Why does mass stay the same

When a chemical reaction occurs, the products should equal the reactants. Why does mass always remain constant? Alex K. Nov 30, The law of conservation of mass. If the number of atoms stays the same no matter how they are rearranged, then their total weight stays the same. In all physical and chemical changes, the total number of atoms remains the same, hence when substances interact with one another, combine or break apart, the total weight of the system remains the same.

Growing plants obtain their new carbon the great majority of their dried weight from carbon dioxide i. When we lose weight by dieting or exercise, most of the loss is from breathing out the carbon atoms metabolised from our fat as carbon dioxide. When a liquid evaporates in a sealed container, the weight stays the same; the gas particles are affected by gravity in the same way as tennis balls and they consequently hit the bottom surface of the container with more force than they hit the top.

In your teaching of conservation of matter and hence weight, students need to be encouraged to change their views from those based on their everyday experiences to more scientific views such as the idea that there are only a fixed number of particles in the world and these building blocks are continually being rearranged into new things.

This is a difficult, abstract idea and we can use analogies to assist students with understanding. One of the difficulties is that closed systems involving changes such as combustion and respiration are almost impossible to set up and weigh in a classroom. You can disprove by experiment only a few of the common alternative conceptions in this area.

It is important to discuss a variety of change situations that appear to involve non-conservation of matter and to revisit this issue in other topics such as ecosystems, food and diet, and sources of energy. Students should be encouraged to make predictions about these processes in a supportive classroom environment where they can be assisted to develop new theories, critically analyse their understandings and those of others, and compare these with scientific views presented by the teacher.

It is easier to try to eliminate the alternative conceptions that can be experimentally tested first and have students then come to an understanding of the scientific model as one which can best explain a wide range of phenomena. Reflection by students on how their views have altered will be a valuable component of achieving long lasting conceptual change in students and an understanding of the scientific explanations.

Students should both discuss and observe phenomena where there is an apparent change in weight and be encouraged to seek alternative explanations.

POE Predict -Observe-Explain activities can be valuable in both eliciting student prior views and leading to reframing of these concepts. The best fit line gives a slope of The slope tells me how the mass should change with a change in the period I know that might seem backwards—but there is a reason.

The intercept tells me the effective mass of the oscillating tray that holds the mass I add. Suppose I want to measure an unknown mass. I just stick it on the inertial balance and oscillate it. After measuring the period, I can find the value of the unknown mass with this equation. That's it. You get the mass without a gravitational force.

Of course this equation only works for THIS inertia balance. If you have one yourself, you need to first calibrate it with some known masses just like I did here. Oh, maybe you will like to see this video. It shows an actual astronaut measuring his weight on the International Space Station using an inertial balance. It's real. So here's a quick breakdown of weight versus mass: Mass: If you could count up the number of protons, neutrons, and electrons in an object which you probably can't , this would be a measure of the mass.

Measuring Mass With a Gravitational Balance. Measuring Mass With an Inertial Balance. Rhett Allain is an associate professor of physics at Southeastern Louisiana University. He enjoys teaching and talking about physics. Sometimes he takes things apart and can't put them back together.

Contributor Twitter. Topics Dot Physics Gravity physics. Sometimes reactions that happen in open beakers are closed systems. These include:. In both examples, the reactants and products stay in the beaker. The total mass of the beaker and the substances it contains stays the same during the reaction.

Substances can enter or leave a non-enclosed system. These systems are often open flasks or crucibles that let gases enter or leave.