The answer is H₂ because there are 2 Hydrogen atoms from the reactant side that are unaccounted for in the products.

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Thus, the amount of matter cannot change. Because mass is always conserved in chemical reactions, balanced chemical equations have the same number and type of each atom on both sides of the equation. Products in chemical reactions are the result of the same number of atoms being rearranged into new substances. 

The law of conservation of mass only applies in “closed systems” meaning that the mass and energy is not able to escape to the environment. For example, if you are boiling water on the stove and you put a lid on your pot, the water vapor will collect inside the pot. This is a closed system, and the amount of mass will not change, though it will be converted from liquid to gas. If you take the lid off your pot, the evaporated water will escape into your kitchen and the total amount of matter will change. You opened your system!

A conductor is a substance that transfers electric or heat energy easily.

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is water because electric and heat energy move easily through it.

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is glass.

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is "the surrounding rubber is an insulator and the plug is a conductor."

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

An insulator is a substance that does not readily transfer energy.

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

Jamal would also need an electrical conductor to complete the circuit

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is "wool"

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is "a conductor."

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.

The answer is an iron nail.

Some types of matter transfer energy more easily than others. These substances are called “conductors.” An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Some examples of conductors are copper, steel, gold, aluminum, and brass. If a substance does NOT transfer energy efficiently, it is called an insulator. Some common insulators are: Glass, Air, Plastic, Rubber, Porcelain, and Wood.