Conservation of mass

• What does the law of conservation of mass state?

  During a chemical reaction, atoms are neither made nor destroyed. Mass is conserved
• What is a chemical change?

  A rearrangement of atoms in the reactants to form the products
• Why is it more difficult to measure the mass of a gas than the mass of a solid or liquid?

  Because a gas can easily escape into the air
• Why might mass increase in a chemical reaction?

  One of the reactants may be a gas that's found in the air and the products aren't.

  Before the reaction, the gas is floating in the air yet not contained in the reaction vessel so you can't account for its mass.

  When the gas reacts to form part of the product, it becomes contained in the reaction vessel so the total mass of the the substances inside the reaction vessel INCREASES
• Why might a mass decrease in a chemical reaction?

  Because some - or all - of the reactants are solids, liquids or aqueous and at least one of the products is a gas.

  Before the reaction, any solid, liquid or aqueous reactants are contained in the reaction vessel.

  If the vessel isn't enclosed, the the gas can escape from the reaction vessel as it's formed and so you can't account for its mass.

  The total mass of the substances of the reaction vessel DECREASES.