Hi i have a question about my science homework can someone help?

Question

how can 2 different substance have the same mass but different volumes ?

how do we know if a substance is undergoing a physical or chemical change?

what do we know about the structure of atoms?

Answer 1

this is based on specifc gravity or density. For example you see that water is lighter than iron. If we take same mass of of water and iron then iron shall have less valume and water more

There are several differences between a physical and chemical change in matter or substances.

A physical change in a substance doesn't change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed.

For example, if a piece of paper is cut up into small pieces it still is paper. This would be a physical change in the shape and size of the paper. If the same piece of paper is burned, it is broken up into different substances that are not paper.

Physical changes can be reversed, chemical changes cannot be reversed with the substance changed back without extraordinary means, if at all. For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated.

If one decided to mix sugar into water to make sugar water, this would be a physical change as the water could be left out to evaporate and the sugar crystals would remain. However, if one made a recipe for a cake with flour, water, sugar and other ingredients and baked them together, it would take extraordinary means to separate the various ingredients out to their original form.

When heat is given off in a chemical change or reaction, it is called an exothermic reaction. When heat is absorbed in a chemical change or reaction, it is called an endothermic reaction. The speed at which chemical reactions take place depend on the temperature pressure and how concentrated the substances involved in the chemical reaction are. Sometimes substances called catalysts are used to speed up or help along a chemical reaction. Light is helpful in the processing of film.

for structure of atom please refer to

http://www.chemguide.co.uk/atoms/propert...

Answer 2

some substances are heavier than others, to it takes less of it to have the same mass as another substance [aka, hydrogen is lighter than oxygen, so you need more hydrogen to have the same mass as oxygen. thus, the hydrogen has a greater volume]

a physical change just changes the appearance...the chemical composition remains the same [like if you rip a piece of paper..that's physical change]

a chemical change changes the chemical composition of the object [like if you burn paper, it becomes soot, which has a different chemical comp. than regular paper]

atoms have protons neutrons and electrons. the electrons have a negative charge, protons are positive, and neutrons are neutral

the atomic number of an element is equal to how many protons it has