.

When hydrogen gas reacts with chlorine gas in the presence of light, it produces hydrogen chloride gas. This reaction is a classic example of a photochemical reaction, as light energy initiates the reaction.

A precipitation reaction occurs when two solutions react to form an insoluble solid, known as a precipitate. The reaction between silver nitrate and sodium chloride produces a white precipitate of silver chloride.

During a chemical reaction, reactants undergo a transformation to create products, which typically have different chemical and physical properties compared to the original substances. This process involves breaking and forming chemical bonds.

A decomposition reaction involves the breakdown of a compound into two or more simpler substances. For instance, mercuric oxide decomposes into mercury and oxygen when heated, illustrating how a single compound can yield multiple simpler products.

In a double displacement reaction, two compounds exchange ions, resulting in the formation of two new compounds. This type of reaction is characterized by the mutual exchange of radicals, exemplified by the reaction between barium chloride and sodium sulfate.

One of the key characteristics of chemical reactions is the evolution of gas. For example, when zinc reacts with dilute sulfuric acid, hydrogen gas is produced, which can often be observed as fizzing or effervescence.

A catalyst is utilized to speed up a chemical reaction without being consumed in the process. For instance, manganese dioxide facilitates the breakdown of hydrogen peroxide more rapidly than would occur without it.

The electrolysis of water is a chemical reaction that requires an electric current to break down water into hydrogen and oxygen gases. This process illustrates how electrical energy can drive chemical changes.

While many reactions can occur at various temperatures, a constant temperature is not a requirement for all chemical reactions. What is essential is that reactant particles collide with enough energy, often facilitated by heat, light, or a catalyst.

In photochemical reactions, light provides the necessary energy to drive the reaction forward. Photosynthesis in plants is a prime example where sunlight is absorbed to convert carbon dioxide and water into glucose and oxygen.

The reaction between iron and copper sulfate results in a noticeable color change. Initially blue due to copper sulfate, the solution turns green as ferrous sulfate forms and settles out copper metal, which appears reddish-brown.

A neutralization reaction occurs when an acid reacts with a base, resulting in the formation of salt and water. This process is essential in various applications, including treating acidity in the stomach and adjusting soil pH for agriculture.

Double displacement reactions often result in the formation of a precipitate when two aqueous solutions react to form an insoluble solid. For example, the reaction between silver nitrate and sodium chloride yields a white precipitate of silver chloride.

A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. For example, manganese dioxide acts as a catalyst in the decomposition of potassium chlorate, lowering the temperature required for the reaction to occur.

A combination reaction, also known as synthesis, occurs when two or more reactants combine to create a single product. An example is the reaction of carbon with oxygen to form carbon dioxide.

In a chemical reaction, the state of reactants and products can change. For example, ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride, demonstrating a change in physical state during the reaction.

An exothermic reaction releases heat to the surroundings, often causing an increase in temperature. A common example is the combustion of carbon, which produces carbon dioxide and releases heat energy.

A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. An example is the reaction of zinc with copper sulfate, where zinc replaces copper, resulting in a new compound and the release of copper metal.

The decomposition of calcium carbonate into calcium oxide and carbon dioxide is an endothermic reaction, meaning it absorbs heat from the surroundings. This is typically observed when calcium carbonate is heated to high temperatures.