A Celsius thermometer with a range of -10°C cannot be used to determine when the soft ball stage is reached in the candy mixture.
Temperature is most commonly measured with a thermometer that has two different scales: Fahrenheit and Celsius. Water freezes at 0°C and boils at 100°C on the Celsius scale and freezes at 32°F and boils at 212°F on the Fahrenheit scale.
Temperature of 236°F corresponds to 113.3°C. Now a Celsius scale has a maximum of 100°C on the thermometer. Hence a Celsius thermometer cannot be used to measure a temperature that corresponds to 236°F.