Chemistry Test: Questions & Answers For Every Level

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings. This means that the reaction requires an input of energy in order to proceed. In an endothermic reaction, the products have a higher energy level than the reactants, and the excess energy is absorbed from the surroundings. This is in contrast to an exothermic reaction, which releases energy to the surroundings.

A catalyst is a substance that is used to speed up a chemical reaction without being consumed or permanently changed in the process. It works by providing an alternative pathway for the reaction to occur with lower activation energy, allowing the reaction to proceed more quickly. Unlike reactants and products, which are directly involved in the reaction and are consumed or produced, a catalyst remains unchanged and can be used again in subsequent reactions.

An atom is considered to be neutral when the number of protons and electrons are equal. In an atom, protons have a positive charge, while electrons have a negative charge. The positive charge of the protons is balanced out by the negative charge of the electrons, resulting in a neutral overall charge for the atom. 

An exothermic reaction is a type of chemical reaction in which energy is released in the form of heat or light. This means that the products of the reaction have less energy than the reactants, resulting in a net release of energy. Examples of exothermic reactions include combustion reactions, where a fuel reacts with oxygen to produce heat and light, and many types of oxidation reactions.

In a chemical reaction, reactants undergo a transformation to form new substances known as products. These products are the end result of the reaction and can have different properties than the reactants. Reactants are the starting materials that participate in the reaction, while catalysts are substances that speed up the reaction without being consumed themselves.

 Reactants are the substances that enter a chemical reaction. They are the starting materials that undergo a chemical change to form new substances called products. Reactants are consumed during the reaction and are usually written on the left side of a chemical equation. In contrast, products are substances that are formed as a result of the reaction and are written on the right side of the equation. Catalysts are substances that speed up the reaction but are not consumed in the process.

A chemical change occurs when a substance undergoes a transformation that alters its chemical composition. Rusting iron is a chemical change because iron reacts with oxygen and moisture to form iron oxide, a new substance. In contrast, melting ice, boiling water, and breaking glass are physical changes as they do not change the substance's chemical identity.

Valence electrons are the electrons in the outermost shell of an atom that are involved in chemical bonding. These electrons determine the atom's reactivity and ability to form chemical bonds with other atoms. The nucleus, which contains protons and neutrons, does not directly participate in chemical bonding. 

A covalent bond is formed when electrons are shared between two atoms. In this type of bond, both atoms contribute electrons to form a shared electron pair. This sharing of electrons allows both atoms to achieve a more stable electron configuration. Covalent bonds are typically found between nonmetal atoms and result in the formation of molecules. Ionic bonds, on the other hand, involve the transfer of electrons from one atom to another, resulting in the formation of ions. 

The atom is the smallest unit of an element that still retains the chemical properties of that element. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. While atoms can combine to form molecules and compounds, it is the individual atom that defines the element. For example, a gold atom is what makes gold gold. If you were to break down a gold atom further, it would no longer be gold. Molecules are formed by the chemical bonding of two or more atoms, and compounds are formed when atoms of two or more different elements chemically combine. Ions are atoms or molecules that have gained or lost electrons, acquiring an electrical charge.

An ionic bond is a type of chemical bond that occurs between metals and nonmetals. In this type of bond, electrons are transferred from the metal atom to the nonmetal atom, resulting in the formation of positive and negative ions. The opposite charges of the ions attract each other, creating a strong bond. This bond is characterized by the electrostatic attraction between the oppositely charged ions.

A covalent bond occurs between nonmetals and nonmetals when they share electrons. In this type of bond, the atoms involved have similar electronegativities, which means they have a similar ability to attract electrons. As a result, they share electrons to achieve a stable electron configuration. This sharing of electrons creates a strong bond between the atoms, forming a molecule. 

An ionic bond is formed when electrons are transferred from one atom to another. In this type of bond, one atom gains electrons to become negatively charged (anion) while the other atom loses electrons to become positively charged (cation). The attraction between these opposite charges holds the atoms together, forming the ionic bond. Covalent bonds, on the other hand, involve the sharing of electrons between atoms.

After a chemical reaction, the properties of the products are different than the properties of the reactants. This is because during a chemical reaction, new substances are formed with different chemical and physical properties compared to the starting materials. The reaction involves the rearrangement of atoms and the formation of new chemical bonds, leading to a change in the properties of the substances involved.