Which choice can be classified as a pure substance? salt water
Iced Tea
pumpkin pie
salt

Salt

Explanation:

It is because it has a defined composition.

Salt

Explanation:

Pure substances are those that contain atoms or molecules of the same type. For eg: elements are pure (gold, hydrogen etcl.) and compounds (such as water, sodium chloride, etc.), etc.

Rest of choice are impure because they contain a mixture of different things, like pie has eggs, or all sorts of other molecules, salt water has water and salt molecules, and iced tea has water and other molecules associated with tea.

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explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.