匿名
匿名 發問於 科學及數學化學 · 1 十年前

science questions

the sentences below are wrong, please write out the places where there are mistakes. Also write out the correct version:

1) Physical change, like burning, are difficult to reverse.

2) A liquid is always formed during a precipitation reaction.

3) The substances you start with in a chemical reaction are called the products.

4) Energy is usually given out when elements are formed from a compound.

5) The new substances formed in chemical reactions are called reactants.

6) All chemical reactions need heat to start then off.

Please answer these questions in english, thank you =)

2 個解答

評分
  • 1 十年前
    最愛解答

    1) Physical change, like burning, are difficult to reverse.

    Physical changes to Chemical

    Since buring is a process react with oxygen, new product is formed and it is not a physical change.

    2) A liquid is always formed during a precipitation reaction.

    Liquid changes to Solid

    Since precipitation reaction is a reaction which gives out insoluble solid, if no solid is formed, it is not a precipitation.

    3) The substances you start with in a chemical reaction are called the products.

    Products change to Reactants

    Products are the substances after a reaction. Reactants are the substances you want them to react, which are before a reaction.

    4) Energy is usually given out when elements are formed from a compound.

    Usually changes to Never

    If elements are formed from a compound, energy must be needed to break the bonding within the compound, so energy is consumed instead of being given out.

    5) The new substances formed in chemical reactions are called reactants.

    Reactants change to Product

    Same reason as question 3.

    6) All chemical reactions need heat to start then off.

    All changes to Some

    Not all chemical reactions need heat to start, for instance, reaction of group 1 elements with water.

還有問題嗎?立即提問即可得到解答。