Please answer the questions for Activity 4: Pure Substances, including questions 5.1, 5.2, and 5.3 using the periodic table.

Here are the answers to Activity 4: Pure Substances.

1. The two known pure substances are:

   • Elements
   • Compounds

2. An element is a pure substance made of atoms of the same kind.

3. Elements and compounds are known as pure substances because they have a constant composition and their properties remain the same or predictable. Elements are made of only one type of atom, and compounds are made of pure atoms combined in a fixed ratio.

4. When we say "its properties remain the same or predictable," it means that a pure substance will always exhibit the same physical and chemical characteristics under specific conditions, regardless of its source or how it was prepared. For example, pure water will always boil at $100^\circ\text{C}$ at standard atmospheric pressure.

5. Using the periodic table:

   5.1 In which group do we find hydrogen?

   • Group 1

   5.2 In which period do we find He, Li, Na and K?

   • He is in Period 1.
   • Li is in Period 2.
   • Na is in Period 3.
   • K is in Period 4.

   5.3 Which element has an atomic number of 20?

   • Calcium (Ca)