No, not all substances are solid at 20 degrees celsius. The state of a substance at a given temperature and pressure depends on its melting point and boiling point. At 20 degrees celsius, some substances will be solid, some will be liquid, and some will be gas.

For example, at 20 degrees celsius, water is a liquid, ice is a solid, and oxygen is a gas. The melting point of water is 0 degrees celsius, and its boiling point is 100 degrees celsius. The melting point of ice is also 0 degrees celsius, and its boiling point is 100 degrees celsius. The melting point of oxygen is -218.4 degrees celsius, and its boiling point is -183 degrees celsius.

Therefore, it is incorrect to say that all substances are solid at 20 degrees celsius. The state of a substance at a given temperature and pressure depends on its melting point and boiling point.